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Dedicated to baba, Who couldn’t be here to see this day...
Page 3 : About the Author, Destiny drew Yashavant Kanetkar towards computers when the IT, industry was just making a beginning in India. Having completed, his education from VJTI Mumbai and IIT Kanpur in Mechanical, Engineering he started his training company in Nagpur., Yashavant has a passion for writing and is an author of several, books in C, C++, VC++, C#, .NET, DirectX and COM, programming. He is a much sought after speaker on various, technology subjects and is a regular columnist for Express, Computers and Developer 2.0. His current affiliations include, being a Director of KICIT, a training company and DCube, Software Technologies, a software development company. In, recognition to his contribution Microsoft awarded him the, prestigious “Best .NET Technical Contributor” award recently., He can be reached at
[email protected].
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Preface to the Fifth Edition, It is mid 2004. World has left behind the DOTCOM bust, 9/11, tragedy, the economic downturn, etc. and moved on. Countless, Indians have relentlessly worked for close to two decades to, successfully establish “India” as a software brand. At times I take, secret pleasure in seeing that a book that I have been part of, has, contributed in its own little way in shaping so many budding, careers that have made the “India” brand acceptable., Computing and the way people use C for doing it keeps changing, as years go by. So overwhelming has been the response to all the, previous editions of “Let Us C” that I have now decided that each, year I would come up with a new edition of it so that I can keep, the readers abreast with the way C is being used at that point in, time., There are two phases in every C programmer’s life. In the first, phase he is a learner trying to understand the language elements, and their nuances. At this stage he wants a simple learning, environment that helps him to master the language. In my opinion,, even today there isn’t any learning environment that can beat, Turbo C/C++ for simplicity. Hence the first fifteen chapters are, written keeping this environment in mind, though a majority of, these programs in these chapters would work with any C compiler., Armed with the knowledge of language elements the C, programmer enters the second phase. Here he wishes to use all that, he has learnt to create programs that match the ability of programs, that he see in today’s world. I am pointing towards programs in, Windows and Linux world. Chapters 16 to 21 are devoted to this. I, would like to your attention the fact that if you want to program, Windows or Linux you need to have a very good grasp over the, programming model used by each of these OS. Windows, messaging architecture and Linux signaling mechanism are the, cases in point. Once you understand these thoroughly rest is just a, vi
Page 5 : matter of time. Chapters 16 to 21 have been written with this, motive., In Linux programming the basic hurdle is in choosing the Linux, distribution, compiler, editor, shell, libraries, etc. To get a headstart you can follow the choices that I found most reasonable and, simple. They have been mentioned in Chapter 20 and Appendix H., Once you are comfortable you can explore other choices., In fourth edition of Let Us C there were chapters on ‘Disk Basics’,, ‘VDU Basics’, ‘Graphics’, ‘Mouse Programming’, ‘C and, Assembly’. Though I used to like these chapters a lot I had to take, a decision to drop them since most of them were DOS-centric and, would not be so useful in modern-day programming. Modern, counterparts of all of these have been covered in Chapters 16 to, 21. However, if you still need the chapters from previous edition, they are available at www.kicit.com/books/letusc/fourthedition., Also, all the programs present in the book are available in source, code form at www.kicit.com/books/letusc/sourcecode. You are, free to download them, improve them, change them, do whatever, with them. If you wish to get solutions for the Exercises in the, book they are available in another book titled ‘Let Us C, Solutions’., ‘Let Us C’ is as much your book as it is mine. So if you feel that I, could have done certain job better than what I have, or you have, any suggestions about what you would like to see in the next, edition, please drop a line to
[email protected]., All the best and happy programming!, , vii
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Contents, 1. Getting Started, , 1, , What is C, Getting Started with C, The C Character Set, Constants, Variables and Keywords, Types of C Constants, Rules for Constructing Integer Constants, Rules for Constructing Real Constants, Rules for Constructing Character Constants, Types of C Variables, Rules for Constructing Variable Names, C Keywords, The First C Program, Compilation and Execution, Receiving Input, C Instructions, Type Declaration Instruction, Arithmetic Instruction, Integer and Float Conversions, Type Conversion in Assignments, Hierarchy of Operations, Associativity of Operators, Control Instructions in C, Summary, Exercise, , 2, 4, 5, 6, 7, 8, 9, 10, 11, 11, 12, 13, 19, 21, 23, 24, 25, 29, 29, 31, 34, 37, 37, 38, , 2. The Decision Control Structure, , 49, , Decisions! Decisions!, The if Statement, The Real Thing, Multiple Statements within if, The if-else Statement, , 50, 51, 55, 56, 58, , viii
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Nested if-elses, Forms of if, Use of Logical Operators, The else if Clause, The ! Operator, Hierarchy of Operators Revisited, A Word of Caution, The Conditional Operators, Summary, Exercise, 3. The Loop Control Structure, Loops, The while Loop, Tips and Traps, More Operators, The for Loop, Nesting of Loops, Multiple Initialisations in the for Loop, The Odd Loop, The break Statement, The continue Statement, The do-while Loop, Summary, Exercise, 4. The Case Control Structure, Decisions Using switch, The Tips and Traps, switch Versus if-else Ladder, The goto Keyword, Summary, Exercise, , ix, , 61, 62, 64, 66, 72, 73, 73, 76, 77, 78, , 97, 98, 99, 101, 105, 107, 114, 115, 116, 118, 120, 121, 124, 124, , 135, 136, 140, 144, 145, 148, 149
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5. Functions & Pointers, , 157, , What is a Function, Why Use Functions, Passing Values between Functions, Scope Rule of Functions, Calling Convention, One Dicey Issue, Advanced Features of Functions, Function Declaration and Prototypes, Call by Value and Call by Reference, An Introduction to Pointers, Pointer Notation, Back to Function Calls, Conclusions, Recursion, Recursion and Stack, Adding Functions to the Library, Summary, Exercise, 6. Data Types Revisited, , 158, 165, 166, 171, 172, 173, 174, 175, 178, 178, 179, 186, 189, 189, 194, 197, 201, 201, , 213, , Integers, long and short, Integers, signed and unsigned, Chars, signed and unsigned, Floats and Doubles, A Few More Issues…, Storage Classes in C, Automatic Storage Class, Register Storage Class, Static Storage Class, External Storage Class, Which to Use When, Summary, Exercise, , x, , 214, 216, 217, 219, 221, 223, 224, 226, 227, 230, 233, 234, 235
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7. The C Preprocessor, , 241, , Features of C Preprocessor, Macro Expansion, Macros with Arguments, Macros versus Functions, File Inclusion, Conditional Compilation, #if and #elif Directives, Miscellaneous Directives, #undef Directive, #pragma Directive, Summary, Exercise, 8. Arrays, , 242, 244, 248, 252, 253, 255, 258, 260, 260, 261, 263, 264, , 269, , What are Arrays, A Simple Program Using Array, More on Arrays, Array Initialization, Bounds Checking, Passing Array Elements to a Function, Pointers and Arrays, Passing an Entire Array to a Function, The Real Thing, Two Dimensional Arrays, Initializing a 2-Dimensional Array, Memory Map of a 2-Dimensional Array, Pointers and 2-Dimensional Arrays, Pointer to an Array, Passing 2-D array to a Function, Array of Pointers, Three Dimensional Array, Summary, , xi, , 270, 272, 275, 275, 276, 277, 279, 286, 287, 289, 290, 291, 292, 295, 297, 300, 302, 304
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Exercise, , 304, , 9. Puppetting On Strings, , 327, , What are Strings, More about Strings, Pointers and Strings, Standard Library String Functions, strlen( ), strcpy( ), strcat( ), strcmp( ), Two-Dimensional Array of Characters, Array of Pointers to Strings, Limitation of Array of Pointers to Strings, Solution, Summary, Exercise, 10. Structures, , 328, 329, 334, 335, 337, 339, 342, 343, 344, 347, 351, 352, 353, 354, , 363, , Why Use Structures, Declaring a Structure, Accessing Structure Elements, How Structure Elements are Stored, Array of Structures, Additional Features of Structures, Uses of Structures, Summary, Exercise, 11. Console Input/Output, , 364, 367, 370, 370, 371, 374, 383, 384, 384, , 393, , Types of I/O, Console I/O Functions, Formatted Console I/O Functions, , xii, , 394, 395, 396
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sprintf( ) and sscanf( ) Functions, Unformatted Console I/O Functions, Summary, Exercise, 12. File Input/Output, , 404, 405, 409, 409, , 415, , Data Organization, File Operations, Opening a File, Reading from a File, Trouble in Opening a File, Closing the File, Counting Characters, Tabs, Spaces, …, A File-copy Program, Writing to a File, File Opening Modes, String (line) I/O in Files, The Awkward Newline, Record I/O in Files, Text Files and Binary Files, Record I/O Revisited, Database Management, Low Level Disk I/O, A Low Level File-copy Program, I/O Under Windows, Summary, Exercise, 13. More Issues In Input/Output, Using argc and argv, Detecting Errors in Reading/Writing, Standard I/O Devices, I/O Redirection, Redirecting the Output, xiii, , 416, 417, 418, 420, 421, 422, 422, 424, 425, 426, 427, 430, 430, 434, 437, 441, 447, 448, 453, 453, 454, 465, 466, 470, 472, 473, 474
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Redirecting the Input, Both Ways at Once, Summary, Exercise, , 476, 477, 478, 478, , 14. Operations On Bits, , 481, , Bitwise Operators, One’s Complement Operator, Right Shift Operator, Left Shift Operator, Bitwise AND Operator, Bitwise OR Operator, Bitwise XOR Operator, The showbits( ) Function, Summary, Exercise, 15. Miscellaneous Features, , 482, 484, 486, 488, 493, 498, 499, 500, 501, 501, 505, , Enumerated Data Type, Uses of Enumerated Data Type, Renaming Data Types with typedef, Typecasting, Bit Fields, Pointers to Functions, Functions Returning Pointers, Functions with Variable Number of Arguments, Unions, Union of Structures, Summary, Exercise, , xiv, , 506, 507, 510, 511, 513, 515, 518, 520, 524, 530, 531, 531
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16. C Under Windows, , 535, , Which Windows…, Integers, The Use of typedef, Pointers in the 32-bit World, Memory Management, Device Access, DOS Programming Model, Windows Programming Model, Event Driven Model, Windows Programming, a Closer Look, The First Windows Program, Hungarian Notation, Summary, Exercise, 17. Windows Programming, , 536, 537, 537, 539, 540, 543, 543, 547, 551, 552, 554, 558, 558, 559, , 561, , The Role of a Message Box, Here Comes the window…, More Windows, A Real-World Window, Creation and Displaying of Window, Interaction with Window, Reacting to Messages, Program Instances, Summary, Exercise, 18. Graphics Under Windows, Graphics as of Now, Device Independent Drawing, , xv, , 562, 563, 566, 567, 569, 570, 572, 575, 575, 576, 579, 580, 580
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Hello Windows, Drawing Shapes, Types of Pens, Types of Brushes, Code and Resources, Freehand Drawing, the Paintbrush Style, Capturing the Mouse, Device Context, a Closer Look, Displaying a Bitmap, Animation at Work, WM_CREATE and OnCreate( ), WM_TIMER and OnTimer( ), A Few More Points…, Windows, the Endless World…, Summary, Exercise, 19. Interaction With Hardware, Hardware Interaction, Hardware Interaction, DOS Perspective, Hardware Interaction, Windows Perspective, Communication with Storage Devices, The ReadSector( ) Function, Accessing Other Storage Devices, Communication with Keyboard, Dynamic Linking, Windows Hooks, Caps Locked, Permanently, Did You Press It TTwwiiccee……, Mangling Keys, KeyLogger, Where is This Leading, Summary, Exercise, , xvi, , 582, 586, 590, 592, 596, 596, 600, 601, 603, 607, 611, 611, 612, 613, 614, 615, , 617, 618, 619, 623, 626, 631, 633, 634, 635, 635, 637, 643, 644, 645, 646, 647, 647
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20. C Under Linux, , 649, , What is Linux, C Programming Under Linux, The ‘Hello Linux’ Program, Processes, Parent and Child Processes, More Processes, Zombies and Orphans, One Interesting Fact, Summary, Exercise, 21. More Linux Programming, , 650, 651, 652, 653, 655, 659, 660, 663, 664, 664, 667, , Communication using Signals, Handling Multiple Signals, Registering a Common Handler, Blocking Signals, Event Driven Programming, Where Do You Go From Here, Summary, Exercise, , 668, 671, 673, 675, 678, 684, 684, 685, , Appendix A – Precedence Table, Appendix B – Standard Library Functions, Appendix C – Chasing the Bugs, Appendix D – Hexadecimal Numbering, Appendix E – ASCII Chart, Appendix F – Helper.h File, Appendix G – Boot Parameters, Appendix H – Linux Installation, Index, , 687, 691, 701, 713, 719, 725, 729, 735, 739, , xvii
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1, , Getting Started, • What is C, • Getting Started with C, The C Character Set, Constants, Variables and Keywords, Types of C Constants, Rules for Constructing Integer Constants, Rules for Constructing Real Constants, Rules for Constructing Character Constants, Types of C Variables, Rules for Constructing Variable Names, C Keywords, • The First C Program, • Compilation and Execution, • Receiving Input, • C Instructions, Type Declaration Instruction, Arithmetic Instruction, Integer and Float Conversions, Hierarchy of Operations, Associativity Of Operators, • Control Instruction in C, • Summary, • Exercise, , 1
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2, , Let Us C, , B, , efore we can begin to write serious programs in C, it would, be interesting to find out what really is C, how it came into, existence and how does it compare with other computer, languages. In this chapter we would briefly outline these issues., Four important aspects of any language are the way it stores data,, the way it operates upon this data, how it accomplishes input and, output and how it lets you control the sequence of execution of, instructions in a program. We would discuss the first three of these, building blocks in this chapter., , What is C, C is a programming language developed at AT & T’s Bell, Laboratories of USA in 1972. It was designed and written by a, man named Dennis Ritchie. In the late seventies C began to, replace the more familiar languages of that time like PL/I,, ALGOL, etc. No one pushed C. It wasn’t made the ‘official’ Bell, Labs language. Thus, without any advertisement C’s reputation, spread and its pool of users grew. Ritchie seems to have been, rather surprised that so many programmers preferred C to older, languages like FORTRAN or PL/I, or the newer ones like Pascal, and APL. But, that's what happened., Possibly why C seems so popular is because it is reliable, simple, and easy to use. Moreover, in an industry where newer languages,, tools and technologies emerge and vanish day in and day out, a, language that has survived for more than 3 decades has to be really, good., An opinion that is often heard today is – “C has been already, superceded by languages like C++, C# and Java, so why bother to
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Chapter 1: Getting Started, , 3, , learn C today”. I seriously beg to differ with this opinion. There, are several reasons for this:, (a) I believe that nobody can learn C++ or Java directly. This is, because while learning these languages you have things like, classes, objects, inheritance, polymorphism, templates,, exception handling, references, etc. do deal with apart from, knowing the actual language elements. Learning these, complicated concepts when you are not even comfortable, with the basic language elements is like putting the cart before, the horse. Hence one should first learn all the language, elements very thoroughly using C language before migrating, to C++, C# or Java. Though this two step learning process, may take more time, but at the end of it you will definitely, find it worth the trouble., (b) C++, C# or Java make use of a principle called Object, Oriented Programming (OOP) to organize the program. This, organizing principle has lots of advantages to offer. But even, while using this organizing principle you would still need a, good hold over the language elements of C and the basic, programming skills., (c) Though many C++ and Java based programming tools and, frameworks have evolved over the years the importance of C, is still unchallenged because knowingly or unknowingly while, using these frameworks and tools you would be still required, to use the core C language elements—another good reason, why one should learn C before C++, C# or Java., (d) Major parts of popular operating systems like Windows,, UNIX, Linux is still written in C. This is because even today, when it comes to performance (speed of execution) nothing, beats C. Moreover, if one is to extend the operating system to, work with new devices one needs to write device driver, programs. These programs are exclusively written in C.
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4, , Let Us C, , (e) Mobile devices like cellular phones and palmtops are, becoming increasingly popular. Also, common consumer, devices like microwave oven, washing machines and digital, cameras are getting smarter by the day. This smartness comes, from a microprocessor, an operating system and a program, embedded in this devices. These programs not only have to, run fast but also have to work in limited amount of memory., No wonder that such programs are written in C. With these, constraints on time and space, C is the language of choice, while building such operating systems and programs., (f) You must have seen several professional 3D computer games, where the user navigates some object, like say a spaceship and, fires bullets at the invaders. The essence of all such games is, speed. Needless to say, such games won't become popular if, they takes a long time to move the spaceship or to fire a, bullet. To match the expectations of the player the game has, to react fast to the user inputs. This is where C language, scores over other languages. Many popular gaming, frameworks have been built using C language., (g) At times one is required to very closely interact with the, hardware devices. Since C provides several language, elements that make this interaction feasible without, compromising the performance it is the preferred choice of, the programmer., I hope that these are very convincing reasons why one should, adopt C as the first and the very important step in your quest for, learning programming languages., , Getting Started with C, Communicating with a computer involves speaking the language, the computer understands, which immediately rules out English as, the language of communication with computer. However, there is
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5, , Chapter 1: Getting Started, , a close analogy between learning English language and learning C, language. The classical method of learning English is to first learn, the alphabets used in the language, then learn to combine these, alphabets to form words, which in turn are combined to form, sentences and sentences are combined to form paragraphs., Learning C is similar and easier. Instead of straight-away learning, how to write programs, we must first know what alphabets,, numbers and special symbols are used in C, then how using them, constants, variables and keywords are constructed, and finally how, are these combined to form an instruction. A group of instructions, would be combined later on to form a program. This is illustrated, in the Figure 1.1., Steps in learning English language:, Alphabets, , Words, , Sentences, , Paragraphs, , Constants, Variables, Keywords, , Instructions, , Program, , Steps in learning C:, Alphabets, Digits, Special symbols, , Figure 1.1, , The C Character Set, A character denotes any alphabet, digit or special symbol used to, represent information. Figure 1.2 shows the valid alphabets,, numbers and special symbols allowed in C.
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6, , Let Us C, , Alphabets, , A, B, ….., Y, Z, a, b, ……, y, z, , Digits, , 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, , Special symbols, , ~‘!@# %^&*()_-+=|\{}, []:; "'<>,.?/, , Figure 1.2, , Constants, Variables and Keywords, The alphabets, numbers and special symbols when properly, combined form constants, variables and keywords. Let us see what, are ‘constants’ and ‘variables’ in C. A constant is an entity that, doesn’t change whereas a variable is an entity that may change., In any program we typically do lots of calculations. The results of, these calculations are stored in computers memory. Like human, memory the computer memory also consists of millions of cells., The calculated values are stored in these memory cells. To make, the retrieval and usage of these values easy these memory cells, (also called memory locations) are given names. Since the value, stored in each location may change the names given to these, locations are called variable names. Consider the following, example., Here 3 is stored in a memory location and a name x is given to it., Then we are assigning a new value 5 to the same memory location, x. This would overwrite the earlier value 3, since a memory, location can hold only one value at a time. This is shown in Figure, 1.3.
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7, , Chapter 1: Getting Started, , x, , 3, x=3, , x, , 5, x=5, , Figure 1.3, Since the location whose name is x can hold different values at, different times x is known as a variable. As against this, 3 or 5 do, not change, hence are known as constants., , Types of C Constants, C constants can be divided into two major categories:, (a) Primary Constants, (b) Secondary Constants, These constants are further categorized as shown in Figure 1.4.
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8, , Let Us C, , C Constants, , Primary Constants, , Secondary Constants, , Integer Constant, , Array, , Real Constant, , Pointer, , Character Constant, , Structure, Union, Enum, etc., , Figure 1.4, At this stage we would restrict our discussion to only Primary, Constants, namely, Integer, Real and Character constants. Let us, see the details of each of these constants. For constructing these, different types of constants certain rules have been laid down., These rules are as under:, , Rules for Constructing Integer Constants, An integer constant must have at least one digit., It must not have a decimal point., It can be either positive or negative., If no sign precedes an integer constant it is assumed to be, positive., (e) No commas or blanks are allowed within an integer constant., (f) The allowable range for integer constants is -32768 to 32767., (a), (b), (c), (d), , Truly speaking the range of an Integer constant depends upon the, compiler. For a 16-bit compiler like Turbo C or Turbo C++ the
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Chapter 1: Getting Started, , 9, , range is –32768 to 32767. For a 32-bit compiler the range would, be even greater. Question like what exactly do you mean by a 16bit or a 32-bit compiler, what range of an Integer constant has to, do with the type of compiler and such questions are discussed in, detail in Chapter 16. Till that time it would be assumed that we are, working with a 16-bit compiler., Ex.: 426, +782, -8000, -7605, , Rules for Constructing Real Constants, Real constants are often called Floating Point constants. The real, constants could be written in two forms—Fractional form and, Exponential form., Following rules must be observed while constructing real, constants expressed in fractional form:, (a), (b), (c), (d), (e), , A real constant must have at least one digit., It must have a decimal point., It could be either positive or negative., Default sign is positive., No commas or blanks are allowed within a real constant., Ex.: +325.34, 426.0, -32.76, -48.5792, , The exponential form of representation of real constants is usually, used if the value of the constant is either too small or too large. It, however doesn’t restrict us in any way from using exponential, form of representation for other real constants.
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10, , Let Us C, , In exponential form of representation, the real constant is, represented in two parts. The part appearing before ‘e’ is called, mantissa, whereas the part following ‘e’ is called exponent., Following rules must be observed while constructing real, constants expressed in exponential form:, (a) The mantissa part and the exponential part should be, , separated by a letter e., (b) The mantissa part may have a positive or negative sign., (c) Default sign of mantissa part is positive., (d) The exponent must have at least one digit, which must be a, , positive or negative integer. Default sign is positive., (e) Range of real constants expressed in exponential form is, -3.4e38 to 3.4e38., Ex.: +3.2e-5, 4.1e8, -0.2e+3, -3.2e-5, , Rules for Constructing Character Constants, (a) A character constant is a single alphabet, a single digit or a, , single special symbol enclosed within single inverted, commas. Both the inverted commas should point to the left., For example, ’A’ is a valid character constant whereas ‘A’ is, not., (b) The maximum length of a character constant can be 1, character., Ex.: 'A', 'I', '5', '='
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Chapter 1: Getting Started, , 11, , Types of C Variables, As we saw earlier, an entity that may vary during program, execution is called a variable. Variable names are names given to, locations in memory. These locations can contain integer, real or, character constants. In any language, the types of variables that it, can support depend on the types of constants that it can handle., This is because a particular type of variable can hold only the same, type of constant. For example, an integer variable can hold only an, integer constant, a real variable can hold only a real constant and a, character variable can hold only a character constant., The rules for constructing different types of constants are different., However, for constructing variable names of all types the same set, of rules apply. These rules are given below., , Rules for Constructing Variable Names, (a) A variable name is any combination of 1 to 31 alphabets,, , digits or underscores. Some compilers allow variable names, whose length could be up to 247 characters. Still, it would be, safer to stick to the rule of 31 characters. Do not create, unnecessarily long variable names as it adds to your typing, effort., (b) The first character in the variable name must be an alphabet or, underscore., (c) No commas or blanks are allowed within a variable name., (d) No special symbol other than an underscore (as in gross_sal), can be used in a variable name., Ex.: si_int, m_hra, pop_e_89, , These rules remain same for all the types of primary and secondary, variables. Naturally, the question follows... how is C able to, differentiate between these variables? This is a rather simple
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12, , Let Us C, , matter. C compiler is able to distinguish between the variable, names by making it compulsory for you to declare the type of any, variable name that you wish to use in a program. This type, declaration is done at the beginning of the program. Following are, the examples of type declaration statements:, Ex.: int si, m_hra ;, float bassal ;, char code ;, , Since, the maximum allowable length of a variable name is 31, characters, an enormous number of variable names can be, constructed using the above-mentioned rules. It is a good practice, to exploit this enormous choice in naming variables by using, meaningful variable names., Thus, if we want to calculate simple interest, it is always advisable, to construct meaningful variable names like prin, roi, noy to, represent Principle, Rate of interest and Number of years rather, than using the variables a, b, c., , C Keywords, Keywords are the words whose meaning has already been, explained to the C compiler (or in a broad sense to the computer)., The keywords cannot be used as variable names because if we do, so we are trying to assign a new meaning to the keyword, which is, not allowed by the computer. Some C compilers allow you to, construct variable names that exactly resemble the keywords., However, it would be safer not to mix up the variable names and, the keywords. The keywords are also called ‘Reserved words’., There are only 32 keywords available in C. Figure 1.5 gives a list, of these keywords for your ready reference. A detailed discussion, of each of these keywords would be taken up in later chapters, wherever their use is relevant.
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13, , Chapter 1: Getting Started, , auto, , double, , int, , struct, , break, , else, , long, , switch, , case, , enum, , register, , typedef, , char, , extern, , return, , union, , const, , float, , short, , unsigned, , continue, , for, , signed, , void, , default, , goto, , sizeof, , volatile, , do, , if, , static, , while, , Figure 1.5, Note that compiler vendors (like Microsoft, Borland, etc.) provide, their own keywords apart from the ones mentioned above. These, include extended keywords like near, far, asm, etc. Though it has, been suggested by the ANSI committee that every such compiler, specific keyword should be preceded by two underscores (as in, __asm ), not every vendor follows this rule., , The First C Program, Armed with the knowledge about the types of variables, constants, & keywords the next logical step is to combine them to form, instructions. However, instead of this, we would write our first C, program now. Once we have done that we would see in detail the, instructions that it made use of., Before we begin with our first C program do remember the, following rules that are applicable to all C programs:, (a) Each instruction in a C program is written as a separate, , statement. Therefore a complete C program would comprise, of a series of statements.
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14, , Let Us C, , (b) The statements in a program must appear in the same order in, , which we wish them to be executed; unless of course the logic, of the problem demands a deliberate ‘jump’ or transfer of, control to a statement, which is out of sequence., (c) Blank spaces may be inserted between two words to improve, , the readability of the statement. However, no blank spaces are, allowed within a variable, constant or keyword., (d) All statements are entered in small case letters., (e) C has no specific rules for the position at which a statement is, , to be written. That’s why it is often called a free-form, language., (f), , Every C statement must end with a ;. Thus ; acts as a, statement terminator., , Let us now write down our first C program. It would simply, calculate simple interest for a set of values representing principle,, number of years and rate of interest., /* Calculation of simple interest */, /* Author gekay Date: 25/05/2004 */, main( ), {, int p, n ;, float r, si ;, p = 1000 ;, n=3;, r = 8.5 ;, /* formula for simple interest */, si = p * n * r / 100 ;, printf ( "%f" , si ) ;
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Chapter 1: Getting Started, , 15, , }, , Now a few useful tips about the program..., −, , Comment about the program should be enclosed within /* */., For example, the first two statements in our program are, comments., , −, , Though comments are not necessary, it is a good practice to, begin a program with a comment indicating the purpose of the, program, its author and the date on which the program was, written., , −, , Any number of comments can be written at any place in the, program. For example, a comment can be written before the, statement, after the statement or within the statement as shown, below:, /* formula */ si = p * n * r / 100 ;, si = p * n * r / 100 ; /* formula */, si = p * n * r / /* formula */ 100 ;, , −, , Sometimes it is not so obvious as to what a particular, statement in a program accomplishes. At such times it is, worthwhile mentioning the purpose of the statement (or a set, of statements) using a comment. For example:, /* formula for simple interest */, si = p * n * r / 100 ;, , −, , Often programmers seem to ignore writing of comments. But, when a team is building big software well commented code is, almost essential for other team members to understand it.
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16, , Let Us C, , −, , Although a lot of comments are probably not necessary in this, program, it is usually the case that programmers tend to use, too few comments rather than too many. An adequate number, of comments can save hours of misery and suffering when you, later try to figure out what the program does., , −, , The normal language rules do not apply to text written within, /* .. */. Thus we can type this text in small case, capital or a, combination. This is because the comments are solely given, for the understanding of the programmer or the fellow, programmers and are completely ignored by the compiler., , −, , Comments cannot be nested. For example,, /* Cal of SI /* Author sam date 01/01/2002 */ */, , is invalid., −, , A comment can be split over more than one line, as in,, /* This is, a jazzy, comment */, , Such a comment is often called a multi-line comment., −, , main( ) is a collective name given to a set of statements. This, name has to be main( ), it cannot be anything else. All, statements that belong to main( ) are enclosed within a pair of, braces { } as shown below., main( ), {, statement 1 ;, statement 2 ;
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Chapter 1: Getting Started, , 17, , statement 3 ;, }, , −, , Technically speaking main( ) is a function. Every function has, a pair of parentheses ( ) associated with it. We would discuss, functions and their working in great detail in Chapter 5., , −, , Any variable used in the program must be declared before, using it. For example,, int p, n ;, float r, si ;, , −, , Any C statement always ends with a ;, For example,, float r, si ;, r = 8.5 ;, , −, , In the statement,, si = p * n * r / 100 ;, , * and / are the arithmetic operators. The arithmetic operators, available in C are +, -, * and /. C is very rich in operators., There are about 45 operators available in C. Surprisingly there, is no operator for exponentiation... a slip, which can be, forgiven considering the fact that C has been developed by an, individual, not by a committee., −, , Once the value of si is calculated it needs to be displayed on, the screen. Unlike other languages, C does not contain any, instruction to display output on the screen. All output to screen, is achieved using readymade library functions. One such
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18, , Let Us C, function is printf( ). We have used it display on the screen the, value contained in si., The general form of printf( ) function is,, printf ( "<format string>", <list of variables> ) ;, <format string> can contain,, %f for printing real values, %d for printing integer values, %c for printing character values, , In addition to format specifiers like %f, %d and %c the, format string may also contain any other characters. These, characters are printed as they are when the printf( ) is, executed., Following are some examples of usage of printf( ) function:, printf ( "%f", si ) ;, printf ( "%d %d %f %f", p, n, r, si ) ;, printf ( "Simple interest = Rs. %f", si ) ;, printf ( "Prin = %d \nRate = %f", p, r ) ;, , The output of the last statement would look like this..., Prin = 1000, Rate = 8.5, , What is ‘\n’ doing in this statement? It is called newline and it, takes the cursor to the next line. Therefore, you get the output, split over two lines. ‘\n’ is one of the several Escape, Sequences available in C. These are discussed in detail in, Chapter 11. Right now, all that we can say is ‘\n’ comes in
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Chapter 1: Getting Started, , 19, , handy when we want to format the output properly on, separate lines., printf( ) can not only print values of variables, it can also, print the result of an expression. An expression is nothing but, a valid combination of constants, variables and operators., Thus, 3, 3 + 2, c and a + b * c – d all are valid expressions., The results of these expressions can be printed as shown, below:, printf ( "%d %d %d %d", 3, 3 + 2, c, a + b * c – d ) ;, , Note that 3 and c also represent valid expressions., , Compilation and Execution, Once you have written the program you need to type it and instruct, the machine to execute it. To type your C program you need, another program called Editor. Once the program has been typed it, needs to be converted to machine language (0s and 1s) before the, machine can execute it. To carry out this conversion we need, another program called Compiler. Compiler vendors provide an, Integrated Development Environment (IDE) which consists of an, Editor as well as the Compiler., There are several such IDEs available in the market targeted, towards different operating systems. For example, Turbo C, Turbo, C++ and Microsoft C are some of the popular compilers that work, under MS-DOS; Visual C++ and Borland C++ are the compilers, that work under Windows, whereas gcc compiler works under, Linux. Note that Turbo C++, Microsoft C++ and Borland C++, software also contain a C compiler bundled with them. If you are a, beginner you would be better off using a simple compiler like, Turbo C or Turbo C++. Once you have mastered the language, elements you can then switch over to more sophisticated compilers, like Visual C++ under Windows or gcc under Linux. Most of the
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20, , Let Us C, , programs in this book would work with all the compilers., Wherever there is a deviation I would point it out that time., Assuming that you are using a Turbo C or Turbo C++ compiler, here are the steps that you need to follow to compile and execute, your first C program…, (a) Start the compiler at C> prompt. The compiler (TC.EXE is, usually present in C:\TC\BIN directory)., (b) Select New from the File menu., (c) Type the program., (d) Save the program using F2 under a proper name (say, Program1.c)., (e) Use Ctrl + F9 to compile and execute the program., (f) Use Alt + F5 to view the output., Note that on compiling the program its machine language, equivalent is stored as an EXE file (Program1.EXE) on the disk., This file is called an executable file. If we copy this file to another, machine we can execute it there without being required to, recompile it. In fact the other machine need not even have a, compiler to be able to execute the file., A word of caution! If you run this program in Turbo C++, compiler, you may get an error — “The function printf should, have a prototype”. To get rid of this error, perform the following, steps and then recompile the program., (a) Select ‘Options’ menu and then select ‘Compiler | C++, Options’. In the dialog box that pops up, select ‘CPP always’, in the ‘Use C++ Compiler’ options., (b) Again select ‘Options’ menu and then select ‘Environment |, Editor’. Make sure that the default extension is ‘C’ rather than, ‘CPP’.
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Chapter 1: Getting Started, , 21, , Receiving Input, In the program discussed above we assumed the values of p, n and, r to be 1000, 3 and 8.5. Every time we run the program we would, get the same value for simple interest. If we want to calculate, simple interest for some other set of values then we are required to, make the relevant change in the program, and again compile and, execute it. Thus the program is not general enough to calculate, simple interest for any set of values without being required to, make a change in the program. Moreover, if you distribute the, EXE file of this program to somebody he would not even be able, to make changes in the program. Hence it is a good practice to, create a program that is general enough to work for any set of, values., To make the program general the program itself should ask the, user to supply the values of p, n and r through the keyboard during, execution. This can be achieved using a function called scanf( )., This function is a counter-part of the printf( ) function. printf( ), outputs the values to the screen whereas scanf( ) receives them, from the keyboard. This is illustrated in the program shown below., /* Calculation of simple interest */, /* Author gekay Date 25/05/2004 */, main( ), {, int p, n ;, float r, si ;, printf ( "Enter values of p, n, r" ) ;, scanf ( "%d %d %f", &p, &n, &r ) ;, si = p * n * r / 100 ;, printf ( "%f" , si ) ;, }
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22, , Let Us C, , The first printf( ) outputs the message ‘Enter values of p, n, r’ on, the screen. Here we have not used any expression in printf( ), which means that using expressions in printf( ) is optional., Note that the ampersand (&) before the variables in the scanf( ), function is a must. & is an ‘Address of’ operator. It gives the, location number used by the variable in memory. When we say, &a, we are telling scanf( ) at which memory location should it, store the value supplied by the user from the keyboard. The, detailed working of the & operator would be taken up in Chapter, 5., Note that a blank, a tab or a new line must separate the values, supplied to scanf( ). Note that a blank is creating using a spacebar,, tab using the Tab key and new line using the Enter key. This is, shown below:, Ex.: The three values separated by blank, 1000 5 15.5, , Ex.: The three values separated by tab., 1000 5, , 15.5, , Ex.: The three values separated by newline., 1000, 5, 15.5, , So much for the tips. How about another program to give you a, feel of things..., /* Just for fun. Author: Bozo */, main( ), {, int num ;, printf ( "Enter a number" ) ;
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23, , Chapter 1: Getting Started, scanf ( "%d", &num ) ;, printf ( "Now I am letting you on a secret..." ) ;, printf ( "You have just entered the number %d", num ) ;, }, , C Instructions, Now that we have written a few programs let us look at the, instructions that we used in these programs. There are basically, three types of instructions in C:, (a) Type Declaration Instruction, (b) Arithmetic Instruction, (c) Control Instruction, , The purpose of each of these instructions is given below:, (a) Type declaration instruction, , −, , To declare the type of, variables used in a C, program., , (b) Arithmetic instruction, , −, , To perform arithmetic, operations between constants and variables., , (c) Control instruction, , −, , To control the sequence of, execution of various statements in a C program., , Since, the elementary C programs would usually contain only the, type declaration and the arithmetic instructions; we would discuss, only these two instructions at this stage. The other types of, instructions would be discussed in detail in the subsequent, chapters.
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24, , Let Us C, , Type Declaration Instruction, This instruction is used to declare the type of variables being used, in the program. Any variable used in the program must be declared, before using it in any statement. The type declaration statement is, written at the beginning of main( ) function., Ex.: int bas ;, float rs, grosssal ;, char name, code ;, , There are several subtle variations of the type declaration, instruction. These are discussed below:, (a) While declaring the type of variable we can also initialize it as, , shown below., int i = 10, j = 25 ;, float a = 1.5, b = 1.99 + 2.4 * 1.44 ;, (b) The order in which we define the variables is sometimes, , important sometimes not. For example,, int i = 10, j = 25 ;, , is same as, int j = 25, j = 10 ;, , However,, float a = 1.5, b = a + 3.1 ;, , is alright, but, float b = a + 3.1, a = 1.5 ;
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Chapter 1: Getting Started, , 25, , is not. This is because here we are trying to use a even before, defining it., (c) The following statements would work, , int a, b, c, d ;, a = b = c = 10 ;, , However, the following statement would not work, int a = b = c = d = 10 ;, , Once again we are trying to use b (to assign to a) before, defining it., , Arithmetic Instruction, A C arithmetic instruction consists of a variable name on the left, hand side of = and variable names & constants on the right hand, side of =. The variables and constants appearing on the right hand, side of = are connected by arithmetic operators like +, -, *, and /., Ex.: int ad ;, float kot, deta, alpha, beta, gamma ;, ad = 3200 ;, kot = 0.0056 ;, deta = alpha * beta / gamma + 3.2 * 2 / 5 ;, , Here,, *, /, -, + are the arithmetic operators., = is the assignment operator., 2, 5 and 3200 are integer constants., 3.2 and 0.0056 are real constants., ad is an integer variable., kot, deta, alpha, beta, gamma are real variables.
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26, , Let Us C, , The variables and constants together are called ‘operands’ that are, operated upon by the ‘arithmetic operators’ and the result is, assigned, using the assignment operator, to the variable on lefthand side., A C arithmetic statement could be of three types. These are as, follows:, (a) Integer mode arithmetic statement - This is an arithmetic, , statement in which all operands are either integer variables or, integer constants., Ex.: int i, king, issac, noteit ;, i=i+1;, king = issac * 234 + noteit - 7689 ;, (b) Real mode arithmetic statement - This is an arithmetic, , statement in which all operands are either real constants or, real variables., Ex.: float qbee, antink, si, prin, anoy, roi ;, qbee = antink + 23.123 / 4.5 * 0.3442 ;, si = prin * anoy * roi / 100.0 ;, (c) Mixed mode arithmetic statement - This is an arithmetic, , statement in which some of the operands are integers and, some of the operands are real., Ex.: float si, prin, anoy, roi, avg ;, int a, b, c, num ;, si = prin * anoy * roi / 100.0 ;, avg = ( a + b + c + num ) / 4 ;, , It is very important to understand how the execution of an, arithmetic statement takes place. Firstly, the right hand side is, evaluated using constants and the numerical values stored in the, variable names. This value is then assigned to the variable on the, left-hand side.
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Chapter 1: Getting Started, , 27, , Though Arithmetic instructions look simple to use one often, commits mistakes in writing them. Let us take a closer look at, these statements. Note the following points carefully., (a) C allows only one variable on left-hand side of =. That is, z =, k * l is legal, whereas k * l = z is illegal., (b) In addition to the division operator C also provides a modular, division operator. This operator returns the remainder on, dividing one integer with another. Thus the expression 10 / 2, yields 5, whereas, 10 % 2 yields 0. Note that the modulus, operator (%) cannot be applied on a float. Also note that on, using % the sign of the remainder is always same as the sign, of the numerator. Thus –5 % 2 yields –1, whereas, 5 % -2, yields 1., (c) An arithmetic instruction is often used for storing character, constants in character variables., char a, b, d ;, a = 'F' ;, b = 'G' ;, d = '+' ;, , When we do this the ASCII values of the characters are stored, in the variables. ASCII values are used to represent any, character in memory. The ASCII values of ‘F’ and ‘G’ are 70, and 71 (refer the ASCII Table in Appendix E)., (d) Arithmetic operations can be performed on ints, floats and, chars., Thus the statements,, char x, y ;, int z ;, x = 'a' ;, y = 'b' ;, z=x+y;
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28, , Let Us C, , are perfectly valid, since the addition is performed on the, ASCII values of the characters and not on characters, themselves. The ASCII values of ‘a’ and ‘b’ are 97 and 98,, and hence can definitely be added., (e) No operator is assumed to be present. It must be written, explicitly. In the following example, the multiplication, operator after b must be explicitly written., a = c.d.b(xy), b=c*d*b*(x*y), , usual arithmetic statement, C statement, , (f) Unlike other high level languages, there is no operator for, performing exponentiation operation. Thus following, statements are invalid., a = 3 ** 2 ;, b=3^2;, , If we want to do the exponentiation we can get it done this, way:, #include <math.h>, main( ), {, int a ;, a = pow ( 3, 2 ) ;, printf ( “%d”, a ) ;, }, , Here pow( ) function is a standard library function. It is being, used to raise 3 to the power of 2. #include <math.h> is a, preprocessor directive. It is being used here to ensure that the, pow( ) function works correctly. We would learn more about, standard library functions in Chapter 5 and about preprocessor, in Chapter 7.
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29, , Chapter 1: Getting Started, , Integer and Float Conversions, In order to effectively develop C programs, it will be necessary to, understand the rules that are used for the implicit conversion of, floating point and integer values in C. These are mentioned below., Note them carefully., (a) An arithmetic operation between an integer and integer, , always yields an integer result., (b) An operation between a real and real always yields a real, , result., (c) An operation between an integer and real always yields a real, , result. In this operation the integer is first promoted to a real, and then the operation is performed. Hence the result is real., I think a few practical examples shown in the following figure, would put the issue beyond doubt., , Operation, , Result, , Operation, , Result, , 5/2, , 2, , 2/5, , 0, , 5.0 / 2, , 2.5, , 2.0 / 5, , 0.4, , 5 / 2.0, , 2.5, , 2 / 5.0, , 0.4, , 5.0 / 2.0, , 2.5, , 2.0 / 5.0, , 0.4, , Figure 1.6, , Type Conversion in Assignments, It may so happen that the type of the expression and the type of the, variable on the left-hand side of the assignment operator may not, be same. In such a case the value of the expression is promoted or
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30, , Let Us C, , demoted depending on the type of the variable on left-hand side of, =., For example, consider the following assignment statements., int i ;, float b ;, i = 3.5 ;, b = 30 ;, , Here in the first assignment statement though the expression’s, value is a float (3.5) it cannot be stored in i since it is an int. In, such a case the float is demoted to an int and then its value is, stored. Hence what gets stored in i is 3. Exactly opposite happens, in the next statement. Here, 30 is promoted to 30.000000 and then, stored in b, since b being a float variable cannot hold anything, except a float value., Instead of a simple expression used in the above examples if a, complex expression occurs, still the same rules apply. For, example, consider the following program fragment., float a, b, c ;, int s ;, s = a * b * c / 100 + 32 / 4 - 3 * 1.1 ;, , Here, in the assignment statement some operands are ints whereas, others are floats. As we know, during evaluation of the expression, the ints would be promoted to floats and the result of the, expression would be a float. But when this float value is assigned, to s it is again demoted to an int and then stored in s., Observe the results of the arithmetic statements shown in Figure, 1.7. It has been assumed that k is an integer variable and a is a real, variable.
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31, , Chapter 1: Getting Started, , Arithmetic Instruction Result Arithmetic Instruction Result, k=2/9, , 0, , a=2/9, , 0.0, , k = 2.0 / 9, , 0, , a = 2.0 / 9, , 0.2222, , k = 2 / 9.0, , 0, , a = 2 / 9.0, , 0.2222, , k = 2.0 / 9.0, , 0, , a = 2.0 / 9.0, , 0.2222, , k=9/2, , 4, , a=9/2, , 4.0, , k = 9.0 / 2, , 4, , a = 9.0 / 2, , 4.5, , k = 9 / 2.0, , 4, , a = 9 / 2.0, , 4.5, , k = 9.0 / 2.0, , 4, , a = 9.0 / 2.0, , 4.5, , Figure 1.7, Note that though the following statements give the same result, 0,, the results are obtained differently., k=2/9;, k = 2.0 / 9 ;, , In the first statement, since both 2 and 9 are integers, the result is, an integer, i.e. 0. This 0 is then assigned to k. In the second, statement 9 is promoted to 9.0 and then the division is performed., Division yields 0.222222. However, this cannot be stored in k, k, being an int. Hence it gets demoted to 0 and then stored in k., , Hierarchy of Operations, While executing an arithmetic statement, which has two or more, operators, we may have some problems as to how exactly does it, get executed. For example, does the expression 2 * x - 3 * y, correspond to (2x)-(3y) or to 2(x-3y)? Similarly, does A / B * C, correspond to A / (B * C) or to (A / B) * C? To answer these, questions satisfactorily one has to understand the ‘hierarchy’ of, operations. The priority or precedence in which the operations in
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32, , Let Us C, , an arithmetic statement are performed is called the hierarchy of, operations. The hierarchy of commonly used operators is shown in, Figure 1.8., , Priority, , Operators, , Description, , 1, 2nd, , */%, +-, , multiplication, division, modular division, addition, subtraction, , 3rd, , =, , assignment, , st, , Figure 1.8, Now a few tips about usage of operators in general., (a) Within parentheses the same hierarchy as mentioned in Figure, , 1.11 is operative. Also, if there are more than one set of, parentheses, the operations within the innermost parentheses, would be performed first, followed by the operations within, the second innermost pair and so on., (b) We must always remember to use pairs of parentheses. A, , careless imbalance of the right and left parentheses is a, common error. Best way to avoid this error is to type ( ) and, then type an expression inside it., A few examples would clarify the issue further., Example 1.1: Determine the hierarchy of operations and evaluate, the following expression:, i=2*3/4+4/4+8-2+5/8, , Stepwise evaluation of this expression is shown below:, i=2*3/4+4/4+8-2+5/8
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33, , Chapter 1: Getting Started, i=6/4+4/4+8-2+5/8, i=1+4/4+8-2+5/8, i = 1 + 1+ 8 - 2 + 5 / 8, i=1+1+8-2+0, i=2+8-2+0, i = 10 - 2 + 0, i=8+0, i=8, , operation: *, operation: /, operation: /, operation: /, operation: +, operation: +, operation : operation: +, , Note that 6 / 4 gives 1 and not 1.5. This so happens because 6 and, 4 both are integers and therefore would evaluate to only an integer, constant. Similarly 5 / 8 evaluates to zero, since 5 and 8 are integer, constants and hence must return an integer value., Example 1.2: Determine the hierarchy of operations and evaluate, the following expression:, kk = 3 / 2 * 4 + 3 / 8 + 3, , Stepwise evaluation of this expression is shown below:, kk = 3 / 2 * 4 + 3 / 8 + 3, kk = 1 * 4 + 3 / 8 + 3, kk = 4 + 3 / 8 + 3, kk = 4 + 0 + 3, kk = 4 + 3, kk = 7, , operation: /, operation: *, operation: /, operation: +, operation: +, , Note that 3 / 8 gives zero, again for the same reason mentioned in, the previous example., All operators in C are ranked according to their precedence. And, mind you there are as many as 45 odd operators in C, and these, can affect the evaluation of an expression in subtle and unexpected, ways if we aren't careful. Unfortunately, there are no simple rules, that one can follow, such as “BODMAS” that tells algebra students, in which order does an expression evaluate. We have not
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34, , Let Us C, , encountered many out of these 45 operators, so we won’t pursue, the subject of precedence any further here. However, it can be, realized at this stage that it would be almost impossible to, remember the precedence of all these operators. So a full-fledged, list of all operators and their precedence is given in Appendix A., This may sound daunting, but when its contents are absorbed in, small bites, it becomes more palatable., So far we have seen how the computer evaluates an arithmetic, statement written in C. But our knowledge would be incomplete, unless we know how to convert a general arithmetic statement to a, C statement. C can handle any complex expression with ease., Some of the examples of C expressions are shown in Figure 1.9., , Algebric Expression, , C Expression, , axb–cxd, , a*b–c*d, (m + n) * (a + b), 3*x*x+2*x+ 5, (a+b+c)/(d+e), , (m + n) (a + b), 3x2 + 2x + 5, a+b+c, d+e, , ⎡ 2 BY, x ⎤, ⎢ d + 1 − 3( z + y ) ⎥, ⎦, ⎣, , 2 * b * y / ( d + 1 ) – x /, 3 * ( z + y ), , Figure 1.9, , Associativity of Operators, When an expression contains two operators of equal priority the tie, between them is settled using the associativity of the operators., Associativity can be of two types—Left to Right or Right to Left., Left to Right associativity means that the left operand must be
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35, , Chapter 1: Getting Started, , unambiguous. Unambiguous in what sense? It must not be, involved in evaluation of any other sub-expression. Similarly, in, case of Right to Left associativity the right operand must be, unambiguous. Let us understand this with an example., Consider the expression, a=3/2*5 ;, Here there is a tie between operators of same priority, that is, between / and *. This tie is settled using the associativity of / and, *. But both enjoy Left to Right associativity. Figure 1.10 shows for, each operator which operand is unambiguous and which is not., , Operator Left, , Right, , Remark, , /, , 3, , 2 or 2 * Left, operand, is, 5, unambiguous, Right is not, , *, , 3 / 2 or 2, , 5, , Right, operand, is, unambiguous, Left is not, , Figure 1.10, Since both / and * have L to R associativity and only / has, unambiguous left operand (necessary condition for L to R, associativity) it is performed earlier., Consider one more expression, a=b=3;, , Here both assignment operators have the same priority and same, associativity (Right to Left). Figure 1.11 shows for each operator, which operand is unambiguous and which is not.
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36, , Let Us C, , Operator Left, , Right, , Remark, , =, , a, , b or b = Left, operand, is, 3, unambiguous, Right is, not, , =, , b or a = b, , 3, , Right, operand, is, unambiguous, Left is not, , Figure 1.11, Since both = have R to L associativity and only the second = has, unambiguous right operand (necessary condition for R to L, associativity) the second = is performed earlier., Consider yet another expression, z=a*b+c/d;, , Here * and / enjoys same priority and same associativity (Left to, Right). Figure 1.12 shows for each operator which operand is, unambiguous and which is not., , Operator Left, , Right Remark, , *, , a, , b, , Both operands are unambiguous, , /, , c, , d, , Both operands are unambiguous, , Figure 1.12, Here since left operands for both operators are unambiguous, Compiler is free to perform * or / operation as per its convenience
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Chapter 1: Getting Started, , 37, , since no matter which is performed earlier the result would be, same., Appendix A gives the associativity of all the operators available in, C., , Control Instructions in C, As the name suggests the ‘Control Instructions’ enable us to, specify the order in which the various instructions in a program are, to be executed by the computer. In other words the control, instructions determine the ‘flow of control’ in a program. There, are four types of control instructions in C. They are:, (a), (b), (c), (d), , Sequence Control Instruction, Selection or Decision Control Instruction, Repetition or Loop Control Instruction, Case Control Instruction, , The Sequence control instruction ensures that the instructions are, executed in the same order in which they appear in the program., Decision and Case control instructions allow the computer to take, a decision as to which instruction is to be executed next. The Loop, control instruction helps computer to execute a group of statements, repeatedly. In the following chapters we are going to learn these, instructions in detail. Try your hand at the Exercise presented on, the following pages before proceeding to the next chapter, which, discusses the decision control instruction., , Summary, (a) The three primary constants and variable types in C are, integer, float and character., (b) A variable name can be of maximum 31 characters., (c) Do not use a keyword as a variable name.
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38, , Let Us C, , (d) An expression may contain any sequence of constants,, variables and operators., (e) Operators having equal precedence are evaluated using, associativity., (f) Left to right associativity means that the left operand of a, operator must be unambiguous whereas right to left, associativity means that the right operand of a operator must, be unambiguous., (g) Input/output in C can be achieved using scanf( ) and printf( ), functions., , Exercise, [A] Which of the following are invalid variable names and why?, BASICSALARY, #MEAN, population in 2006, FLOAT, team’svictory, , _basic, group., over time, hELLO, Plot # 3, , basic-hra, 422, mindovermatter, queue., 2015_DDay, , [B] Point out the errors, if any, in the following C statements:, , (a) int = 314.562 * 150 ;, (b) name = ‘Ajay’ ;, (c) varchar = ‘3’ ;, (d) 3.14 * r * r * h = vol_of_cyl ;, (e) k = ( a * b ) ( c + ( 2.5a + b ) ( d + e ) ;, (f) m_inst = rate of interest * amount in rs ;
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Chapter 1: Getting Started, , 39, , (g) si = principal * rateofinterest * numberofyears / 100 ;, (h) area = 3.14 * r ** 2 ;, (i) volume = 3.14 * r ^ 2 * h ;, (j) k = ( (a * b ) + c ) ( 2.5 * a + b ) ;, (k) a = b = 3 = 4 ;, (l) count = count + 1 ;, (m) date = '2 Mar 04' ;, [C] Evaluate the following expressions and show their hierarchy., , (a) g = big / 2 + big * 4 / big - big + abc / 3 ;, (abc = 2.5, big = 2, assume g to be a float), (b) on = ink * act / 2 + 3 / 2 * act + 2 + tig ;, (ink = 4, act = 1, tig = 3.2, assume on to be an int), (c) s = qui * add / 4 - 6 / 2 + 2 / 3 * 6 / god ;, (qui = 4, add = 2, god = 2, assume s to be an int), (d) s = 1 / 3 * a / 4 - 6 / 2 + 2 / 3 * 6 / g ;, (a = 4, g = 3, assume s to be an int), [D] Fill the following table for the expressions given below and, then evaluate the result. A sample entry has been filled in the, table for expression (a).
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40, , Let Us C, , Operator, , Left, , Right, , Remark, , /, , 10, , 5 or 5 / 2 Left, operand, is, /1, unambiguous, Right, is not, , .., , .., , .., , .., , (a) g = 10 / 5 /2 / 1 ;, (b) b = 3 / 2 + 5 * 4 / 3 ;, (c) a = b = c = 3 + 4 ;, , [E] Convert the following equations into corresponding C, statements., , (a), , Z=, , 8.8 ( a + b ) 2 / c - 0.5 + 2 a / ( q + r ), ( a + b ) * (1 / m ), , (b), , X=, , - b + ( b * b ) + 2 4ac, 2a, , (c), , R=, , 2v + 6.22 ( c + d ), g+v, , (d), , A=, , 7.7b ( xy + a ) / c - 0.8 + 2b, ( x + a ) (1 / y ), , [F] What would be the output of the following programs:, (a) main( ), {
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Chapter 1: Getting Started, int i = 2, j = 3, k, l ;, float a, b ;, k=i/j*j;, l=j/i*i;, a=i/j*j;, b=j/i*i;, printf( "%d %d %f %f", k, l, a, b ) ;, }, (b) main( ), {, int a, b ;, a = -3 - - 3 ;, b = -3 - - ( - 3 ) ;, printf ( "a = %d b = %d", a, b ) ;, }, (c), , main( ), {, float a = 5, b = 2 ;, int c ;, c=a%b;, printf ( "%d", c ) ;, }, , (d) main( ), {, printf ( "nn \n\n nn\n" ) ;, printf ( "nn /n/n nn/n" ) ;, }, (e) main( ), {, int a, b ;, printf ( "Enter values of a and b" ) ;, scanf ( " %d %d ", &a, &b ) ;, printf ( "a = %d b = %d", a, b ) ;, }, , 41
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42, (f), , Let Us C, main( ), {, int p, q ;, printf ( "Enter values of p and q" ) ;, scanf ( " %d %d ", p, q ) ;, printf ( "p = %d q =%d", p, q ) ;, }, , [G] Pick up the correct alternative for each of the following, questions:, , (a) C language has been developed by, (1) Ken Thompson, (2) Dennis Ritchie, (3) Peter Norton, (4) Martin Richards, (b) C can be used on, (1) Only MS-DOS operating system, (2) Only Linux operating system, (3) Only Windows operating system, (4) All the above, (c) C programs are converted into machine language with the, help of, (1) An Editor, (2) A compiler, (3) An operating system, (4) None of the above, (d) The real constant in C can be expressed in which of the, following forms, (1) Fractional form only, (2) Exponential form only, (3) ASCII form only
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Chapter 1: Getting Started, (4) Both fractional and exponential forms, (e) A character variable can at a time store, (1) 1 character, (2) 8 characters, (3) 254 characters, (4) None of the above, (f) The statement char ch = ‘Z’ would store in ch, (1) The character Z, (2) ASCII value of Z, (3) Z along with the single inverted commas, (4) Both (1) and (2), (g) Which of the following is NOT a character constant, (1) ‘Thank You’, (2) ‘Enter values of P, N, R’, (3) ‘23.56E-03’, (4) All the above, (h) The maximum value that an integer constant can have is, (1) -32767, (2) 32767, (3) 1.7014e+38, (4) –1.7014e+38, (i) A C variable cannot start with, (1) An alphabet, (2) A number, (3) A special symbol other than underscore, (4) Both (2) & (3) above, (j) Which of the following statement is wrong, (1) mes = 123.56 ;, (2) con = 'T' * 'A' ;, (3) this = 'T' * 20 ;, (4) 3 + a = b ;, , 43
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44, , Let Us C, , (k) Which of the following shows the correct hierarchy of, arithmetic operators in C, (1) **, * or /, + or (2) **, *, /, +, (3) **, /, *, +, (4) / or *, - or +, (l) In b = 6.6 / a + 2 * n ; which operation will be performed, first?, (1) 6.6 / a, (2) a + 2, (3) 2 * n, (4) Depends upon compiler, (m) Which of the following is allowed in a C Arithmetic, instruction, (1) [ ], (2) { }, (3) ( ), (4) None of the above, (n) Which of the following statements is false, (1) Each new C instruction has to be written on a separate, line, (2) Usually all C statements are entered in small case letters, (3) Blank spaces may be inserted between two words in a C, statement, (4) Blank spaces cannot be inserted within a variable name, (o) If a is an integer variable, a = 5 / 2 ; will return a value, (1) 2.5, (2) 3, (3) 2, (4) 0, (p) The expression, a = 7 / 22 * ( 3.14 + 2 ) * 3 / 5 ; evaluates to
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Chapter 1: Getting Started, (1), (2), (3), (4), , 8.28, 6.28, 3.14, 0, , (q) The expression, a = 30 * 1000 + 2768 ; evaluates to, (1) 32768, (2) -32768, (3) 113040, (4) 0, (r) The expression x = 4 + 2 % - 8 evaluates to, (1) -6, (2) 6, (3) 4, (4) None of the above, (s) Hierarchy decides which operator, (1) is most important, (2) is used first, (3) is fastest, (4) operates on largest numbers, (t) An integer constant in C must have:, (1) At least one digit, (2) Atleast one decimal point, (3) A comma along with digits, (4) Digits separated by commas, (u) A character variable can never store more than, (1) 32 characters, (2) 8 characters, (3) 254 characters, (4) 1 character, (v) In C a variable cannot contain, (1) Blank spaces, , 45
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46, , Let Us C, (2) Hyphen, (3) Decimal point, (4) All the above, , (w) Which of the following is FALSE in C, (1) Keywords can be used as variable names, (2) Variable names can contain a digit, (3) Variable names do not contain a blank space, (4) Capital letters can be used in variable names, (x) In C, Arithmetic instruction cannot contain, (1) variables, (2) constants, (3) variable names on right side of =, (4) constants on left side of =, (y) Which of the following shows the correct hierarchy of, arithmetic operations in C, (1) / + * (2) * - / +, (3) + - / *, (4) * / + (z) What will be the value of d if d is a float after the operation, d = 2 / 7.0?, (1) 0, (2) 0.2857, (3) Cannot be determined, (4) None of the above, [H] Write C programs for the following:, , (a) Ramesh’s basic salary is input through the keyboard. His, dearness allowance is 40% of basic salary, and house rent, allowance is 20% of basic salary. Write a program to calculate, his gross salary.
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Chapter 1: Getting Started, , 47, , (b) The distance between two cities (in km.) is input through the, keyboard. Write a program to convert and print this distance, in meters, feet, inches and centimeters., (c) If the marks obtained by a student in five different subjects, are input through the keyboard, find out the aggregate marks, and percentage marks obtained by the student. Assume that, the maximum marks that can be obtained by a student in each, subject is 100., (d) Temperature of a city in Fahrenheit degrees is input through, the keyboard. Write a program to convert this temperature, into Centigrade degrees., (e) The length & breadth of a rectangle and radius of a circle are, input through the keyboard. Write a program to calculate the, area & perimeter of the rectangle, and the area &, circumference of the circle., (f) Two numbers are input through the keyboard into two, locations C and D. Write a program to interchange the, contents of C and D., (g) If a five-digit number is input through the keyboard, write a, program to calculate the sum of its digits., (Hint: Use the modulus operator ‘%’), (h) If a five-digit number is input through the keyboard, write a, program to reverse the number., (i) If a four-digit number is input through the keyboard, write a, program to obtain the sum of the first and last digit of this, number., (j) In a town, the percentage of men is 52. The percentage of, total literacy is 48. If total percentage of literate men is 35 of, the total population, write a program to find the total number
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48, , Let Us C, of illiterate men and women if the population of the town is, 80,000., , (k) A cashier has currency notes of denominations 10, 50 and, 100. If the amount to be withdrawn is input through the, keyboard in hundreds, find the total number of currency notes, of each denomination the cashier will have to give to the, withdrawer., (l) If the total selling price of 15 items and the total profit earned, on them is input through the keyboard, write a program to, find the cost price of one item., (m) If a five-digit number is input through the keyboard, write a, program to print a new number by adding one to each of its, digits. For example if the number that is input is 12391 then, the output should be displayed as 23402.
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2, , The Decision, Control Structure, • Decisions! Decisions!, • The if Statement, The Real Thing, Multiple Statements within if, • The if-else Statement, Nested if-elses, Forms of if, • Use of Logical Operators, The else if Clause, The ! Operator, Hierarchy of Operators Revisited, • A Word of Caution, • The Conditional Operators, • Summary, • Exercise, , 49
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50, , Let Us C, , W, , e all need to alter our actions in the face of changing, circumstances. If the weather is fine, then I will go for a, stroll. If the highway is busy I would take a diversion., If the pitch takes spin, we would win the match. If she says no, I, would look elsewhere. If you like this book, I would write the next, edition. You can notice that all these decisions depend on some, condition being met., C language too must be able to perform different sets of actions, depending on the circumstances. In fact this is what makes it worth, its salt. C has three major decision making instructions—the if, statement, the if-else statement, and the switch statement. A, fourth, somewhat less important structure is the one that uses, conditional operators. In this chapter we will explore all these, ways (except switch, which has a separate chapter devoted to it,, later) in which a C program can react to changing circumstances., , Decisions! Decisions!, In the programs written in Chapter 1 we have used sequence, control structure in which the various steps are executed, sequentially, i.e. in the same order in which they appear in the, program. In fact to execute the instructions sequentially, we don’t, have to do anything at all. By default the instructions in a program, are executed sequentially. However, in serious programming, situations, seldom do we want the instructions to be executed, sequentially. Many a times, we want a set of instructions to be, executed in one situation, and an entirely different set of, instructions to be executed in another situation. This kind of, situation is dealt in C programs using a decision control, instruction. As mentioned earlier, a decision control instruction, can be implemented in C using:, (a) The if statement, (b) The if-else statement, (c) The conditional operators
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Chapter 2: The Decision Control Structure, , 51, , Now let us learn each of these and their variations in turn., , The if Statement, Like most languages, C uses the keyword if to implement the, decision control instruction. The general form of if statement looks, like this:, if ( this condition is true ), execute this statement ;, , The keyword if tells the compiler that what follows is a decision, control instruction. The condition following the keyword if is, always enclosed within a pair of parentheses. If the condition,, whatever it is, is true, then the statement is executed. If the, condition is not true then the statement is not executed; instead the, program skips past it. But how do we express the condition itself, in C? And how do we evaluate its truth or falsity? As a general, rule, we express a condition using C’s ‘relational’ operators. The, relational operators allow us to compare two values to see whether, they are equal to each other, unequal, or whether one is greater, than the other. Here’s how they look and how they are evaluated in, C., , this expression, , is true if, , x == y, , x is equal to y, , x != y, , x is not equal to y, , x<y, , x is less than y, , x>y, , x is greater than y, , x <= y, , x is less than or equal to y, , x >= y, , x is greater than or equal to y, , Figure 2.1
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52, , Let Us C, , The relational operators should be familiar to you except for the, equality operator == and the inequality operator !=. Note that = is, used for assignment, whereas, == is used for comparison of two, quantities. Here is a simple program, which demonstrates the use, of if and the relational operators., /* Demonstration of if statement */, main( ), {, int num ;, printf ( "Enter a number less than 10 " ) ;, scanf ( "%d", &num ) ;, if ( num <= 10 ), printf ( "What an obedient servant you are !" ) ;, }, , On execution of this program, if you type a number less than or, equal to 10, you get a message on the screen through printf( ). If, you type some other number the program doesn’t do anything. The, following flowchart would help you understand the flow of control, in the program.
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Chapter 2: The Decision Control Structure, , 53, , START, PRINT enter a num, less than 10, INPUT num, , no, , is, num > 10, , yes, , PRINT What an obedient, servant you are !, , STOP, , Figure 2.2, To make you comfortable with the decision control instruction one, more example has been given below. Study it carefully before, reading further. To help you understand it easily, the program is, accompanied by an appropriate flowchart., Example 2.1: While purchasing certain items, a discount of 10%, is offered if the quantity purchased is more than 1000. If quantity, and price per item are input through the keyboard, write a program, to calculate the total expenses.
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54, , Let Us C, , START, dis = 0, INPUT, qty, rate, , no, , is, qty > 1000, , yes, , dis = 10, , tot = qty * rate – qty * rate * dis / 100, PRINT, tot, STOP, , Figure 2.3, /* Calculation of total expenses */, main( ), {, int qty, dis = 0 ;, float rate, tot ;, printf ( "Enter quantity and rate " ) ;, scanf ( "%d %f", &qty, &rate) ;, if ( qty > 1000 ), dis = 10 ;
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Chapter 2: The Decision Control Structure, , 55, , tot = ( qty * rate ) - ( qty * rate * dis / 100 ) ;, printf ( "Total expenses = Rs. %f", tot ) ;, }, , Here is some sample interaction with the program., Enter quantity and rate 1200 15.50, Total expenses = Rs. 16740.000000, Enter quantity and rate 200 15.50, Total expenses = Rs. 3100.000000, , In the first run of the program, the condition evaluates to true, as, 1200 (value of qty) is greater than 1000. Therefore, the variable, dis, which was earlier set to 0, now gets a new value 10. Using this, new value total expenses are calculated and printed., In the second run the condition evaluates to false, as 200 (the value, of qty) isn’t greater than 1000. Thus, dis, which is earlier set to 0,, remains 0, and hence the expression after the minus sign evaluates, to zero, thereby offering no discount., Is the statement dis = 0 necessary? The answer is yes, since in C, a, variable if not specifically initialized contains some unpredictable, value (garbage value)., , The Real Thing, We mentioned earlier that the general form of the if statement is as, follows, if ( condition ), statement ;, , Truly speaking the general form is as follows:
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56, , Let Us C, , if ( expression ), statement ;, , Here the expression can be any valid expression including a, relational expression. We can even use arithmetic expressions in, the if statement. For example all the following if statements are, valid, if ( 3 + 2 % 5 ), printf ( "This works" ) ;, if ( a = 10 ), printf ( "Even this works" ) ;, if ( -5 ), printf ( "Surprisingly even this works" ) ;, , Note that in C a non-zero value is considered to be true, whereas a, 0 is considered to be false. In the first if, the expression evaluates, to 5 and since 5 is non-zero it is considered to be true. Hence the, printf( ) gets executed., In the second if, 10 gets assigned to a so the if is now reduced to if, ( a ) or if ( 10 ). Since 10 is non-zero, it is true hence again, printf( ) goes to work., In the third if, -5 is a non-zero number, hence true. So again, printf( ) goes to work. In place of -5 even if a float like 3.14 were, used it would be considered to be true. So the issue is not whether, the number is integer or float, or whether it is positive or negative., Issue is whether it is zero or non-zero., , Multiple Statements within if, It may so happen that in a program we want more than one, statement to be executed if the expression following if is satisfied., If such multiple statements are to be executed then they must be
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Chapter 2: The Decision Control Structure, , 57, , placed within a pair of braces as illustrated in the following, example., Example 2.2: The current year and the year in which the, employee joined the organization are entered through the, keyboard. If the number of years for which the employee has, served the organization is greater than 3 then a bonus of Rs. 2500/is given to the employee. If the years of service are not greater, than 3, then the program should do nothing., /* Calculation of bonus */, main( ), {, int bonus, cy, yoj, yr_of_ser ;, printf ( "Enter current year and year of joining " ) ;, scanf ( "%d %d", &cy, &yoj ) ;, yr_of_ser = cy - yoj ;, if ( yr_of_ser > 3 ), {, bonus = 2500 ;, printf ( "Bonus = Rs. %d", bonus ) ;, }, }, , Observe that here the two statements to be executed on satisfaction, of the condition have been enclosed within a pair of braces. If a, pair of braces is not used then the C compiler assumes that the, programmer wants only the immediately next statement after the if, to be executed on satisfaction of the condition. In other words we, can say that the default scope of the if statement is the immediately, next statement after it.
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58, , Let Us C, , START, INPUT, cy, yoj, , yr_of_ser = cy - yoj, , no, , yr_of_ser > 3, , yes, , bonus = 2500, , PRINT, bonus, , STOP, , Figure 2.4, , The if-else Statement, The if statement by itself will execute a single statement, or a, group of statements, when the expression following if evaluates to, true. It does nothing when the expression evaluates to false. Can, we execute one group of statements if the expression evaluates to, true and another group of statements if the expression evaluates to, false? Of course! This is what is the purpose of the else statement, that is demonstrated in the following example:, Example 2.3: In a company an employee is paid as under:
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Chapter 2: The Decision Control Structure, , 59, , If his basic salary is less than Rs. 1500, then HRA = 10% of basic, salary and DA = 90% of basic salary. If his salary is either equal to, or above Rs. 1500, then HRA = Rs. 500 and DA = 98% of basic, salary. If the employee's salary is input through the keyboard write, a program to find his gross salary., /* Calculation of gross salary */, main( ), {, float bs, gs, da, hra ;, printf ( "Enter basic salary " ) ;, scanf ( "%f", &bs ) ;, if ( bs < 1500 ), {, hra = bs * 10 / 100 ;, da = bs * 90 / 100 ;, }, else, {, hra = 500 ;, da = bs * 98 / 100 ;, }, gs = bs + hra + da ;, printf ( "gross salary = Rs. %f", gs ) ;, }
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60, , Let Us C, , START, INPUT, bs, is, bs < 1500, hra = 500, , hra = bs * 10 / 100, , da = bs * 98 / 100, , da = bs * 90 / 100, , gs = bs + hra + da, PRINT, gs, , Figure 2.5, , STOP, , Figure 2.5, A few points worth noting..., (a) The group of statements after the if upto and not including the, , else is called an ‘if block’. Similarly, the statements after the, else form the ‘else block’., (b) Notice that the else is written exactly below the if. The, statements in the if block and those in the else block have, been indented to the right. This formatting convention is
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Chapter 2: The Decision Control Structure, , 61, , followed throughout the book to enable you to understand the, working of the program better., (c) Had there been only one statement to be executed in the if, , block and only one statement in the else block we could have, dropped the pair of braces., (d) As with the if statement, the default scope of else is also the, , statement immediately after the else. To override this default, scope a pair of braces as shown in the above example must be, used., , Nested if-elses, It is perfectly all right if we write an entire if-else construct within, either the body of the if statement or the body of an else statement., This is called ‘nesting’of ifs. This is shown in the following, program., /* A quick demo of nested if-else */, main( ), {, int i ;, printf ( "Enter either 1 or 2 " ) ;, scanf ( "%d", &i ) ;, if ( i == 1 ), printf ( "You would go to heaven !" ) ;, else, {, if ( i == 2 ), printf ( "Hell was created with you in mind" ) ;, else, printf ( "How about mother earth !" ) ;, }, }
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62, , Let Us C, , Note that the second if-else construct is nested in the first else, statement. If the condition in the first if statement is false, then the, condition in the second if statement is checked. If it is false as, well, then the final else statement is executed., You can see in the program how each time a if-else construct is, nested within another if-else construct, it is also indented to add, clarity to the program. Inculcate this habit of indentation,, otherwise you would end up writing programs which nobody (you, included) can understand easily at a later date., In the above program an if-else occurs within the else block of the, first if statement. Similarly, in some other program an if-else may, occur in the if block as well. There is no limit on how deeply the, ifs and the elses can be nested., , Forms of if, The if statement can take any of the following forms:, (a) if ( condition ), do this ;, (b) if ( condition ), {, do this ;, and this ;, }, (c), , if ( condition ), do this ;, else, do this ;, (d) if ( condition ), {, do this ;
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Chapter 2: The Decision Control Structure, and this ;, }, else, {, do this ;, and this ;, }, (e) if ( condition ), do this ;, else, {, if ( condition ), do this ;, else, {, do this ;, and this ;, }, }, (f), , if ( condition ), {, if ( condition ), do this ;, else, {, do this ;, and this ;, }, }, else, do this ;, , 63
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64, , Let Us C, , Use of Logical Operators, C allows usage of three logical operators, namely, &&, || and !., These are to be read as ‘AND’ ‘OR’ and ‘NOT’ respectively., There are several things to note about these logical operators. Most, obviously, two of them are composed of double symbols: || and, &&. Don’t use the single symbol | and &. These single symbols, also have a meaning. They are bitwise operators, which we would, examine in Chapter 14., The first two operators, && and ||, allow two or more conditions, to be combined in an if statement. Let us see how they are used in, a program. Consider the following example., Example 2.4: The marks obtained by a student in 5 different, subjects are input through the keyboard. The student gets a, division as per the following rules:, Percentage above or equal to 60 - First division, Percentage between 50 and 59 - Second division, Percentage between 40 and 49 - Third division, Percentage less than 40 - Fail, Write a program to calculate the division obtained by the student., There are two ways in which we can write a program for this, example. These methods are given below., /* Method – I */, main( ), {, int m1, m2, m3, m4, m5, per ;, printf ( "Enter marks in five subjects " ) ;, scanf ( "%d %d %d %d %d", &m1, &m2, &m3, &m4, &m5 ) ;, per = ( m1 + m2 + m3 + m4 + m5 ) / 5 ;
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Chapter 2: The Decision Control Structure, , 65, , if ( per >= 60 ), printf ( "First division ") ;, else, {, if ( per >= 50 ), printf ( "Second division" ) ;, else, {, if ( per >= 40 ), printf ( "Third division" ) ;, else, printf ( "Fail" ) ;, }, }, }, , This is a straight forward program. Observe that the program uses, nested if-elses. This leads to three disadvantages:, (a) As the number of conditions go on increasing the level of, , indentation also goes on increasing. As a result the whole, program creeps to the right., (b) Care needs to be exercised to match the corresponding ifs and, elses., (c) Care needs to be exercised to match the corresponding pair of, braces., All these three problems can be eliminated by usage of ‘Logical, operators’. The following program illustrates this., /* Method – II */, main( ), {, int m1, m2, m3, m4, m5, per ;, printf ( "Enter marks in five subjects " ) ;, scanf ( "%d %d %d %d %d", &m1, &m2, &m3, &m4, &m5 ) ;, per = ( m1 + m2 + m3 + m4 + m5 ) / 5 ;
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66, , Let Us C, if ( per >= 60 ), printf ( "First division" ) ;, if ( ( per >= 50 ) && ( per < 60 ) ), printf ( "Second division" ) ;, if ( ( per >= 40 ) && ( per < 50 ) ), printf ( "Third division" ) ;, if ( per < 40 ), printf ( "Fail" ) ;, , }, , As can be seen from the second if statement, the && operator is, used to combine two conditions. ‘Second division’ gets printed if, both the conditions evaluate to true. If one of the conditions, evaluate to false then the whole thing is treated as false., Two distinct advantages can be cited in favour of this program:, (a) The matching (or do I say mismatching) of the ifs with their, , corresponding elses gets avoided, since there are no elses in, this program., (b) In spite of using several conditions, the program doesn't creep, to the right. In the previous program the statements went on, creeping to the right. This effect becomes more pronounced as, the number of conditions go on increasing. This would make, the task of matching the ifs with their corresponding elses and, matching of opening and closing braces that much more, difficult., , The else if Clause, There is one more way in which we can write program for, Example 2.4. This involves usage of else if blocks as shown, below:
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Chapter 2: The Decision Control Structure, , 67, , /* else if ladder demo */, main( ), {, int m1, m2, m3, m4, m5, per ;, per = ( m1+ m2 + m3 + m4+ m5 ) / per ;, if ( per >= 60 ), printf ( "First division" ) ;, else if ( per >= 50 ), printf ( "Second division" ) ;, else if ( per >= 40 ), printf ( "Third division" ) ;, else, printf ( "fail" ) ;, }, , You can note that this program reduces the indentation of the, statements. In this case every else is associated with its previous if., The last else goes to work only if all the conditions fail. Even in, else if ladder the last else is optional., Note that the else if clause is nothing different. It is just a way of, rearranging the else with the if that follows it. This would be, evident if you look at the following code:, if ( i == 2 ), printf ( "With you…" ) ;, else, {, if ( j == 2 ), printf ( "…All the time" ) ;, }, , if ( i == 2 ), printf ( "With you…" ) ;, else if ( j == 2 ), printf ( "…All the time " ) ;, , Another place where logical operators are useful is when we want, to write programs for complicated logics that ultimately boil down
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68, , Let Us C, , to only two answers. For example, consider the following, example:, Example 2.5: A company insures its drivers in the following, cases:, −, −, −, , If the driver is married., If the driver is unmarried, male & above 30 years of age., If the driver is unmarried, female & above 25 years of age., , In all other cases the driver is not insured. If the marital status, sex, and age of the driver are the inputs, write a program to determine, whether the driver is to be insured or not., Here after checking a complicated set of instructions the final, output of the program would be one of the two—Either the driver, should be ensured or the driver should not be ensured. As, mentioned above, since these are the only two outcomes this, problem can be solved using logical operators. But before we do, that let us write a program that does not make use of logical, operators., /* Insurance of driver - without using logical operators */, main( ), {, char sex, ms ;, int age ;, printf ( "Enter age, sex, marital status " ) ;, scanf ( "%d %c %c", &age, &sex, &ms ) ;, if ( ms == 'M' ), printf ( "Driver is insured" ) ;, else, {, if ( sex == 'M' ), {
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Chapter 2: The Decision Control Structure, , 69, , if ( age > 30 ), printf ( "Driver is insured" ) ;, else, printf ( "Driver is not insured" ) ;, }, else, {, if ( age > 25 ), printf ( "Driver is insured" ) ;, else, printf ( "Driver is not insured" ) ;, }, }, }, , From the program it is evident that we are required to match, several ifs and elses and several pairs of braces. In a more real-life, situation there would be more conditions to check leading to the, program creeping to the right. Let us now see how to avoid these, problems by using logical operators., As mentioned above, in this example we expect the answer to be, either ‘Driver is insured’ or ‘Driver is not insured’. If we list down, all those cases in which the driver is insured, then they would be:, (a) Driver is married., (b) Driver is an unmarried male above 30 years of age., (c) Driver is an unmarried female above 25 years of age., , Since all these cases lead to the driver being insured, they can be, combined together using && and || as shown in the program, below:, /* Insurance of driver - using logical operators */, main( ), {, char sex, ms ;
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70, , Let Us C, int age ;, printf ( "Enter age, sex, marital status " ) ;, scanf ( "%d %c %c" &age, &sex, &ms ) ;, if ( ( ms == 'M') || ( ms == 'U' && sex == 'M' && age > 30 ) ||, ( ms == 'U' && sex == 'F' && age > 25 ) ), printf ( "Driver is insured" ) ;, else, printf ( "Driver is not insured" ) ;, , }, , In this program it is important to note that:, −, , The driver will be insured only if one of the conditions, enclosed in parentheses evaluates to true., , −, , For the second pair of parentheses to evaluate to true, each, condition in the parentheses separated by && must evaluate to, true., , −, , Even if one of the conditions in the second parentheses, evaluates to false, then the whole of the second parentheses, evaluates to false., , −, , The last two of the above arguments apply to third pair of, parentheses as well., , Thus we can conclude that the && and || are useful in the, following programming situations:, (a) When it is to be tested whether a value falls within a, , particular range or not., (b) When after testing several conditions the outcome is only one, of the two answers (This problem is often called yes/no, problem).
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Chapter 2: The Decision Control Structure, , 71, , There can be one more situation other than checking ranges or, yes/no problem where you might find logical operators useful. The, following program demonstrates it., Example 2.6: Write a program to calculate the salary as per the, following table:, , Gender, , Years of Service, , Qualifications, , Salary, , Male, , >= 10, , Post-Graduate, , 15000, , >= 10, , Graduate, , 10000, , < 10, , Post-Graduate, , 10000, , < 10, , Graduate, , 7000, , >= 10, , Post-Graduate, , 12000, , >= 10, , Graduate, , 9000, , < 10, , Post-Graduate, , 10000, , < 10, , Graduate, , 6000, , Female, , Figure 2.6, main( ), {, char g ;, int yos, qual, sal ;, printf ( "Enter Gender, Years of Service and, Qualifications ( 0 = G, 1 = PG ):" ) ;, scanf ( "%c%d%d", &g, &yos, &qual ) ;, if ( g == 'm' && yos >= 10 && qual == 1 ), sal = 15000 ;, else if ( ( g == 'm' && yos >= 10 && qual == 0 ) ||, ( g == 'm' && yos < 10 && qual == 1 ) ), sal = 10000 ;
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72, , Let Us C, else if ( g == 'm' && yos < 10 && qual == 0 ), sal = 7000 ;, else if ( g == 'f' && yos >= 10 && qual == 1 ), sal = 12000 ;, else if ( g == 'f' && yos >= 10 && qual == 0 ), sal = 9000 ;, else if ( g == 'f' && yos < 10 && qual == 1 ), sal = 10000 ;, else if ( g == 'f' && yos < 10 && qual == 0 ), sal = 6000 ;, printf ( "\nSalary of Employee = %d", sal ) ;, , }, , The ! Operator, So far we have used only the logical operators && and ||. The, third logical operator is the NOT operator, written as !. This, operator reverses the result of the expression it operates on. For, example, if the expression evaluates to a non-zero value, then, applying ! operator to it results into a 0. Vice versa, if the, expression evaluates to zero then on applying ! operator to it, makes it 1, a non-zero value. The final result (after applying !) 0 or, 1 is considered to be false or true respectively. Here is an example, of the NOT operator applied to a relational expression., ! ( y < 10 ), , This means “not y less than 10”. In other words, if y is less than, 10, the expression will be false, since ( y < 10 ) is true. We can, express the same condition as ( y >= 10 )., The NOT operator is often used to reverse the logical value of a, single variable, as in the expression, if ( ! flag )
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Chapter 2: The Decision Control Structure, , 73, , This is another way of saying, if ( flag == 0 ), , Does the NOT operator sound confusing? Avoid it if you want, as, the same thing can be achieved without using the NOT operator., , Hierarchy of Operators Revisited, Since we have now added the logical operators to the list of, operators we know, it is time to review these operators and their, priorities. Figure 2.7 summarizes the operators we have seen so, far. The higher the position of an operator is in the table, higher is, its priority. (A full-fledged precedence table of operators is given, in Appendix A.), Operators, , Type, , !, , Logical NOT, , * / %, , Arithmetic and modulus, , + -, , Arithmetic, , < > <= >=, , Relational, , == !=, , Relational, , &&, , Logical AND, , ||, , Logical OR, , =, , Assignment, , Figure 2.7, , A Word of Caution, What will be the output of the following program:
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74, , Let Us C, , main( ), {, int i ;, printf ( "Enter value of i " ) ;, scanf ( "%d", &i ) ;, if ( i = 5 ), printf ( "You entered 5" ) ;, else, printf ( "You entered something other than 5" ) ;, }, , And here is the output of two runs of this program..., Enter value of i 200, You entered 5, Enter value of i 9999, You entered 5, , Surprising? You have entered 200 and 9999, and still you find in, either case the output is ‘You entered 5’. This is because we have, written the condition wrongly. We have used the assignment, operator = instead of the relational operator ==. As a result, the, condition gets reduced to if ( 5 ), irrespective of what you supply, as the value of i. And remember that in C ‘truth’ is always nonzero, whereas ‘falsity’ is always zero. Therefore, if ( 5 ) always, evaluates to true and hence the result., Another common mistake while using the if statement is to write a, semicolon (;) after the condition, as shown below:, main( ), {, int i ;, printf ( "Enter value of i " ) ;, scanf ( "%d", &i ) ;
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75, , Chapter 2: The Decision Control Structure, if ( i == 5 ) ;, printf ( "You entered 5" ) ;, }, , The ; makes the compiler to interpret the statement as if you have, written it in following manner:, if ( i == 5 ), ;, printf ( "You entered 5" ) ;, , Here, if the condition evaluates to true the ; (null statement, which, does nothing on execution) gets executed, following which the, printf( ) gets executed. If the condition fails then straightaway the, printf( ) gets executed. Thus, irrespective of whether the condition, evaluates to true or false the printf( ) is bound to get executed., Remember that the compiler would not point out this as an error,, since as far as the syntax is concerned nothing has gone wrong, but, the logic has certainly gone awry. Moral is, beware of such, pitfalls., The following figure summarizes the working of all the three, logical operators., , Operands, , Results, , x, , y, , !x, , !y, , x && y, , x || y, , 0, , 0, , 1, , 1, , 0, , 0, , 0, , non-zero, , 1, , 0, , 0, , 0, , non-zero, , 0, , 0, , 1, , 0, , 1, , non-zero, , non-zero, , 0, , 0, , 1, , 1, , Figure 2.8
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76, , Let Us C, , The Conditional Operators, The conditional operators ? and : are sometimes called ternary, operators since they take three arguments. In fact, they form a kind, of foreshortened if-then-else. Their general form is,, expression 1 ? expression 2 : expression 3, , What this expression says is: “if expression 1 is true (that is, if its, value is non-zero), then the value returned will be expression 2,, otherwise the value returned will be expression 3”. Let us, understand this with the help of a few examples:, (a) int x, y ;, scanf ( "%d", &x ) ;, y=(x>5?3:4);, , This statement will store 3 in y if x is greater than 5,, otherwise it will store 4 in y., The equivalent if statement will be,, if ( x > 5 ), y=3;, else, y=4;, (b) char a ;, int y ;, scanf ( "%c", &a ) ;, y = ( a >= 65 && a <= 90 ? 1 : 0 ) ;, , Here 1 would be assigned to y if a >=65 && a <=90 evaluates to, true, otherwise 0 would be assigned., The following points may be noted about the conditional, operators:
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Chapter 2: The Decision Control Structure, , 77, , (a) It’s not necessary that the conditional operators should be, , used only in arithmetic statements. This is illustrated in the, following examples:, Ex.:, , int i ;, scanf ( "%d", &i ) ;, ( i == 1 ? printf ( "Amit" ) : printf ( "All and sundry" ) ) ;, , Ex.:, , char a = 'z' ;, printf ( "%c" , ( a >= 'a' ? a : '!' ) ) ;, , (b) The conditional operators can be nested as shown below., , int big, a, b, c ;, big = ( a > b ? ( a > c ? 3: 4 ) : ( b > c ? 6: 8 ) ) ;, (c) Check out the following conditional expression:, , a>b?g=a:g=b;, , This will give you an error ‘Lvalue Required’. The error can, be overcome by enclosing the statement in the : part within a, pair of parenthesis. This is shown below:, a>b?g=a:(g=b);, , In absence of parentheses the compiler believes that b is being, assigned to the result of the expression to the left of second =., Hence it reports an error., The limitation of the conditional operators is that after the ? or, after the : only one C statement can occur. In practice rarely is this, the requirement. Therefore, in serious C programming conditional, operators aren’t as frequently used as the if-else., , Summary, (a) There are three ways for taking decisions in a program. First, way is to use the if-else statement, second way is to use the
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78, , (b), , (c), , (d), , (e), (f), , (g), , Let Us C, conditional operators and third way is to use the switch, statement., The default scope of the if statement is only the next, statement. So, to execute more than one statement they must, be written in a pair of braces., An if block need not always be associated with an else block., However, an else block is always associated with an if, statement., If the outcome of an if-else ladder is only one of two answers, then the ladder should be replaced either with an else-if clause, or by logical operators., && and || are binary operators, whereas, ! is a unary operator., In C every test expression is evaluated in terms of zero and, non-zero values. A zero value is considered to be false and a, non-zero value is considered to be true., Assignment statements used with conditional operators must, be enclosed within a pair of parenthesis., , Exercise, if, if-else, Nested if-elses, [A] What would be the output of the following programs:, (a) main( ), {, int a = 300, b, c ;, if ( a >= 400 ), b = 300 ;, c = 200 ;, printf ( "\n%d %d", b, c ) ;, }, (b) main( ), {, int a = 500, b, c ;, if ( a >= 400 )
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Chapter 2: The Decision Control Structure, b = 300 ;, c = 200 ;, printf ( "\n%d %d", b, c ) ;, }, (c), , main( ), {, int x = 10, y = 20 ;, if ( x == y ) ;, printf ( "\n%d %d", x, y ) ;, }, , (d) main( ), {, int x = 3, y = 5 ;, if ( x == 3 ), printf ( "\n%d", x ) ;, else ;, printf ( "\n%d", y ) ;, }, (e) main( ), {, int x = 3 ;, float y = 3.0 ;, if ( x == y ), printf ( "\nx and y are equal" ) ;, else, printf ( "\nx and y are not equal" ) ;, (f), , }, main( ), {, int x = 3, y, z ;, y = x = 10 ;, z = x < 10 ;, printf ( "\nx = %d y = %d z = %d", x, y, z ) ;, }, , 79
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80, , Let Us C, , (g) main( ), {, int k = 35 ;, printf ( "\n%d %d %d", k == 35, k = 50, k > 40 ) ;, }, (h) main( ), {, int i = 65 ;, char j = ‘A’ ;, if ( i == j ), printf ( “C is WOW” ) ;, else, printf( "C is a headache" ) ;, }, (i), , main( ), {, int a = 5, b, c ;, b = a = 15 ;, c = a < 15 ;, printf ( "\na = %d b = %d c = %d", a, b, c ) ;, }, , (j), , main( ), {, int x = 15 ;, printf ( "\n%d %d %d", x != 15, x = 20, x < 30 ) ;, }, , [B] Point out the errors, if any, in the following programs:, (a) main( ), {, float a = 12.25, b = 12.52 ;, if ( a = b ), printf ( "\na and b are equal" ) ;
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Chapter 2: The Decision Control Structure, }, (b) main( ), {, int j = 10, k = 12 ;, if ( k >= j ), {, {, k=j;, j=k;, }, }, }, (c), , main( ), {, if ( 'X' < 'x' ), printf ( "\nascii value of X is smaller than that of x" ) ;, }, , (d) main( ), {, int x = 10 ;, if ( x >= 2 ) then, printf ( "\n%d", x ) ;, }, (e) main( ), {, int x = 10 ;, if x >= 2, printf ( "\n%d", x ) ;, }, (f), , main( ), {, int x = 10, y = 15 ;, if ( x % 2 = y % 3 ), , 81
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82, , Let Us C, printf ( "\nCarpathians" ) ;, }, , (g) main( ), {, int x = 30 , y = 40 ;, if ( x == y ), printf( "x is equal to y" ) ;, elseif ( x > y ), printf( "x is greater than y" ) ;, elseif ( x < y ), printf( "x is less than y" ) ;, }, (h) main( ), {, int x = 10 ;, if ( x >= 2 ) then, printf ( "\n%d", x ) ;, }, (i), , main( ), {, int a, b ;, scanf ( "%d %d",a, b ) ;, if ( a > b ) ;, printf ( "This is a game" ) ;, else, printf ( "You have to play it" ) ;, }, , [C] Attempt the following:, (a) If cost price and selling price of an item is input through the, , keyboard, write a program to determine whether the seller has, made profit or incurred loss. Also determine how much profit, he made or loss he incurred.
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Chapter 2: The Decision Control Structure, , 83, , (b) Any integer is input through the keyboard. Write a program to, , find out whether it is an odd number or even number., (c) Any year is input through the keyboard. Write a program to, , determine whether the year is a leap year or not., (Hint: Use the % (modulus) operator), (d) According to the Gregorian calendar, it was Monday on the, , date 01/01/1900. If any year is input through the keyboard, write a program to find out what is the day on 1st January of, this year., (e) A five-digit number is entered through the keyboard. Write a, , program to obtain the reversed number and to determine, whether the original and reversed numbers are equal or not., (f), , If the ages of Ram, Shyam and Ajay are input through the, keyboard, write a program to determine the youngest of the, three., , (g) Write a program to check whether a triangle is valid or not,, , when the three angles of the triangle are entered through the, keyboard. A triangle is valid if the sum of all the three angles, is equal to 180 degrees., (h) Find the absolute value of a number entered through the, , keyboard., (i), , Given the length and breadth of a rectangle, write a program to, find whether the area of the rectangle is greater than its, perimeter. For example, the area of the rectangle with length = 5, and breadth = 4 is greater than its perimeter., , (j), , Given three points (x1, y1), (x2, y2) and (x3, y3), write a, program to check if all the three points fall on one straight line.
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84, , Let Us C, , (k) Given the coordinates (x, y) of a center of a circle and it’s radius,, , write a program which will determine whether a point lies inside, the circle, on the circle or outside the circle., , (Hint: Use sqrt( ) and pow( ) functions), (l), , Given a point (x, y), write a program to find out if it lies on the, x-axis, y-axis or at the origin, viz. (0, 0)., , Logical Operators, If a = 10, b = 12, c = 0, find the values of the expressions in, the following table:, Expression, , Value, , a != 6 && b > 5, a == 9 || b < 3, ! ( a < 10 ), ! ( a > 5 && c ), 5 && c != 8 || !c, , 1, , [D] What would be the output of the following programs:, (a) main( ), {, int i = 4, z = 12 ;, if ( i = 5 || z > 50 ), printf ( "\nDean of students affairs" ) ;, else, printf ( "\nDosa" ) ;, }, (b) main( ), {, int i = 4, z = 12 ;
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Chapter 2: The Decision Control Structure, if ( i = 5 && z > 5 ), printf ( "\nLet us C" ) ;, else, printf ( "\nWish C was free !" ) ;, }, (c), , main( ), {, int i = 4, j = -1, k = 0, w, x, y, z ;, w = i || j || k ;, x = i && j && k ;, y = i || j && k ;, z = i && j || k ;, printf ( "\nw = %d x = %d y = %d z = %d", w, x, y, z ) ;, }, , (d) main( ), {, int i = 4, j = -1, k = 0, y, z ;, y = i + 5 && j + 1 || k + 2 ;, z = i + 5 || j + 1 && k + 2 ;, printf ( "\ny = %d z = %d", y, z ) ;, }, (e) main( ), {, int i = -3, j = 3 ;, if ( !i + !j * 1 ), printf ( "\nMassaro" ) ;, else, printf ( "\nBennarivo" ) ;, }, (f), , main( ), {, int a = 40 ;, if ( a > 40 && a < 45 ), printf ( "a is greater than 40 and less than 45" ) ;, , 85
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86, , Let Us C, else, printf ( "%d", a ) ;, }, , (g) main( ), {, int p = 8, q = 20 ;, if ( p == 5 && q > 5 ), printf ( "\nWhy not C" ) ;, else, printf ( "\nDefinitely C !" ) ;, }, (h) main( ), {, int i = -1, j = 1, k ,l ;, k = i && j ;, l = i || j ;, printf ( "%d %d", I, j ) ;, }, (i), , main( ), {, int x = 20 , y = 40 , z = 45 ;, if ( x > y && x > z ), printf( "x is big" ) ;, else if ( y > x && y > z ), printf( "y is big" ) ;, else if ( z > x && z > y ), printf( "z is big" ) ;, }, , (j), , main( ), {, int i = -1, j = 1, k ,l ;, k = !i && j ;, l = !i || j ;, printf ( "%d %d", i, j ) ;
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Chapter 2: The Decision Control Structure, }, (k), , main( ), {, int j = 4, k ;, k = !5 && j ;, printf ( "\nk = %d", k ) ;, }, , [E] Point out the errors, if any, in the following programs:, (a) /* This program, /* is an example of, /* using Logical operators */, main( ), {, int i = 2, j = 5 ;, if ( i == 2 && j == 5 ), printf ( "\nSatisfied at last" ) ;, }, (b) main( ), {, int code, flag ;, if ( code == 1 & flag == 0 ), printf ( "\nThe eagle has landed" ) ;, }, (c), , main( ), {, char spy = 'a', password = 'z' ;, if ( spy == 'a' or password == 'z' ), printf ( "\nAll the birds are safe in the nest" ) ;, }, , (d) main( ), {, , 87
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88, , Let Us C, int i = 10, j = 20 ;, if ( i = 5 ) && if ( j = 10 ), printf ( "\nHave a nice day" ) ;, }, , (a) main( ), {, int x = 10 , y = 20;, if ( x >= 2 and y <=50 ), printf ( "\n%d", x ) ;, }, (b) main( ), {, int a, b ;, if ( a == 1 & b == 0 ), printf ( "\nGod is Great" ) ;, }, (c), , main( ), {, int x = 2;, if ( x == 2 && x != 0 ) ;, {, printf ( "\nHi" ) ;, printf( "\nHello" ) ;, }, else, printf( "Bye" ) ;, }, , (d) main( ), {, int i = 10, j = 10 ;, if ( i && j == 10), printf ( "\nHave a nice day" ) ;, }
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Chapter 2: The Decision Control Structure, , 89, , [F] Attempt the following:, (a) Any year is entered through the keyboard, write a program to, , determine whether the year is leap or not. Use the logical, operators && and ||., (b) Any character is entered through the keyboard, write a, , program to determine whether the character entered is a, capital letter, a small case letter, a digit or a special symbol., The following table shows the range of ASCII values for, various characters., Characters, , ASCII Values, , A–Z, , 65 – 90, , a–z, , 97 – 122, , 0–9, , 48 – 57, , special symbols, , 0 - 47, 58 - 64, 91 - 96, 123 - 127, , (c) An Insurance company follows following rules to calculate, , premium., (1) If a person’s health is excellent and the person is between, 25 and 35 years of age and lives in a city and is a male, then the premium is Rs. 4 per thousand and his policy, amount cannot exceed Rs. 2 lakhs., (2) If a person satisfies all the above conditions except that, the sex is female then the premium is Rs. 3 per thousand, and her policy amount cannot exceed Rs. 1 lakh., (3) If a person’s health is poor and the person is between 25, and 35 years of age and lives in a village and is a male
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90, , Let Us C, then the premium is Rs. 6 per thousand and his policy, cannot exceed Rs. 10,000., (4) In all other cases the person is not insured., Write a program to output whether the person should be, insured or not, his/her premium rate and maximum amount, for which he/she can be insured., , (d) A certain grade of steel is graded according to the following, , conditions:, (i) Hardness must be greater than 50, (ii) Carbon content must be less than 0.7, (iii) Tensile strength must be greater than 5600, The grades are as follows:, Grade is 10 if all three conditions are met, Grade is 9 if conditions (i) and (ii) are met, Grade is 8 if conditions (ii) and (iii) are met, Grade is 7 if conditions (i) and (iii) are met, Grade is 6 if only one condition is met, Grade is 5 if none of the conditions are met, Write a program, which will require the user to give values of, hardness, carbon content and tensile strength of the steel, under consideration and output the grade of the steel., (e) A library charges a fine for every book returned late. For first, , 5 days the fine is 50 paise, for 6-10 days fine is one rupee and, above 10 days fine is 5 rupees. If you return the book after 30, days your membership will be cancelled. Write a program to, accept the number of days the member is late to return the, book and display the fine or the appropriate message.
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Chapter 2: The Decision Control Structure, (f), , 91, , If the three sides of a triangle are entered through the, keyboard, write a program to check whether the triangle is, valid or not. The triangle is valid if the sum of two sides is, greater than the largest of the three sides., , (g) If the three sides of a triangle are entered through the, , keyboard, write a program to check whether the triangle is, isosceles, equilateral, scalene or right angled triangle., (h) In a company, worker efficiency is determined on the basis of, , the time required for a worker to complete a particular job. If, the time taken by the worker is between 2 – 3 hours, then the, worker is said to be highly efficient. If the time required by, the worker is between 3 – 4 hours, then the worker is ordered, to improve speed. If the time taken is between 4 – 5 hours, the, worker is given training to improve his speed, and if the time, taken by the worker is more than 5 hours, then the worker has, to leave the company. If the time taken by the worker is input, through the keyboard, find the efficiency of the worker., (i), , A university has the following rules for a student to qualify, for a degree with A as the main subject and B as the, subsidiary subject:, (a) He should get 55 percent or more in A and 45 percent or, more in B., (b) If he gets than 55 percent in A he should get 55 percent or, more in B. However, he should get at least 45 percent in, A., (c) If he gets less than 45 percent in B and 65 percent or more, in A he is allowed to reappear in an examination in B to, qualify., (d) In all other cases he is declared to have failed., Write a program to receive marks in A and B and Output, whether the student has passed, failed or is allowed to, reappear in B.
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92, (j), , Let Us C, , The policy followed by a company to process customer orders, is given by the following rules:, (a) If a customer order is less than or equal to that in stock, and has credit is OK, supply has requirement., (b) If has credit is not OK do not supply. Send him, intimation., (c) If has credit is Ok but the item in stock is less than has, order, supply what is in stock. Intimate to him data the, balance will be shipped., Write a C program to implement the company policy., , Conditional operators, [G] What would be the output of the following programs:, (a) main( ), {, int i = -4, j, num ;, j = ( num < 0 ? 0 : num * num ) ;, printf ( "\n%d", j ) ;, }, (b) main( ), {, int k, num = 30 ;, k = ( num > 5 ? ( num <= 10 ? 100 : 200 ) : 500 ) ;, printf ( "\n%d", num ) ;, }, (c), , main( ), {, int j = 4 ;, ( !j != 1 ? printf ( "\nWelcome") : printf ( "\nGood Bye") ) ;
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Chapter 2: The Decision Control Structure, , 93, , }, [H] Point out the errors, if any, in the following programs:, (a) main( ), {, int tag = 0, code = 1 ;, if ( tag == 0 ), ( code > 1 ? printf ( "\nHello" ) ? printf ( "\nHi" ) ) ;, else, printf ( "\nHello Hi !!" ) ;, }, (b) main( ), {, int ji = 65 ;, printf ( "\nji >= 65 ? %d : %c", ji ) ;, }, (c), , main( ), {, int i = 10, j ;, i >= 5 ? ( j = 10 ) : ( j = 15 ) ;, printf ( "\n%d %d", i, j ) ;, }, , (d) main( ), {, int a = 5 , b = 6 ;, ( a == b ? printf( "%d",a) ) ;, }, (e) main( ), {, int n = 9 ;, ( n == 9 ? printf( "You are correct" ) ; : printf( "You are wrong" ) ;) ;, }
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94, (f), , Let Us C, main( ), {, int kk = 65 ,ll ;, ll = ( kk == 65 : printf ( "\n kk is equal to 65" ) : printf ( "\n kk is not, equal to 65" ) ) ;, printf( "%d", ll ) ;, }, , (g) main( ), {, int x = 10, y = 20 ;, x == 20 && y != 10 ? printf( "True" ) : printf( "False" ) ;, }, [I] Rewrite the following programs using conditional operators., (a) main( ), {, int x, min, max ;, scanf ( "\n%d %d", &max, &x ) ;, if ( x > max ), max = x ;, else, min = x ;, }, (b), , main( ), {, int code ;, scanf ( "%d", &code ) ;, if ( code > 1 ), printf ( "\nJerusalem" ) ;, else, if ( code < 1 ), printf ( "\nEddie" ) ;, else, printf ( "\nC Brain" ) ;, }
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Chapter 2: The Decision Control Structure, (c), , 95, , main( ), {, float sal ;, printf ("Enter the salary" ) ;, scanf ( "%f", &sal ) ;, if ( sal < 40000 && sal > 25000 ), printf ( "Manager" ) ;, else, if ( sal < 25000 && sal > 15000 ), printf ( "Accountant" ) ;, else, printf ( "Clerk" ) ;, }, , [J] Attempt the following:, (a) Using conditional operators determine:, , (1) Whether the character entered through the keyboard is a, lower case alphabet or not., (2) Whether a character entered through the keyboard is a, special symbol or not., (b) Write a program using conditional operators to determine, , whether a year entered through the keyboard is a leap year or, not., (c) Write a program to find the greatest of the three numbers, , entered through the keyboard using conditional operators.
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96, , Let Us C
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• Loops, • The while Loop, Tips and Traps, More Operators, • The for Loop, Nesting of Loops, Multiple Initialisations in the for Loop, • The Odd Loop, • The break Statement, The continue Statement, • The do-while Loop, • Summary, • Exercise, , T, , he programs that we have developed so far used either a, sequential or a decision control instruction. In the first one,, the calculations were carried out in a fixed order, while in, the second, an appropriate set of instructions were executed, depending upon the outcome of the condition being tested (or a, logical decision being taken)., , 97
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98, , Let Us C, , These programs were of limited nature, because when executed,, they always performed the same series of actions, in the same way,, exactly once. Almost always, if something is worth doing, it’s, worth doing more than once. You can probably think of several, examples of this from real life, such as eating a good dinner or, going for a movie. Programming is the same; we frequently need, to perform an action over and over, often with variations in the, details each time. The mechanism, which meets this need, is the, ‘loop’, and loops are the subject of this chapter., , Loops, The versatility of the computer lies in its ability to perform a set of, instructions repeatedly. This involves repeating some portion of, the program either a specified number of times or until a particular, condition is being satisfied. This repetitive operation is done, through a loop control instruction., There are three methods by way of which we can repeat a part of a, program. They are:, (a) Using a for statement, (b) Using a while statement, (c) Using a do-while statement, Each of these methods is discussed in the following pages., , The while Loop, It is often the case in programming that you want to do something, a fixed number of times. Perhaps you want to calculate gross, salaries of ten different persons, or you want to convert, temperatures from centigrade to fahrenheit for 15 different cities.
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99, , Chapter 3: The Loop Control Structure, , The while loop is ideally suited for such cases. Let us look at a, simple example, which uses a while loop. The flowchart shown, below would help you to understand the operation of the while, loop., START, , count = 1, , is, count <= 3, , No, , Yes, STOP, INPUT, p, n, r, , si = p * n * r / 100, PRINT, si, count = count + 1, , Figure 3.1, /* Calculation of simple interest for 3 sets of p, n and r */, main( ), {, int p, n, count ;, float r, si ;, count = 1 ;
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100, , Let Us C, , while ( count <= 3 ), {, printf ( "\nEnter values of p, n and r " ) ;, scanf ( "%d %d %f", &p, &n, &r ) ;, si = p * n * r / 100 ;, printf ( "Simple interest = Rs. %f", si ) ;, count = count + 1 ;, }, }, , And here are a few sample runs..., Enter values of p, n and r 1000 5 13.5, Simple interest = Rs. 675.000000, Enter values of p, n and r 2000 5 13.5, Simple interest = Rs. 1350.000000, Enter values of p, n and r 3500 5 3.5, Simple interest = Rs. 612.500000, , The program executes all statements after the while 3 times. The, logic for calculating the simple interest is written within a pair of, braces immediately after the while keyword. These statements, form what is called the ‘body’ of the while loop. The parentheses, after the while contain a condition. So long as this condition, remains true all statements within the body of the while loop keep, getting executed repeatedly. To begin with the variable count is, initialized to 1 and every time the simple interest logic is executed, the value of count is incremented by one. The variable count is, many a times called either a ‘loop counter’ or an ‘index variable’., The operation of the while loop is illustrated in the following, figure.
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101, , Chapter 3: The Loop Control Structure, , START, , initialise, , False, , test, True, , STOP, , body of loop, , increment, , Figure 3.2, , Tips and Traps, The general form of while is as shown below:, initialise loop counter ;, while ( test loop counter using a condition ), {, do this ;, and this ;, increment loop counter ;, }, , Note the following points about while..., −, , The statements within the while loop would keep on getting, executed till the condition being tested remains true. When the
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102, , Let Us C, , condition becomes false, the control passes to the first, statement that follows the body of the while loop., In place of the condition there can be any other valid, expression. So long as the expression evaluates to a non-zero, value the statements within the loop would get executed., −, , The condition being tested may use relational or logical, operators as shown in the following examples:, while ( i <= 10 ), while ( i >= 10 && j <= 15 ), while ( j > 10 && ( b < 15 || c < 20 ) ), , −, , The statements within the loop may be a single line or a block, of statements. In the first case the parentheses are optional. For, example,, while ( i <= 10 ), i=i+1;, , is same as, while ( i <= 10 ), {, i=i+1;, }, , −, , As a rule the while must test a condition that will eventually, become false, otherwise the loop would be executed forever,, indefinitely., main( ), {, int i = 1 ;, while ( i <= 10 ), printf ( "%d\n", i ) ;, }
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Chapter 3: The Loop Control Structure, , 103, , This is an indefinite loop, since i remains equal to 1 forever., The correct form would be as under:, main( ), {, int i = 1 ;, while ( i <= 10 ), {, printf ( "%d\n", i ) ;, i=i+1;, }, }, , −, , Instead of incrementing a loop counter, we can even decrement, it and still manage to get the body of the loop executed, repeatedly. This is shown below:, main( ), {, int i = 5 ;, while ( i >= 1 ), {, printf ( "\nMake the computer literate!" ) ;, i=i-1;, }, }, , −, , It is not necessary that a loop counter must only be an int. It, can even be a float., main( ), {, float a = 10.0 ;, while ( a <= 10.5 ), {, printf ( "\nRaindrops on roses..." ) ;, printf ( "...and whiskers on kittens" ) ;, a = a + 0.1 ;
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104, , Let Us C, }, , }, , −, , Even floating point loop counters can be decremented. Once, again the increment and decrement could be by any value, not, necessarily 1., What do you think would be the output of the following, program?, main( ), {, int i = 1 ;, while ( i <= 32767 ), {, printf ( "%d\n", i ) ;, i=i+1;, }, }, , No, it doesn’t print numbers from 1 to 32767. It’s an, indefinite loop. To begin with, it prints out numbers from 1 to, 32767. After that value of i is incremented by 1, therefore it, tries to become 32768, which falls outside the valid integer, range, so it goes to other side and becomes -32768 which, would certainly satisfy the condition in the while. This, process goes on indefinitely., −, , What will be the output of the following program?, main( ), {, int i = 1 ;, while ( i <= 10 ) ;, {, printf ( "%d\n", i ) ;, i=i+1;, }, }
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Chapter 3: The Loop Control Structure, , 105, , This is another indefinite loop, and it doesn’t give any output, at all. The reason is, we have carelessly given a ; after the, while. This would make the loop work like this..., while ( i <= 10 ), ;, {, printf ( "%d\n", i ) ;, i=i+1;, }, , Since the value of i is not getting incremented the control, would keep rotating within the loop, eternally. Note that, enclosing printf( ) and i = i +1 within a pair of braces is not, an error. In fact we can put a pair of braces around any, individual statement or set of statements without affecting the, execution of the program., , More Operators, There are variety of operators which are frequently used with, while. To illustrate their usage let us consider a problem wherein, numbers from 1 to 10 are to be printed on the screen. The program, for performing this task can be written using while in the, following different ways:, (a) main( ), {, int i = 1 ;, while ( i <= 10 ), {, printf ( "%d\n", i ) ;, i=i+1;, }, }
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106, , Let Us C, , (b) main( ), {, int i = 1 ;, while ( i <= 10 ), {, printf ( "%d\n", i ) ;, i++ ;, }, }, , Note that the increment operator ++ increments the value of i, by 1, every time the statement i++ gets executed. Similarly, to, reduce the value of a variable by 1 a decrement operator -- is, also available., However, never use n+++ to increment the value of n by 2,, since C doesn’t recognize the operator +++., (c), , main( ), {, int i = 1 ;, while ( i <= 10 ), {, printf ( "%d\n", i ) ;, i += 1 ;, }, }, , Note that += is a compound assignment operator. It, increments the value of i by 1. Similarly, j = j + 10 can also, be written as j += 10. Other compound assignment operators, are -=, *=, / = and %=., (d) main( ), {, int i = 0 ;, while ( i++ < 10 )
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Chapter 3: The Loop Control Structure, , 107, , printf ( "%d\n", i ) ;, }, , In the statement while ( i++ < 10 ), firstly the comparison of, value of i with 10 is performed, and then the incrementation, of i takes place. Since the incrementation of i happens after its, usage, here the ++ operator is called a post-incrementation, operator. When the control reaches printf( ), i has already, been incremented, hence i must be initialized to 0., (e) main( ), {, int i = 0 ;, while ( ++i <= 10 ), printf ( "%d\n", i ) ;, }, , In the statement while ( ++i <= 10 ), firstly incrementation of, i takes place, then the comparison of value of i with 10 is, performed. Since the incrementation of i happens before its, usage, here the ++ operator is called a pre-incrementation, operator., , The for Loop, Perhaps one reason why few programmers use while is that they, are too busy using the for, which is probably the most popular, looping instruction. The for allows us to specify three things about, a loop in a single line:, (a) Setting a loop counter to an initial value., (b) Testing the loop counter to determine whether its value has, reached the number of repetitions desired., (c) Increasing the value of loop counter each time the program, segment within the loop has been executed.
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108, , Let Us C, , The general form of for statement is as under:, for ( initialise counter ; test counter ; increment counter ), {, do this ;, and this ;, and this ;, }, , Let us write down the simple interest program using for. Compare, this program with the one, which we wrote using while. The, flowchart is also given below for a better understanding.
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109, , Chapter 3: The Loop Control Structure, , START, , count INPU, =1 T, bs, , count = count + 1, , is, count <= 3, , No, , Yes, INPUT, p, n, r, , si = p * n * r / 100, , PRINT, si, , Figure 3.3, /* Calculation of simple interest for 3 sets of p, n and r */, main ( ), {, int p, n, count ;, float r, si ;, for ( count = 1 ; count <= 3 ; count = count + 1 ), {, printf ( "Enter values of p, n, and r " ) ;, scanf ( "%d %d %f", &p, &n, &r ) ;, si = p * n * r / 100 ;, printf ( "Simple Interest = Rs.%f\n", si ) ;, }, }, , STOP
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110, , Let Us C, , If this program is compared with the one written using while, it, can be seen that the three steps—initialization, testing and, incrementation—required for the loop construct have now been, incorporated in the for statement., Let us now examine how the for statement gets executed:, −, , When the for statement is executed for the first time, the value, of count is set to an initial value 1., , −, , Now the condition count <= 3 is tested. Since count is 1 the, condition is satisfied and the body of the loop is executed for, the first time., , −, , Upon reaching the closing brace of for, control is sent back to, the for statement, where the value of count gets incremented, by 1., , −, , Again the test is performed to check whether the new value of, count exceeds 3., , −, , If the value of count is still within the range 1 to 3, the, statements within the braces of for are executed again., , −, , The body of the for loop continues to get executed till count, doesn’t exceed the final value 3., , −, , When count reaches the value 4 the control exits from the loop, and is transferred to the statement (if any) immediately after, the body of for., , The following figure would help in further clarifying the concept, of execution of the for loop.
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111, , Chapter 3: The Loop Control Structure, , START, , initialise, , test, , False, , True, body of loop, , STOP, , increment, , Figure 3.4, It is important to note that the initialization, testing and, incrementation part of a for loop can be replaced by any valid, expression. Thus the following for loops are perfectly ok., for ( i = 10 ; i ; i -- ), printf ( "%d", i ) ;, for ( i < 4 ; j = 5 ; j = 0 ), printf ( "%d", i ) ;, for ( i = 1; i <=10 ; printf ( "%d",i++ ), ;, for ( scanf ( "%d", &i ) ; i <= 10 ; i++ ), printf ( "%d", i ) ;, , Let us now write down the program to print numbers from 1 to 10, in different ways. This time we would use a for loop instead of a, while loop.
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112, , Let Us C, , (a) main( ), {, int i ;, for ( i = 1 ; i <= 10 ; i = i + 1 ), printf ( "%d\n", i ) ;, }, , Note that the initialisation, testing and incrementation of loop, counter is done in the for statement itself. Instead of i = i + 1,, the statements i++ or i += 1 can also be used., Since there is only one statement in the body of the for loop,, the pair of braces have been dropped. As with the while, the, default scope of for is the immediately next statement after, for., (b) main( ), {, int i ;, for ( i = 1 ; i <= 10 ; ), {, printf ( "%d\n", i ) ;, i=i+1;, }, }, , Here, the incrementation is done within the body of the for, loop and not in the for statement. Note that inspite of this the, semicolon after the condition is necessary., (c), , main( ), {, int i = 1 ;, for ( ; i <= 10 ; i = i + 1 ), printf ( "%d\n", i ) ;, }
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Chapter 3: The Loop Control Structure, , 113, , Here the initialisation is done in the declaration statement, itself, but still the semicolon before the condition is necessary., (d) main( ), {, int i = 1 ;, for ( ; i <= 10 ; ), {, printf ( "%d\n", i ) ;, i=i+1;, }, }, , Here, neither the initialisation, nor the incrementation is done, in the for statement, but still the two semicolons are, necessary., (e) main( ), {, int i ;, for ( i = 0 ; i++ < 10 ; ), printf ( "%d\n", i ) ;, }, , Here, the comparison as well as the incrementation is done, through the same statement, i++ < 10. Since the ++ operator, comes after i firstly comparison is done, followed by, incrementation. Note that it is necessary to initialize i to 0., (f), , main( ), {, int i ;, for ( i = 0 ; ++i <= 10 ; ), printf ( "%d\n", i ) ;, }
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114, , Let Us C, , Here, both, the comparison and the incrementation is done, through the same statement, ++i <= 10. Since ++ precedes i, firstly incrementation is done, followed by comparison. Note, that it is necessary to initialize i to 0., , Nesting of Loops, The way if statements can be nested, similarly whiles and fors can, also be nested. To understand how nested loops work, look at the, program given below:, /* Demonstration of nested loops */, main( ), {, int r, c, sum ;, for ( r = 1 ; r <= 3 ; r++ ) /* outer loop */, {, for ( c = 1 ; c <= 2 ; c++ ) /* inner loop */, {, sum = r + c ;, printf ( "r = %d c = %d sum = %d\n", r, c, sum ) ;, }, }, }, , When you run this program you will get the following output:, r = 1 c = 1 sum = 2, r = 1 c = 2 sum = 3, r = 2 c = 1 sum = 3, r = 2 c = 2 sum = 4, r = 3 c = 1 sum = 4, r = 3 c = 2 sum = 5, , Here, for each value of r the inner loop is cycled through twice,, with the variable c taking values from 1 to 2. The inner loop
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Chapter 3: The Loop Control Structure, , 115, , terminates when the value of c exceeds 2, and the outer loop, terminates when the value of r exceeds 3., As you can see, the body of the outer for loop is indented, and the, body of the inner for loop is further indented. These multiple, indentations make the program easier to understand., Instead of using two statements, one to calculate sum and another, to print it out, we can compact this into one single statement by, saying:, printf ( "r = %d c = %d sum = %d\n", r, c, r + c ) ;, , The way for loops have been nested here, similarly, two while, loops can also be nested. Not only this, a for loop can occur within, a while loop, or a while within a for., , Multiple Initialisations in the for Loop, The initialisation expression of the for loop can contain more than, one statement separated by a comma. For example,, for ( i = 1, j = 2 ; j <= 10 ; j++ ), , Multiple statements can also be used in the incrementation, expression of for loop; i.e., you can increment (or decrement) two, or more variables at the same time. However, only one expression, is allowed in the test expression. This expression may contain, several conditions linked together using logical operators., Use of multiple statements in the initialisation expression also, demonstrates why semicolons are used to separate the three, expressions in the for loop. If commas had been used, they could, not also have been used to separate multiple statements in the, initialisation expression, without confusing the compiler.
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116, , Let Us C, , The Odd Loop, The loops that we have used so far executed the statements within, them a finite number of times. However, in real life programming, one comes across a situation when it is not known beforehand how, many times the statements in the loop are to be executed. This, situation can be programmed as shown below:, /* Execution of a loop an unknown number of times */, main( ), {, char another ;, int num ;, do, {, printf ( "Enter a number " ) ;, scanf ( "%d", &num ) ;, printf ( "square of %d is %d", num, num * num ) ;, printf ( "\nWant to enter another number y/n " ) ;, scanf ( " %c", &another ) ;, } while ( another == 'y' ) ;, }, , And here is the sample output..., Enter a number 5, square of 5 is 25, Want to enter another number y/n y, Enter a number 7, square of 7 is 49, Want to enter another number y/n n, , In this program the do-while loop would keep getting executed till, the user continues to answer y. The moment he answers n, the loop, terminates, since the condition ( another == 'y' ) fails. Note that, this loop ensures that statements within it are executed at least, once even if n is supplied first time itself.
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Chapter 3: The Loop Control Structure, , 117, , Though it is simpler to program such a requirement using a dowhile loop, the same functionality if required, can also be, accomplished using for and while loops as shown below:, /* odd loop using a for loop */, main( ), {, char another = 'y' ;, int num ;, for ( ; another == 'y' ; ), {, printf ( "Enter a number " ) ;, scanf ( "%d", &num ) ;, printf ( "square of %d is %d", num, num * num ) ;, printf ( "\nWant to enter another number y/n " ) ;, scanf ( " %c", &another ) ;, }, }, /* odd loop using a while loop */, main( ), {, char another = 'y' ;, int num ;, while ( another == 'y' ), {, printf ( "Enter a number " ) ;, scanf ( "%d", &num ) ;, printf ( "square of %d is %d", num, num * num ) ;, printf ( "\nWant to enter another number y/n " ) ;, scanf ( " %c", &another ) ;, }, }
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118, , Let Us C, , The break Statement, We often come across situations where we want to jump out of a, loop instantly, without waiting to get back to the conditional test., The keyword break allows us to do this. When break is, encountered inside any loop, control automatically passes to the, first statement after the loop. A break is usually associated with an, if. As an example, let’s consider the following example., Example: Write a program to determine whether a number is, prime or not. A prime number is one, which is divisible only by 1, or itself., All we have to do to test whether a number is prime or not, is to, divide it successively by all numbers from 2 to one less than itself., If remainder of any of these divisions is zero, the number is not a, prime. If no division yields a zero then the number is a prime, number. Following program implements this logic., main( ), {, int num, i ;, printf ( "Enter a number " ) ;, scanf ( "%d", &num ) ;, i=2;, while ( i <= num - 1 ), {, if ( num % i == 0 ), {, printf ( "Not a prime number" ) ;, break ;, }, i++ ;, }
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Chapter 3: The Loop Control Structure, , 119, , if ( i == num ), printf ( "Prime number" ) ;, }, , In this program the moment num % i turns out to be zero, (i.e., num is exactly divisible by i) the message “Not a prime number”, is printed and the control breaks out of the while loop. Why does, the program require the if statement after the while loop at all?, Well, there are two ways the control could have reached outside, the while loop:, (a) It jumped out because the number proved to be not a prime., (b) The loop came to an end because the value of i became equal, to num., When the loop terminates in the second case, it means that there, was no number between 2 to num - 1 that could exactly divide, num. That is, num is indeed a prime. If this is true, the program, should print out the message “Prime number”., The keyword break, breaks the control only from the while in, which it is placed. Consider the following program, which, illustrates this fact., main( ), {, int i = 1 , j = 1 ;, while ( i++ <= 100 ), {, while ( j++ <= 200 ), {, if ( j == 150 ), break ;, else, printf ( "%d %d\n", i, j ) ;, }
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120, , Let Us C, , }, }, , In this program when j equals 150, break takes the control outside, the inner while only, since it is placed inside the inner while., , The continue Statement, In some programming situations we want to take the control to the, beginning of the loop, bypassing the statements inside the loop,, which have not yet been executed. The keyword continue allows, us to do this. When continue is encountered inside any loop,, control automatically passes to the beginning of the loop., A continue is usually associated with an if. As an example, let's, consider the following program., main( ), {, int i, j ;, for ( i = 1 ; i <= 2 ; i++ ), {, for ( j = 1 ; j <= 2 ; j++ ), {, if ( i == j ), continue ;, printf ( "\n%d %d\n", i, j ) ;, }, }, }, , The output of the above program would be..., 12, 21
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Chapter 3: The Loop Control Structure, , 121, , Note that when the value of i equals that of j, the continue, statement takes the control to the for loop (inner) bypassing rest of, the statements pending execution in the for loop (inner)., , The do-while Loop, The do-while loop looks like this:, do, {, this ;, and this ;, and this ;, and this ;, } while ( this condition is true ) ;, , There is a minor difference between the working of while and dowhile loops. This difference is the place where the condition is, tested. The while tests the condition before executing any of the, statements within the while loop. As against this, the do-while, tests the condition after having executed the statements within the, loop. Figure 3.5 would clarify the execution of do-while loop still, further.
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122, , Let Us C, , START, initialise, , body of loop, , increment, , True, , test, False, STOP, , Figure 3.5, This means that do-while would execute its statements at least, once, even if the condition fails for the first time. The while, on, the other hand will not execute its statements if the condition fails, for the first time. This difference is brought about more clearly by, the following program., main( ), {, while ( 4 < 1 ), printf ( "Hello there \n") ;, }
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Chapter 3: The Loop Control Structure, , 123, , Here, since the condition fails the first time itself, the printf( ) will, not get executed at all. Let's now write the same program using a, do-while loop., main( ), {, do, {, printf ( "Hello there \n") ;, } while ( 4 < 1 ) ;, }, , In this program the printf( ) would be executed once, since first, the body of the loop is executed and then the condition is tested., There are some occasions when we want to execute a loop at least, once no matter what. This is illustrated in the following example:, break and continue are used with do-while just as they would be, in a while or a for loop. A break takes you out of the do-while, bypassing the conditional test. A continue sends you straight to, the test at the end of the loop.
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124, , Let Us C, , Summary, (a) The three type of loops available in C are for, while, and dowhile., (b) A break statement takes the execution control out of the loop., (c) A continue statement skips the execution of the statements, after it and takes the control to the beginning of the loop., (d) A do-while loop is used to ensure that the statements within, the loop are executed at least once., (e) The ++ operator increments the operand by 1, whereas, the -operator decrements it by 1., (f) The operators +=, -=, *=, /=, %= are compound assignment, operators. They modify the value of the operand to the left of, them., , Exercise, while Loop, [A] What would be the output of the following programs:, (a) main( ), {, int j ;, while ( j <= 10 ), {, printf ( "\n%d", j ) ;, j=j+1;, }, }, (b) main( ), {, int i = 1 ;, while ( i <= 10 ) ;, {, printf ( "\n%d", i ) ;
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Chapter 3: The Loop Control Structure, i++ ;, }, }, , (c), , main( ), {, int j ;, while ( j <= 10 ), {, printf ( "\n%d", j ) ;, j=j+1;, }, }, , (d) main( ), {, int x = 1 ;, while ( x == 1 ), {, x=x-1;, printf ( "\n%d", x ) ;, }, }, (e) main( ), {, int x = 1 ;, while ( x == 1 ), x=x-1;, printf ( "\n%d", x ) ;, }, (f), , main( ), {, char x ;, , 125
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126, , Let Us C, while ( x = 0 ; x <= 255 ; x++ ), printf ( "\nAscii value %d Character %c", x, x ) ;, , }, (g) main( ), {, int x = 4, y, z ;, y = --x ;, z = x-- ;, printf ( "\n%d %d %d", x, y, z ) ;, }, (h) main( ), {, int x = 4, y = 3, z ;, z = x-- -y ;, printf ( "\n%d %d %d", x, y, z ) ;, }, (i), , main( ), {, while ( 'a' < 'b' ), printf ( "\nmalyalam is a palindrome" ) ;, }, , (j), , main( ), {, int i = 10 ;, while ( i = 20 ), printf ( "\nA computer buff!" ) ;, }, , (k), , main( ), {, int i ;, while ( i = 10 ), {
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Chapter 3: The Loop Control Structure, printf ( "\n%d", i ) ;, i=i+1;, }, }, , (l), , main( ), {, float x = 1.1 ;, while ( x == 1.1 ), {, printf ( "\n%f", x ) ;, x = x – 0.1 ;, }, }, , (m) main( ), {, while ( '1' < '2' ), printf ( "\nIn while loop" ) ;, }, (n) main( ), {, char x ;, for ( x = 0 ; x <= 255 ; x++ ), printf ( "\nAscii value %d Character %c", x, x ) ;, }, (o) main( ), {, int x = 4, y = 0, z ;, while ( x >= 0 ), {, x-- ;, y++ ;, if ( x == y ), , 127
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128, , Let Us C, continue ;, else, printf ( “\n%d %d”, x, y ) ;, }, , }, (p) main( ), {, int x = 4, y = 0, z ;, while ( x >= 0 ), {, if ( x == y ), break ;, else, printf ( “\n%d %d”, x, y ) ;, x-- ;, y++ ;, }, }, [B] Attempt the following:, , (a) Write a program to calculate overtime pay of 10 employees., Overtime is paid at the rate of Rs. 12.00 per hour for every, hour worked above 40 hours. Assume that employees do not, work for fractional part of an hour., (b) Write a program to find the factorial value of any number, entered through the keyboard., (c) Two numbers are entered through the keyboard. Write a, program to find the value of one number raised to the power, of another., (d) Write a program to print all the ASCII values and their, equivalent characters using a while loop. The ASCII values, vary from 0 to 255.
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Chapter 3: The Loop Control Structure, , 129, , (e) Write a program to print out all Armstrong numbers between, 1 and 500. If sum of cubes of each digit of the number is, equal to the number itself, then the number is called an, Armstrong number. For example, 153 = ( 1 * 1 * 1 ) + ( 5 * 5, *5)+(3*3*3), (f) Write a program for a matchstick game being played between, the computer and a user. Your program should ensure that the, computer always wins. Rules for the game are as follows:, −, −, −, −, , There are 21 matchsticks., The computer asks the player to pick 1, 2, 3, or 4, matchsticks., After the person picks, the computer does its, picking., Whoever is forced to pick up the last matchstick, loses the game., , (g) Write a program to enter the numbers till the user wants and, at the end it should display the count of positive, negative and, zeros entered., (h) Write a program to find the octal equivalent of the entered, number., (i) Write a program to find the range of a set of numbers. Range, is the difference between the smallest and biggest number in, the list., , for, break, continue, do-while, [C] What would be the output of the following programs:, (a) main( ), {, int i = 0 ;, for ( ; i ; )
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130, , Let Us C, printf ( "\nHere is some mail for you" ) ;, , }, (b) main( ), {, int i ;, for ( i = 1 ; i <= 5 ; printf ( "\n%d", i ) ) ;, i++ ;, }, (c), , main( ), {, int i = 1, j = 1 ;, for ( ; ; ), {, if ( i > 5 ), break ;, else, j += i ;, printf ( "\n%d", j ) ;, i += j ;, }, }, , (d) main( ), {, int i ;, for ( i = 1 ; i <= 5 ; printf ( "\n%c", 65 ) ) ;, i++ ;, }, [D] Answer the following:, (a) The three parts of the loop expression in the for loop are:, , the i____________ expression, the t____________ expression, the i____________ expression
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Chapter 3: The Loop Control Structure, , 131, , (b) An expression contains relational operators, assignment, , operators, and arithmetic operators. In the absence of, parentheses, they will be evaluated in which of the following, order:, 1., 2., 3., 4., , assignment, relational, arithmetic, arithmetic, relational, assignment, relational, arithmetic, assignment, assignment, arithmetic, relational, , (c) The break statement is used to exit from:, , 1., 2., 3., 4., , an if statement, a for loop, a program, the main( ) function, , (d) A do-while loop is useful when we want that the statements, , within the loop must be executed:, 1., 2., 3., 4., , Only once, At least once, More than once, None of the above, , (e) In what sequence the initialization, testing and execution of, , body is done in a do-while loop, 1., 2., 3., 4., (f), , Initialization, execution of body, testing, Execution of body, initialization, testing, Initialization, testing, execution of body, None of the above, , Which of the following is not an infinite loop., 1. int i = 1 ;, while ( 1 ), {, i++ ;, }, , 2. for ( ; ; ) ;
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132, , Let Us C, , 3. int True = 0, false ;, while ( True ), {, False = 1 ;, }, , 4. int y, x = 0 ;, do, {, y=x;, } while ( x == 0 ) ;, , (g) Which of the following statement is used to take the control to, , the beginning of the loop?, 1., 2., 3., 4., , exit, break, continue, None of the above, , [E] Attempt the following:, (a) Write a program to print all prime numbers from 1 to 300., , (Hint: Use nested loops, break and continue), (b) Write a program to fill the entire screen with a smiling face., , The smiling face has an ASCII value 1., (c) Write a program to add first seven terms of the following, , series using a for loop:, 1, 1!, , 2, 2!, , 3, 3!, , ……, , (d) Write a program to generate all combinations of 1, 2 and 3, , using for loop., (e) According to a study, the approximate level of intelligence of, , a person can be calculated using the following formula:, i = 2 + ( y + 0.5 x )
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Chapter 3: The Loop Control Structure, , 133, , Write a program, which will produce a table of values of i, y, and x, where y varies from 1 to 6, and, for each value of y, x, varies from 5.5 to 12.5 in steps of 0.5., (f), , Write a program to produce the following output:, A, A, A, A, A, A, A, , B, B, B, B, B, B, , C, C, C, C, C, , D E F G F E D, D E F, F E D, D E, E D, D, D, , C, C, C, C, C, , B, B, B, B, B, B, , A, A, A, A, A, A, A, , (g) Write a program to fill the entire screen with diamond and, , heart alternatively. The ASCII value for heart is 3 and that of, diamond is 4., (h) Write a program to print the multiplication table of the, , number entered by the user. The table should get displayed in, the following form., 29 * 1 = 29, 29 * 2 = 58, …, (i), , Write a program to produce the following output:, 1, 2, 4, 7, , 3, 5, , 8, , 6, 9, , 10
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134, (j), , Let Us C, , Write a program to produce the following output:, 1, 1, , 1, , 1, 1, 1, , 2, 3, , 1, 3, , 4, , 6, , 1, 4, , 1, , (k) A machine is purchased which will produce earning of Rs., , 1000 per year while it lasts. The machine costs Rs. 6000 and, will have a salvage of Rs. 2000 when it is condemned. If 12, percent per annum can be earned on alternate investments, what would be the minimum life of the machine to make it a, more attractive investment compared to alternative, investment?, (l), , When interest compounds q times per year at an annual rate of, r % for n years, the principle p compounds to an amount a as per, the following formula, a = p ( 1 + r / q ) nq, , Write a program to read 10 sets of p, r, n & q and calculate, the corresponding as., (m) The natural logarithm can be approximated by the following, , series., , x −1 1 ⎛ x −1 ⎞ 1 ⎛ x −1⎞ 1 ⎛ x −1⎞, + ⎜, ⎟+ ⎜, ⎟+ ⎜, ⎟ + ...., 2⎝ x ⎠ 2⎝ x ⎠ 2⎝ x ⎠, x, 2, , 3, , 4, , If x is input through the keyboard, write a program to, calculate the sum of first seven terms of this series.
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4, , The Case Control, Structure, • Decisions Using switch, The Tips and Traps, • switch Versus if-else Ladder, • The goto Keyword, • Summary, • Exercise, , 135
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136, , Let Us C, , I, , n real life we are often faced with situations where we are, required to make a choice between a number of alternatives, rather than only one or two. For example, which school to join, or which hotel to visit or still harder which girl to marry (you, almost always end up making a wrong decision is a different, matter altogether!). Serious C programming is same; the choice we, are asked to make is more complicated than merely selecting, between two alternatives. C provides a special control statement, that allows us to handle such cases effectively; rather than using a, series of if statements. This control instruction is in fact the topic, of this chapter. Towards the end of the chapter we would also, study a keyword called goto, and understand why we should avoid, its usage in C programming., , Decisions Using switch, The control statement that allows us to make a decision from the, number of choices is called a switch, or more correctly a switchcase-default, since these three keywords go together to make up, the control statement. They most often appear as follows:, switch ( integer expression ), {, case constant 1 :, do this ;, case constant 2 :, do this ;, case constant 3 :, do this ;, default :, do this ;, }, , The integer expression following the keyword switch is any C, expression that will yield an integer value. It could be an integer, constant like 1, 2 or 3, or an expression that evaluates to an
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Chapter 4: The Case Control Structure, , 137, , integer. The keyword case is followed by an integer or a character, constant. Each constant in each case must be different from all the, others. The “do this” lines in the above form of switch represent, any valid C statement., What happens when we run a program containing a switch? First,, the integer expression following the keyword switch is evaluated., The value it gives is then matched, one by one, against the, constant values that follow the case statements. When a match is, found, the program executes the statements following that case,, and all subsequent case and default statements as well. If no, match is found with any of the case statements, only the, statements following the default are executed. A few examples, will show how this control structure works., Consider the following program:, main( ), {, int i = 2 ;, switch ( i ), {, case 1 :, printf ( "I am in case 1 \n" ) ;, case 2 :, printf ( "I am in case 2 \n" ) ;, case 3 :, printf ( "I am in case 3 \n" ) ;, default :, printf ( "I am in default \n" ) ;, }, }, , The output of this program would be:, I am in case 2
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138, , Let Us C, , I am in case 3, I am in default, , The output is definitely not what we expected! We didn’t expect, the second and third line in the above output. The program prints, case 2 and 3 and the default case. Well, yes. We said the switch, executes the case where a match is found and all the subsequent, cases and the default as well., If you want that only case 2 should get executed, it is upto you to, get out of the switch then and there by using a break statement., The following example shows how this is done. Note that there is, no need for a break statement after the default, since the control, comes out of the switch anyway., main( ), {, int i = 2 ;, switch ( i ), {, case 1 :, printf ( "I am in case 1 \n" ) ;, break ;, case 2 :, printf ( "I am in case 2 \n" ) ;, break ;, case 3 :, printf ( "I am in case 3 \n" ) ;, break ;, default :, printf ( "I am in default \n" ) ;, }, }, , The output of this program would be:, I am in case 2
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Chapter 4: The Case Control Structure, , 139, , The operation of switch is shown below in the form of a flowchart, for a better understanding., START, , case 1, , Yes, , statement 1, , No, case 2, , Yes, , statement 2, , No, case 3, , Yes, , statement 3, , No, case 4, , Yes, , statement 4, , No, , STOP, switch ( choice ), {, case 1 :, statement 1 ;, break ;, case 2 :, statement 2 ;, break ;, case 3 :, statement 3 ;, break ;, case 4 :, statement 4 ;, }, , Figure 4.1
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140, , Let Us C, , The Tips and Traps, A few useful tips about the usage of switch and a few pitfalls to be, avoided:, (a) The earlier program that used switch may give you the wrong, impression that you can use only cases arranged in ascending, order, 1, 2, 3 and default. You can in fact put the cases in any, order you please. Here is an example of scrambled case order:, main( ), {, int i = 22 ;, switch ( i ), {, case 121 :, printf ( "I am in case 121 \n" ) ;, break ;, case 7 :, printf ( "I am in case 7 \n" ) ;, break ;, case 22 :, printf ( "I am in case 22 \n" ) ;, break ;, default :, printf ( "I am in default \n" ) ;, }, }, , The output of this program would be:, I am in case 22, , (b) You are also allowed to use char values in case and switch as, shown in the following program:, main( )
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Chapter 4: The Case Control Structure, , 141, , {, char c = 'x' ;, switch ( c ), {, case 'v' :, printf ( "I am in case v \n" ) ;, break ;, case 'a' :, printf ( "I am in case a \n" ) ;, break ;, case 'x' :, printf ( "I am in case x \n" ) ;, break ;, default :, printf ( "I am in default \n" ) ;, }, }, , The output of this program would be:, I am in case x, , In fact here when we use ‘v’, ‘a’, ‘x’ they are actually, replaced by the ASCII values (118, 97, 120) of these character, constants., (c) At times we may want to execute a common set of statements, for multiple cases. How this can be done is shown in the, following example., main( ), {, char ch ;, printf ( "Enter any of the alphabet a, b, or c " ) ;, scanf ( "%c", &ch ) ;
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142, , Let Us C, switch ( ch ), {, case 'a' :, case 'A' :, printf ( "a as in ashar" ) ;, break ;, case 'b' :, case 'B' :, printf ( "b as in brain" ) ;, break ;, case 'c' :, case 'C' :, printf ( "c as in cookie" ) ;, break ;, default :, printf ( "wish you knew what are alphabets" ) ;, }, , }, , Here, we are making use of the fact that once a case is, satisfied the control simply falls through the case till it, doesn’t encounter a break statement. That is why if an, alphabet a is entered the case ‘a’ is satisfied and since there, are no statements to be executed in this case the control, automatically reaches the next case i.e. case ‘A’ and executes, all the statements in this case., (d) Even if there are multiple statements to be executed in each, case there is no need to enclose them within a pair of braces, (unlike if, and else)., (e) Every statement in a switch must belong to some case or the, other. If a statement doesn’t belong to any case the compiler, won’t report an error. However, the statement would never get, executed. For example, in the following program the printf( ), never goes to work.
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Chapter 4: The Case Control Structure, , 143, , main( ), {, int i, j ;, printf ( "Enter value of i" ) ;, scanf ( "%d”, &i ) ;, switch ( i ), {, printf ( "Hello" ) ;, case 1 :, j = 10 ;, break ;, case 2 :, j = 20 ;, break ;, }, }, , (f) If we have no default case, then the program simply falls, through the entire switch and continues with the next, instruction (if any,) that follows the closing brace of switch., (g) Is switch a replacement for if? Yes and no. Yes, because it, offers a better way of writing programs as compared to if, and, no because in certain situations we are left with no choice but, to use if. The disadvantage of switch is that one cannot have a, case in a switch which looks like:, case i <= 20 :, , All that we can have after the case is an int constant or a char, constant or an expression that evaluates to one of these, constants. Even a float is not allowed., The advantage of switch over if is that it leads to a more, structured program and the level of indentation is manageable,
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144, , Let Us C, , more so if there are multiple statements within each case of a, switch., (h) We can check the value of any expression in a switch. Thus, the following switch statements are legal., switch ( i + j * k ), switch ( 23 + 45 % 4 * k ), switch ( a < 4 && b > 7 ), , Expressions can also be used in cases provided they are, constant expressions. Thus case 3 + 7 is correct, however,, case a + b is incorrect., (i) The break statement when used in a switch takes the control, outside the switch. However, use of continue will not take, the control to the beginning of switch as one is likely to, believe., (j) In principle, a switch may occur within another, but in, practice it is rarely done. Such statements would be called, nested switch statements., (k) The switch statement is very useful while writing menu, driven programs. This aspect of switch is discussed in the, exercise at the end of this chapter., , switch Versus if-else Ladder, There are some things that you simply cannot do with a switch., These are:, (a) A float expression cannot be tested using a switch, (b) Cases can never have variable expressions (for example it is, wrong to say case a +3 : ), (c) Multiple cases cannot use same expressions. Thus the, following switch is illegal:
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Chapter 4: The Case Control Structure, , 145, , switch ( a ), {, case 3 :, ..., case 1 + 2 :, ..., }, , (a), (b) and (c) above may lead you to believe that these are, obvious disadvantages with a switch, especially since there, weren’t any such limitations with if-else. Then why use a switch at, all? For speed—switch works faster than an equivalent if-else, ladder. How come? This is because the compiler generates a jump, table for a switch during compilation. As a result, during, execution it simply refers the jump table to decide which case, should be executed, rather than actually checking which case is, satisfied. As against this, if-elses are slower because they are, evaluated at execution time. A switch with 10 cases would work, faster than an equivalent if-else ladder. Also, a switch with 2 cases, would work slower than if-else ladder. Why? If the 10th case is, satisfied then jump table would be referred and statements for the, 10th case would be executed. As against this, in an if-else ladder 10, conditions would be evaluated at execution time, which makes it, slow. Note that a lookup in the jump table is faster than evaluation, of a condition, especially if the condition is complex., If on the other hand the conditions in the if-else were simple and, less in number then if-else would work out faster than the lookup, mechanism of a switch. Hence a switch with two cases would, work slower than an equivalent if-else. Thus, you as a programmer, should take a decision which of the two should be used when., , The goto Keyword, Avoid goto keyword! They make a C programmer’s life miserable., There is seldom a legitimate reason for using goto, and its use is
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146, , Let Us C, , one of the reasons that programs become unreliable, unreadable,, and hard to debug. And yet many programmers find goto, seductive., In a difficult programming situation it seems so easy to use a goto, to take the control where you want. However, almost always, there, is a more elegant way of writing the same program using if, for,, while and switch. These constructs are far more logical and easy, to understand., The big problem with gotos is that when we do use them we can, never be sure how we got to a certain point in our code. They, obscure the flow of control. So as far as possible skip them. You, can always get the job done without them. Trust me, with good, programming skills goto can always be avoided. This is the first, and last time that we are going to use goto in this book. However,, for sake of completeness of the book, the following program, shows how to use goto., main( ), {, int goals ;, printf ( "Enter the number of goals scored against India" ) ;, scanf ( "%d", &goals ) ;, if ( goals <= 5 ), goto sos ;, else, {, printf ( "About time soccer players learnt C\n" ) ;, printf ( "and said goodbye! adieu! to soccer" ) ;, exit( ) ; /* terminates program execution */, }, sos :, printf ( "To err is human!" ) ;
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Chapter 4: The Case Control Structure, , 147, , }, , And here are two sample runs of the program..., Enter the number of goals scored against India 3, To err is human!, Enter the number of goals scored against India 7, About time soccer players learnt C, and said goodbye! adieu! to soccer, , A few remarks about the program would make the things clearer., −, , If the condition is satisfied the goto statement transfers control, to the label ‘sos’, causing printf( ) following sos to be, executed., , −, , The label can be on a separate line or on the same line as the, statement following it, as in,, sos : printf ( "To err is human!" ) ;, , −, , Any number of gotos can take the control to the same label., , −, , The exit( ) function is a standard library function which, terminates the execution of the program. It is necessary to use, this function since we don't want the statement, printf ( "To err is human!" ), , to get executed after execution of the else block., −, , The only programming situation in favour of using goto is, when we want to take the control out of the loop that is, contained in several other loops. The following program, illustrates this.
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148, , Let Us C, , main( ), {, int i, j, k ;, for ( i = 1 ; i <= 3 ; i++ ), {, for ( j = 1 ; j <= 3 ; j++ ), {, for ( k = 1 ; k <= 3 ; k++ ), {, if ( i == 3 && j == 3 && k == 3 ), goto out ;, else, printf ( "%d %d %d\n", i, j, k ) ;, }, }, }, out :, printf ( "Out of the loop at last!" ) ;, }, , Go through the program carefully and find out how it works. Also, write down the same program without using goto., , Summary, (a) When we need to choose one among number of alternatives, a, switch statement is used., (b) The switch keyword is followed by an integer or an, expression that evaluates to an integer., (c) The case keyword is followed by an integer or a character, constant., (d) The control falls through all the cases unless the break, statement is given., (e) The usage of the goto keyword should be avoided as it usually, violets the normal flow of execution.
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Chapter 4: The Case Control Structure, , Exercise, [A] What would be the output of the following programs:, (a) main( ), {, char suite = 3 ;, switch ( suite ), {, case 1 :, printf ( "\nDiamond" ) ;, case 2 :, printf ( "\nSpade" ) ;, default :, printf ( "\nHeart") ;, }, printf ( "\nI thought one wears a suite" ) ;, }, (b) main( ), {, int c = 3 ;, switch ( c ), {, case 'v' :, printf ( "I am in case v \n" ) ;, break ;, case 3 :, printf ( "I am in case 3 \n" ) ;, break ;, case 12 :, printf ( "I am in case 12 \n" ) ;, break ;, default :, printf ( "I am in default \n" ) ;, }, , 149
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152, , Let Us C, }, , }, [B] Point out the errors, if any, in the following programs:, (a) main( ), {, int suite = 1 ;, switch ( suite ) ;, {, case 0 ;, printf ( "\nClub" ) ;, case 1 ;, printf ( "\nDiamond" ) ;, }, }, (b) main( ), {, int temp ;, scanf ( "%d", &temp ) ;, switch ( temp ), {, case ( temp <= 20 ) :, printf ( "\nOoooooohhhh! Damn cool!" ) ;, case ( temp > 20 && temp <= 30 ) :, printf ( "\nRain rain here again!" ) ;, case ( temp > 30 && temp <= 40 ) :, printf ( "\nWish I am on Everest" ) ;, default :, printf ( "\nGood old nagpur weather" ) ;, }, }, (c), , main( ), {, float a = 3.5 ;, switch ( a )
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Chapter 4: The Case Control Structure, , 153, , {, case 0.5 :, printf ( "\nThe art of C" ) ;, break ;, case 1.5 :, printf ( "\nThe spirit of C" ) ;, break ;, case 2.5 :, printf ( "\nSee through C" ) ;, break ;, case 3.5 :, printf ( "\nSimply c" ) ;, }, }, (d) main( ), {, int a = 3, b = 4, c ;, c=b–a;, switch ( c ), {, case 1 || 2 :, printf ( "God give me an opportunity to change things" ) ;, break ;, case a || b :, printf ( "God give me an opportunity to run my show" ) ;, break ;, }, }, [C] Write a menu driven program which has following options:, , 1., 2., 3., 4., , Factorial of a number., Prime or not, Odd or even, Exit
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154, , Let Us C, , Make use of switch statement., The outline of this program is given below:, /* A menu driven program */, main( ), {, int choice ;, while ( 1 ), {, printf ( "\n1. Factorial" ) ;, printf ( "\n2. Prime" ) ;, printf ( "\n3. Odd/Even" ) ;, printf ( "\n4. Exit" ) ;, printf ( "\nYour choice? " ) ;, scanf ( "%d", &choice ) ;, switch ( choice ), {, case 1 :, /* logic for factorial of a number */, break ;, case 2 :, /* logic for deciding prime number */, break ;, case 3 :, /* logic for odd/even */, break ;, case 4 :, exit( ) ;, }, }, }, , Note:
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Chapter 4: The Case Control Structure, , 155, , The statement while ( 1 ) puts the entire logic in an infinite loop., This is necessary since the menu must keep reappearing on the, screen once an item is selected and an appropriate action taken., [D] Write a program which to find the grace marks for a student, using switch. The user should enter the class obtained by the, student and the number of subjects he has failed in., , −, , If the student gets first class and the number of subjects he, failed in is greater than 3, then he does not get any grace., If the number of subjects he failed in is less than or equal, to 3 then the grace is of 5 marks per subject., , −, , If the student gets second class and the number of subjects, he failed in is greater than 2, then he does not get any, grace. If the number of subjects he failed in is less than or, equal to 2 then the grace is of 4 marks per subject., , −, , If the student gets third class and the number of subjects, he failed in is greater than 1, then he does not get any, grace. If the number of subjects he failed in is equal to 1, then the grace is of 5 marks per subject
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156, , Let Us C
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5, , Functions &, Pointers, • What is a Function, Why Use Functions, • Passing Values between Functions, • Scope Rule of Functions, • Calling Convention, • One Dicey Issue, • Advanced Features of Functions, Function Declaration and Prototypes, Call by Value and Call by Reference, An Introduction to Pointers, Pointer Notation, Back to Function Calls, Conclusions, Recursion, • Adding Functions to the Library, • Summary, • Exercise, , 157
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158, , Let Us C, , K, , nowingly or unknowingly we rely on so many persons for, so many things. Man is an intelligent species, but still, cannot perform all of life’s tasks all alone. He has to rely, on others. You may call a mechanic to fix up your bike, hire a, gardener to mow your lawn, or rely on a store to supply you, groceries every month. A computer program (except for the, simplest one) finds itself in a similar situation. It cannot handle all, the tasks by itself. Instead, it requests other program like, entities—called ‘functions’ in C—to get its tasks done. In this, chapter we will study these functions. We will look at a variety of, features of these functions, starting with the simplest one and then, working towards those that demonstrate the power of C functions., , What is a Function, A function is a self-contained block of statements that perform a, coherent task of some kind. Every C program can be thought of as, a collection of these functions. As we noted earlier, using a, function is something like hiring a person to do a specific job for, you. Sometimes the interaction with this person is very simple;, sometimes it’s complex., Suppose you have a task that is always performed exactly in the, same way—say a bimonthly servicing of your motorbike. When, you want it to be done, you go to the service station and say, “It’s, time, do it now”. You don’t need to give instructions, because the, mechanic knows his job. You don’t need to be told when the job is, done. You assume the bike would be serviced in the usual way, the, mechanic does it and that’s that., Let us now look at a simple C function that operates in much the, same way as the mechanic. Actually, we will be looking at two, things—a function that calls or activates the function and the, function itself.
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Chapter 5: Functions & Pointers, , 159, , main( ), {, message( ) ;, printf ( "\nCry, and you stop the monotony!" ) ;, }, message( ), {, printf ( "\nSmile, and the world smiles with you..." ) ;, }, , And here’s the output..., Smile, and the world smiles with you..., Cry, and you stop the monotony!, , Here, main( ) itself is a function and through it we are calling the, function message( ). What do we mean when we say that main( ), ‘calls’ the function message( )? We mean that the control passes to, the function message( ). The activity of main( ) is temporarily, suspended; it falls asleep while the message( ) function wakes up, and goes to work. When the message( ) function runs out of, statements to execute, the control returns to main( ), which comes, to life again and begins executing its code at the exact point where, it left off. Thus, main( ) becomes the ‘calling’ function, whereas, message( ) becomes the ‘called’ function., If you have grasped the concept of ‘calling’ a function you are, prepared for a call to more than one function. Consider the, following example:, main( ), {, printf ( "\nI am in main" ) ;, italy( ) ;, brazil( ) ;, argentina( ) ;, }
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160, , Let Us C, , italy( ), {, printf ( "\nI am in italy" ) ;, }, brazil( ), {, printf ( "\nI am in brazil" ) ;, }, argentina( ), {, printf ( "\nI am in argentina" ) ;, }, , The output of the above program when executed would be as, under:, I am in main, I am in italy, I am in brazil, I am in argentina, , From this program a number of conclusions can be drawn:, −, , Any C program contains at least one function., , −, , If a program contains only one function, it must be main( )., , −, , If a C program contains more than one function, then one (and, only one) of these functions must be main( ), because program, execution always begins with main( )., , −, , There is no limit on the number of functions that might be, present in a C program., , −, , Each function in a program is called in the sequence specified, by the function calls in main( ).
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Chapter 5: Functions & Pointers, −, , 161, , After each function has done its thing, control returns to, main( ).When main( ) runs out of function calls, the program, ends., , As we have noted earlier the program execution always begins, with main( ). Except for this fact all C functions enjoy a state of, perfect equality. No precedence, no priorities, nobody is nobody’s, boss. One function can call another function it has already called, but has in the meantime left temporarily in order to call a third, function which will sometime later call the function that has called, it, if you understand what I mean. No? Well, let’s illustrate with an, example., main( ), {, printf ( "\nI am in main" ) ;, italy( ) ;, printf ( "\nI am finally back in main" ) ;, }, italy( ), {, printf ( "\nI am in italy" ) ;, brazil( ) ;, printf ( "\nI am back in italy" ) ;, }, brazil( ), {, printf ( "\nI am in brazil" ) ;, argentina( ) ;, }, argentina( ), {, printf ( "\nI am in argentina" ) ;, }, , And the output would look like...
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162, , Let Us C, , I am in main, I am in italy, I am in brazil, I am in argentina, I am back in italy, I am finally back in main, , Here, main( ) calls other functions, which in turn call still other, functions. Trace carefully the way control passes from one, function to another. Since the compiler always begins the program, execution with main( ), every function in a program must be, called directly or indirectly by main( ). In other words, the main( ), function drives other functions., Let us now summarize what we have learnt so far., (a) C program is a collection of one or more functions., (b) A function gets called when the function name is followed by, a semicolon. For example,, main( ), {, argentina( ) ;, }, , (c) A function is defined when function name is followed by a, pair of braces in which one or more statements may be, present. For example,, argentina( ), {, statement 1 ;, statement 2 ;, statement 3 ;, }
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Chapter 5: Functions & Pointers, , 163, , (d) Any function can be called from any other function. Even, main( ) can be called from other functions. For example,, main( ), {, message( ) ;, }, message( ), {, printf ( "\nCan't imagine life without C" ) ;, main( ) ;, }, , (e) A function can be called any number of times. For example,, main( ), {, message( ) ;, message( ) ;, }, message( ), {, printf ( "\nJewel Thief!!" ) ;, }, , (f) The order in which the functions are defined in a program and, the order in which they get called need not necessarily be, same. For example,, main( ), {, message1( ) ;, message2( ) ;, }, message2( ), {, printf ( "\nBut the butter was bitter" ) ;
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164, , Let Us C, , }, message1( ), {, printf ( "\nMary bought some butter" ) ;, }, , Here, even though message1( ) is getting called before, message2( ), still, message1( ) has been defined after, message2( ). However, it is advisable to define the functions, in the same order in which they are called. This makes the, program easier to understand., (g) A function can call itself. Such a process is called ‘recursion’., We would discuss this aspect of C functions later in this, chapter., (h) A function can be called from other function, but a function, cannot be defined in another function. Thus, the following, program code would be wrong, since argentina( ) is being, defined inside another function, main( )., main( ), {, printf ( "\nI am in main" ) ;, argentina( ), {, printf ( "\nI am in argentina" ) ;, }, }, , (i) There are basically two types of functions:, Library functions Ex. printf( ), scanf( ) etc., User-defined functions Ex. argentina( ), brazil( ) etc., As the name suggests, library functions are nothing but, commonly required functions grouped together and stored in
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Chapter 5: Functions & Pointers, , 165, , what is called a Library. This library of functions is present on, the disk and is written for us by people who write compilers, for us. Almost always a compiler comes with a library of, standard functions. The procedure of calling both types of, functions is exactly same., , Why Use Functions, Why write separate functions at all? Why not squeeze the entire, logic into one function, main( )? Two reasons:, (a) Writing functions avoids rewriting the same code over and, , over. Suppose you have a section of code in your program, that calculates area of a triangle. If later in the program you, want to calculate the area of a different triangle, you won’t, like it if you are required to write the same instructions all, over again. Instead, you would prefer to jump to a ‘section of, code’ that calculates area and then jump back to the place, from where you left off. This section of code is nothing but a, function., (b) Using functions it becomes easier to write programs and keep, , track of what they are doing. If the operation of a program can, be divided into separate activities, and each activity placed in, a different function, then each could be written and checked, more or less independently. Separating the code into modular, functions also makes the program easier to design and, understand., What is the moral of the story? Don’t try to cram the entire logic in, one function. It is a very bad style of programming. Instead, break, a program into small units and write functions for each of these, isolated subdivisions. Don’t hesitate to write functions that are, called only once. What is important is that these functions perform, some logically isolated task.
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166, , Let Us C, , Passing Values between Functions, The functions that we have used so far haven’t been very flexible., We call them and they do what they are designed to do. Like our, mechanic who always services the motorbike in exactly the same, way, we haven’t been able to influence the functions in the way, they carry out their tasks. It would be nice to have a little more, control over what functions do, in the same way it would be nice, to be able to tell the mechanic, “Also change the engine oil, I am, going for an outing”. In short, now we want to communicate, between the ‘calling’ and the ‘called’ functions., The mechanism used to convey information to the function is the, ‘argument’. You have unknowingly used the arguments in the, printf( ) and scanf( ) functions; the format string and the list of, variables used inside the parentheses in these functions are, arguments. The arguments are sometimes also called ‘parameters’., Consider the following program. In this program, in main( ) we, receive the values of a, b and c through the keyboard and then, output the sum of a, b and c. However, the calculation of sum is, done in a different function called calsum( ). If sum is to be, calculated in calsum( ) and values of a, b and c are received in, main( ), then we must pass on these values to calsum( ), and once, calsum( ) calculates the sum we must return it from calsum( ), back to main( )., /* Sending and receiving values between functions */, main( ), {, int a, b, c, sum ;, printf ( "\nEnter any three numbers " ) ;, scanf ( "%d %d %d", &a, &b, &c ) ;, sum = calsum ( a, b, c ) ;
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Chapter 5: Functions & Pointers, , 167, , printf ( "\nSum = %d", sum ) ;, }, calsum ( x, y, z ), int x, y, z ;, {, int d ;, d=x+y+z;, return ( d ) ;, }, , And here is the output..., Enter any three numbers 10 20 30, Sum = 60, , There are a number of things to note about this program:, (a) In this program, from the function main( ) the values of a, b, and c are passed on to the function calsum( ), by making a, call to the function calsum( ) and mentioning a, b and c in the, parentheses:, sum = calsum ( a, b, c ) ;, , In the calsum( ) function these values get collected in three, variables x, y and z:, calsum ( x, y, z ), int x, y, z ;, , (b) The variables a, b and c are called ‘actual arguments’,, whereas the variables x, y and z are called ‘formal, arguments’. Any number of arguments can be passed to a, function being called. However, the type, order and number of, the actual and formal arguments must always be same.
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168, , Let Us C, , Instead of using different variable names x, y and z, we could, have used the same variable names a, b and c. But the, compiler would still treat them as different variables since, they are in different functions., (c) There are two methods of declaring the formal arguments., The one that we have used in our program is known as, Kernighan and Ritchie (or just K & R) method., calsum ( x, y, z ), int x, y, z ;, , Another method is,, calsum ( int x, int y, int z ), , This method is called ANSI method and is more commonly, used these days., (d) In the earlier programs the moment closing brace ( } ) of the, called function was encountered the control returned to the, calling function. No separate return statement was necessary, to send back the control., This approach is fine if the called function is not going to, return any meaningful value to the calling function. In the, above program, however, we want to return the sum of x, y, and z. Therefore, it is necessary to use the return statement., The return statement serves two purposes:, (1) On executing the return statement it immediately, transfers the control back to the calling program., (2) It returns the value present in the parentheses after, return, to th3e calling program. In the above program, the value of sum of three numbers is being returned.
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Chapter 5: Functions & Pointers, , 169, , (e) There is no restriction on the number of return statements, that may be present in a function. Also, the return statement, need not always be present at the end of the called function., The following program illustrates these facts., fun( ), {, char ch ;, printf ( "\nEnter any alphabet " ) ;, scanf ( "%c", &ch ) ;, if ( ch >= 65 && ch <= 90 ), return ( ch ) ;, else, return ( ch + 32 ) ;, }, , In this function different return statements will be executed, depending on whether ch is capital or not., (f) Whenever the control returns from a function some value is, definitely returned. If a meaningful value is returned then it, should be accepted in the calling program by equating the, called function to some variable. For example,, sum = calsum ( a, b, c ) ;, , (g) All the following are valid return statements., return ( a ) ;, return ( 23 ) ;, return ( 12.34 ) ;, return ;
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170, , Let Us C, , In the last statement a garbage value is returned to the calling, function since we are not returning any specific value. Note, that in this case the parentheses after return are dropped., (h) If we want that a called function should not return any value,, in that case, we must mention so by using the keyword void, as shown below., void display( ), {, printf ( "\nHeads I win..." ) ;, printf ( "\nTails you lose" ) ;, }, , (i) A function can return only one value at a time. Thus, the, following statements are invalid., return ( a, b ) ;, return ( x, 12 ) ;, , There is a way to get around this limitation, which would be, discussed later in this chapter when we learn pointers., (j) If the value of a formal argument is changed in the called, function, the corresponding change does not take place in the, calling function. For example,, main( ), {, int a = 30 ;, fun ( a ) ;, printf ( "\n%d", a ) ;, }, fun ( int b ), {, b = 60 ;
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Chapter 5: Functions & Pointers, , 171, , printf ( "\n%d", b ) ;, }, , The output of the above program would be:, 60, 30, , Thus, even though the value of b is changed in fun( ), the, value of a in main( ) remains unchanged. This means that, when values are passed to a called function the values present, in actual arguments are not physically moved to the formal, arguments; just a photocopy of values in actual argument is, made into formal arguments., , Scope Rule of Functions, Look at the following program, main( ), {, int i = 20 ;, display ( i ) ;, }, display ( int j ), {, int k = 35 ;, printf ( "\n%d", j ) ;, printf ( "\n%d", k ) ;, }, , In this program is it necessary to pass the value of the variable i to, the function display( )? Will it not become automatically available, to the function display( )? No. Because by default the scope of a, variable is local to the function in which it is defined. The presence
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172, , Let Us C, , of i is known only to the function main( ) and not to any other, function. Similarly, the variable k is local to the function, display( ) and hence it is not available to main( ). That is why to, make the value of i available to display( ) we have to explicitly, pass it to display( ). Likewise, if we want k to be available to, main( ) we will have to return it to main( ) using the return, statement. In general we can say that the scope of a variable is, local to the function in which it is defined., , Calling Convention, Calling convention indicates the order in which arguments are, passed to a function when a function call is encountered. There are, two possibilities here:, (a) Arguments might be passed from left to right., (b) Arguments might be passed from right to left., C language follows the second order., Consider the following function call:, fun (a, b, c, d ) ;, , In this call it doesn’t matter whether the arguments are passed, from left to right or from right to left. However, in some function, call the order of passing arguments becomes an important, consideration. For example:, int a = 1 ;, printf ( "%d %d %d", a, ++a, a++ ) ;, , It appears that this printf( ) would output 1 2 3., This however is not the case. Surprisingly, it outputs 3 3 1. This is, because C’s calling convention is from right to left. That is, firstly
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Chapter 5: Functions & Pointers, , 173, , 1 is passed through the expression a++ and then a is incremented, to 2. Then result of ++a is passed. That is, a is incremented to 3, and then passed. Finally, latest value of a, i.e. 3, is passed. Thus in, right to left order 1, 3, 3 get passed. Once printf( ) collects them it, prints them in the order in which we have asked it to get them, printed (and not the order in which they were passed). Thus 3 3 1, gets printed., , One Dicey Issue, Consider the following function calls:, #include <conio.h>, clrscr ( ) ;, gotoxy ( 10, 20 ) ;, ch = getch ( a ) ;, , Here we are calling three standard library functions. Whenever we, call the library functions we must write their prototype before, making the call. This helps the compiler in checking whether the, values being passed and returned are as per the prototype, declaration. But since we don’t define the library functions (we, merely call them) we may not know the prototypes of library, functions. Hence when the library of functions is provided a set of, ‘.h’ files is also provided. These files contain the prototypes of, library functions. But why multiple files? Because the library, functions are divided into different groups and one file is provided, for each group. For example, prototypes of all input/output, functions are provided in the file ‘stdio.h’, prototypes of all, mathematical functions are provided in the file ‘math.h’, etc., On compilation of the above code the compiler reports all errors, due to the mismatch between parameters in function call and their, corresponding prototypes declared in the file ‘conio.h’. You can, even open this file and look at the prototypes. They would appear, as shown below:
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174, , Let Us C, , void clrscr( ) ;, void gotoxy ( int, int ) ;, int getch( ) ;, , Now consider the following function calls:, #include <stdio.h>, int i = 10, j = 20 ;, printf ( "%d %d %d ", i, j ) ;, printf ( "%d", i, j ) ;, , The above functions get successfully compiled even though there, is a mismatch in the format specifiers and the variables in the list., This is because printf( ) accepts variable number of arguments, (sometimes 2 arguments, sometimes 3 arguments, etc.), and even, with the mismatch above the call still matches with the prototype, of printf( ) present in ‘stdio.h’. At run-time when the first printf( ), is executed, since there is no variable matching with the last, specifier %d, a garbage integer gets printed. Similarly, in the, second printf( ) since the format specifier for j has not been, mentioned its value does not get printed., , Advanced Features of Functions, With a sound basis of the preliminaries of C functions, let us now, get into their intricacies. Following advanced topics would be, considered here., (a) Function Declaration and Prototypes, (b) Calling functions by value or by reference, (c) Recursion, , Let us understand these features one by one.
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Chapter 5: Functions & Pointers, , 175, , Function Declaration and Prototypes, Any C function by default returns an int value. More specifically,, whenever a call is made to a function, the compiler assumes that, this function would return a value of the type int. If we desire that a, function should return a value other than an int, then it is necessary, to explicitly mention so in the calling function as well as in the, called function. Suppose we want to find out square of a number, using a function. This is how this simple program would look like:, main( ), {, float a, b ;, printf ( "\nEnter any number " ) ;, scanf ( "%f", &a ) ;, b = square ( a ) ;, printf ( "\nSquare of %f is %f", a, b ) ;, }, square ( float x ), {, float y ;, y=x*x;, return ( y ) ;, }, , And here are three sample runs of this program..., Enter any number 3, Square of 3 is 9.000000, Enter any number 1.5, Square of 1.5 is 2.000000, Enter any number 2.5, Square of 2.5 is 6.000000
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176, , Let Us C, , The first of these answers is correct. But square of 1.5 is definitely, not 2. Neither is 6 a square of 2.5. This happened because any C, function, by default, always returns an integer value. Therefore,, even though the function square( ) calculates the square of 1.5 as, 2.25, the problem crops up when this 2.25 is to be returned to, main( ). square( ) is not capable of returning a float value. How, do we overcome this? The following program segment illustrates, how to make square( ) capable of returning a float value., main( ), {, float square ( float ) ;, float a, b ;, printf ( "\nEnter any number " ) ;, scanf ( "%f", &a ) ;, b = square ( a ) ;, printf ( "\nSquare of %f is %f", a, b ) ;, }, float square ( float x ), {, float y ;, y=x*x;, return ( y ) ;, }, , And here is the output..., Enter any number 1.5, Square of 1.5 is 2.250000, Enter any number 2.5, Square of 2.5 is 6.250000
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Chapter 5: Functions & Pointers, , 177, , Now the expected answers i.e. 2.25 and 6.25 are obtained. Note, that the function square( ) must be declared in main( ) as, float square ( float ) ;, , This statement is often called the prototype declaration of the, square( ) function. What it means is square( ) is a function that, receives a float and returns a float. We have done the prototype, declaration in main( ) because we have called it from main( )., There is a possibility that we may call square( ) from several other, functions other than main( ). Does this mean that we would need, prototype declaration of square( ) in all these functions. No, in, such a case we would make only one declaration outside all the, functions at the beginning of the program., In practice you may seldom be required to return a value other, than an int, but just in case you are required to, employ the above, method. In some programming situations we want that a called, function should not return any value. This is made possible by, using the keyword void. This is illustrated in the following, program., main( ), {, void gospel( ) ;, gospel( ) ;, }, void gospel( ), {, printf ( "\nViruses are electronic bandits..." ) ;, printf ( "\nwho eat nuggets of information..." ) ;, printf ( "\nand chunks of bytes..." ) ;, printf ( "\nwhen you least expect..." ) ;, }
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178, , Let Us C, , Here, the gospel( ) function has been defined to return void; means, it would return nothing. Therefore, it would just flash the four, messages about viruses and return the control back to the main( ), function., , Call by Value and Call by Reference, By now we are well familiar with how to call functions. But, if you, observe carefully, whenever we called a function and passed, something to it we have always passed the ‘values’ of variables to, the called function. Such function calls are called ‘calls by value’., By this what we mean is, on calling a function we are passing, values of variables to it. The examples of call by value are shown, below:, sum = calsum ( a, b, c ) ;, f = factr ( a ) ;, , We have also learnt that variables are stored somewhere in, memory. So instead of passing the value of a variable, can we not, pass the location number (also called address) of the variable to a, function? If we were able to do so it would become a ‘call by, reference’. What purpose a ‘call by reference’ serves we would, find out a little later. First we must equip ourselves with, knowledge of how to make a ‘call by reference’. This feature of C, functions needs at least an elementary knowledge of a concept, called ‘pointers’. So let us first acquire the basics of pointers after, which we would take up this topic once again., , An Introduction to Pointers, Which feature of C do beginners find most difficult to understand?, The answer is easy: pointers. Other languages have pointers but, few use them so frequently as C does. And why not? It is C’s, clever use of pointers that makes it the excellent language it is.
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Chapter 5: Functions & Pointers, , 179, , The difficulty beginners have with pointers has much to do with, C’s pointer terminology than the actual concept. For instance,, when a C programmer says that a certain variable is a “pointer”,, what does that mean? It is hard to see how a variable can point to, something, or in a certain direction., It is hard to get a grip on pointers just by listening to programmer’s, jargon. In our discussion of C pointers, therefore, we will try to, avoid this difficulty by explaining pointers in terms of, programming concepts we already understand. The first thing we, want to do is explain the rationale of C’s pointer notation., , Pointer Notation, Consider the declaration,, int i = 3 ;, , This declaration tells the C compiler to:, (a) Reserve space in memory to hold the integer value., (b) Associate the name i with this memory location., (c) Store the value 3 at this location., , We may represent i’s location in memory by the following, memory map., , i, , location name, , 3, , value at location, , 65524, , Figure 5.1, , location number
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180, , Let Us C, , We see that the computer has selected memory location 65524 as, the place to store the value 3. The location number 65524 is not a, number to be relied upon, because some other time the computer, may choose a different location for storing the value 3. The, important point is, i’s address in memory is a number., We can print this address number through the following program:, main( ), {, int i = 3 ;, printf ( "\nAddress of i = %u", &i ) ;, printf ( "\nValue of i = %d", i ) ;, }, , The output of the above program would be:, Address of i = 65524, Value of i = 3, , Look at the first printf( ) statement carefully. ‘&’ used in this, statement is C’s ‘address of’ operator. The expression &i returns, the address of the variable i, which in this case happens to be, 65524. Since 65524 represents an address, there is no question of a, sign being associated with it. Hence it is printed out using %u,, which is a format specifier for printing an unsigned integer. We, have been using the ‘&’ operator all the time in the scanf( ), statement., The other pointer operator available in C is ‘*’, called ‘value at, address’ operator. It gives the value stored at a particular address., The ‘value at address’ operator is also called ‘indirection’, operator., Observe carefully the output of the following program:
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181, , Chapter 5: Functions & Pointers, main( ), {, int i = 3 ;, printf ( "\nAddress of i = %u", &i ) ;, printf ( "\nValue of i = %d", i ) ;, printf ( "\nValue of i = %d", *( &i ) ) ;, }, , The output of the above program would be:, Address of i = 65524, Value of i = 3, Value of i = 3, , Note that printing the value of *( &i ) is same as printing the value, of i., The expression &i gives the address of the variable i. This address, can be collected in a variable, by saying,, j = &i ;, , But remember that j is not an ordinary variable like any other, integer variable. It is a variable that contains the address of other, variable (i in this case). Since j is a variable the compiler must, provide it space in the memory. Once again, the following memory, map would illustrate the contents of i and j., , i, , j, , 3, , 65524, , 65524, , Figure 5.2, , 65522
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182, , Let Us C, , As you can see, i’s value is 3 and j’s value is i’s address., But wait, we can’t use j in a program without declaring it. And, since j is a variable that contains the address of i, it is declared as,, int *j ;, , This declaration tells the compiler that j will be used to store the, address of an integer value. In other words j points to an integer., How do we justify the usage of * in the declaration,, int *j ;, , Let us go by the meaning of *. It stands for ‘value at address’., Thus, int *j would mean, the value at the address contained in j is, an int., Here is a program that demonstrates the relationships we have, been discussing., main( ), {, int i = 3 ;, int *j ;, j = &i ;, printf ( "\nAddress of i = %u", &i ) ;, printf ( "\nAddress of i = %u", j ) ;, printf ( "\nAddress of j = %u", &j ) ;, printf ( "\nValue of j = %u", j ) ;, printf ( "\nValue of i = %d", i ) ;, printf ( "\nValue of i = %d", *( &i ) ) ;, printf ( "\nValue of i = %d", *j ) ;, }, , The output of the above program would be:
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Chapter 5: Functions & Pointers, , 183, , Address of i = 65524, Address of i = 65524, Address of j = 65522, Value of j = 65524, Value of i = 3, Value of i = 3, Value of i = 3, , Work through the above program carefully, taking help of the, memory locations of i and j shown earlier. This program, summarizes everything that we have discussed so far. If you don’t, understand the program’s output, or the meanings of &i, &j, *j, and *( &i ), re-read the last few pages. Everything we say about C, pointers from here onwards will depend on your understanding, these expressions thoroughly., Look at the following declarations,, int *alpha ;, char *ch ;, float *s ;, , Here, alpha, ch and s are declared as pointer variables, i.e., variables capable of holding addresses. Remember that, addresses, (location nos.) are always going to be whole numbers, therefore, pointers always contain whole numbers. Now we can put these two, facts together and say—pointers are variables that contain, addresses, and since addresses are always whole numbers, pointers, would always contain whole numbers., The declaration float *s does not mean that s is going to contain a, floating-point value. What it means is, s is going to contain the, address of a floating-point value. Similarly, char *ch means that, ch is going to contain the address of a char value. Or in other, words, the value at address stored in ch is going to be a char.
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184, , Let Us C, , The concept of pointers can be further extended. Pointer, we know, is a variable that contains address of another variable. Now this, variable itself might be another pointer. Thus, we now have a, pointer that contains another pointer’s address. The following, example should make this point clear., main( ), {, int i = 3, *j, **k ;, j = &i ;, k = &j ;, printf ( "\nAddress of i = %u", &i ) ;, printf ( "\nAddress of i = %u", j ) ;, printf ( "\nAddress of i = %u", *k ) ;, printf ( "\nAddress of j = %u", &j ) ;, printf ( "\nAddress of j = %u", k ) ;, printf ( "\nAddress of k = %u", &k ) ;, printf ( "\nValue of j = %u", j ) ;, printf ( "\nValue of k = %u", k ) ;, printf ( "\nValue of i = %d", i ) ;, printf ( "\nValue of i = %d", * ( &i ) ) ;, printf ( "\nValue of i = %d", *j ) ;, printf ( "\nValue of i = %d", **k ) ;, }, , The output of the above program would be:, Address of i = 65524, Address of i = 65524, Address of i = 65524, Address of j = 65522, Address of j = 65522, Address of k = 65520, Value of j = 65524, Value of k = 65522
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185, , Chapter 5: Functions & Pointers, Value of i, Value of i, Value of i, Value of i, , =3, =3, =3, =3, , Figure 5.3 would help you in tracing out how the program prints, the above output., Remember that when you run this program the addresses that get, printed might turn out to be something different than the ones, shown in the figure. However, with these addresses too the, relationship between i, j and k can be easily established., , i, , j, , k, , 3, , 65524, , 65522, , 65524, , 65522, , 65520, , Figure 5.3, Observe how the variables j and k have been declared,, int i, *j, **k ;, , Here, i is an ordinary int, j is a pointer to an int (often called an, integer pointer), whereas k is a pointer to an integer pointer. We, can extend the above program still further by creating a pointer to, a pointer to an integer pointer. In principle, you would agree that, likewise there could exist a pointer to a pointer to a pointer to a, pointer to a pointer. There is no limit on how far can we go on, extending this definition. Possibly, till the point we can, comprehend it. And that point of comprehension is usually a, pointer to a pointer. Beyond this one rarely requires to extend the, definition of a pointer. But just in case...
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186, , Let Us C, , Back to Function Calls, Having had the first tryst with pointers let us now get back to what, we had originally set out to learn—the two types of function, calls—call by value and call by reference. Arguments can, generally be passed to functions in one of the two ways:, (a) sending the values of the arguments, (b) sending the addresses of the arguments, , In the first method the ‘value’ of each of the actual arguments in, the calling function is copied into corresponding formal arguments, of the called function. With this method the changes made to the, formal arguments in the called function have no effect on the, values of actual arguments in the calling function. The following, program illustrates the ‘Call by Value’., main( ), {, int a = 10, b = 20 ;, swapv ( a, b ) ;, printf ( "\na = %d b = %d", a, b ) ;, }, swapv ( int x, int y ), {, int t ;, t=x;, x=y;, y=t;, printf ( "\nx = %d y = %d", x, y ) ;, }, , The output of the above program would be:
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Chapter 5: Functions & Pointers, , 187, , x = 20 y = 10, a = 10 b = 20, , Note that values of a and b remain unchanged even after, exchanging the values of x and y., In the second method (call by reference) the addresses of actual, arguments in the calling function are copied into formal arguments, of the called function. This means that using these addresses we, would have an access to the actual arguments and hence we would, be able to manipulate them. The following program illustrates this, fact., main( ), {, int a = 10, b = 20 ;, swapr ( &a, &b ) ;, printf ( "\na = %d b = %d", a, b ) ;, }, swapr( int *x, int *y ), {, int t ;, t = *x ;, *x = *y ;, *y = t ;, }, , The output of the above program would be:, a = 20 b = 10, , Note that this program manages to exchange the values of a and b, using their addresses stored in x and y.
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188, , Let Us C, , Usually in C programming we make a call by value. This means, that in general you cannot alter the actual arguments. But if, desired, it can always be achieved through a call by reference., Using a call by reference intelligently we can make a function, return more than one value at a time, which is not possible, ordinarily. This is shown in the program given below., main( ), {, int radius ;, float area, perimeter ;, printf ( "\nEnter radius of a circle " ) ;, scanf ( "%d", &radius ) ;, areaperi ( radius, &area, &perimeter ) ;, printf ( "Area = %f", area ) ;, printf ( "\nPerimeter = %f", perimeter ) ;, }, areaperi ( int r, float *a, float *p ), {, *a = 3.14 * r * r ;, *p = 2 * 3.14 * r ;, }, , And here is the output..., Enter radius of a circle 5, Area = 78.500000, Perimeter = 31.400000, , Here, we are making a mixed call, in the sense, we are passing the, value of radius but, addresses of area and perimeter. And since, we are passing the addresses, any change that we make in values, stored at addresses contained in the variables a and p, would make
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Chapter 5: Functions & Pointers, , 189, , the change effective in main( ). That is why when the control, returns from the function areaperi( ) we are able to output the, values of area and perimeter., Thus, we have been able to indirectly return two values from a, called function, and hence, have overcome the limitation of the, return statement, which can return only one value from a function, at a time., , Conclusions, From the programs that we discussed here we can draw the, following conclusions:, (a) If we want that the value of an actual argument should not get, changed in the function being called, pass the actual argument, by value., (b) If we want that the value of an actual argument should get, changed in the function being called, pass the actual argument, by reference., (c) If a function is to be made to return more than one value at a, time then return these values indirectly by using a call by, reference., , Recursion, In C, it is possible for the functions to call themselves. A function, is called ‘recursive’ if a statement within the body of a function, calls the same function. Sometimes called ‘circular definition’,, recursion is thus the process of defining something in terms of, itself., Let us now see a simple example of recursion. Suppose we want to, calculate the factorial value of an integer. As we know, the
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190, , Let Us C, , factorial of a number is the product of all the integers between 1, and that number. For example, 4 factorial is 4 * 3 * 2 * 1. This can, also be expressed as 4! = 4 * 3! where ‘!’ stands for factorial. Thus, factorial of a number can be expressed in the form of itself. Hence, this can be programmed using recursion. However, before we try, to write a recursive function for calculating factorial let us take a, look at the non-recursive function for calculating the factorial, value of an integer., main( ), {, int a, fact ;, printf ( "\nEnter any number " ) ;, scanf ( "%d", &a ) ;, fact = factorial ( a ) ;, printf ( "Factorial value = %d", fact ) ;, }, factorial ( int x ), {, int f = 1, i ;, for ( i = x ; i >= 1 ; i-- ), f=f*i;, return ( f ) ;, }, , And here is the output..., Enter any number 3, Factorial value = 6
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Chapter 5: Functions & Pointers, , 191, , Work through the above program carefully, till you understand the, logic of the program properly. Recursive factorial function can be, understood only if you are thorough with the above logic., Following is the recursive version of the function to calculate the, factorial value., main( ), {, int a, fact ;, printf ( "\nEnter any number " ) ;, scanf ( "%d", &a ) ;, fact = rec ( a ) ;, printf ( "Factorial value = %d", fact ) ;, }, rec ( int x ), {, int f ;, if ( x == 1 ), return ( 1 ) ;, else, f = x * rec ( x - 1 ) ;, return ( f ) ;, }, , And here is the output for four runs of the program, Enter any number 1, Factorial value = 1, Enter any number 2, Factorial value = 2, Enter any number 3
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192, , Let Us C, , Factorial value = 6, Enter any number 5, Factorial value = 120, , Let us understand this recursive factorial function thoroughly. In, the first run when the number entered through scanf( ) is 1, let us, see what action does rec( ) take. The value of a (i.e. 1) is copied, into x. Since x turns out to be 1 the condition if ( x == 1 ) is, satisfied and hence 1 (which indeed is the value of 1 factorial) is, returned through the return statement., When the number entered through scanf( ) is 2, the ( x == 1 ) test, fails, so we reach the statement,, f = x * rec ( x - 1 ) ;, , And here is where we meet recursion. How do we handle the, expression x * rec ( x - 1 )? We multiply x by rec ( x - 1 ). Since, the current value of x is 2, it is same as saying that we must, calculate the value (2 * rec ( 1 )). We know that the value returned, by rec ( 1 ) is 1, so the expression reduces to (2 * 1), or simply 2., Thus the statement,, x * rec ( x - 1 ) ;, , evaluates to 2, which is stored in the variable f, and is returned to, main( ), where it is duly printed as, Factorial value = 2, , Now perhaps you can see what would happen if the value of a is 3,, 4, 5 and so on., In case the value of a is 5, main( ) would call rec( ) with 5 as its, actual argument, and rec( ) will send back the computed value. But, before sending the computed value, rec( ) calls rec( ) and waits for, a value to be returned. It is possible for the rec( ) that has just been
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Chapter 5: Functions & Pointers, , 193, , called to call yet another rec( ), the argument x being decreased in, value by 1 for each of these recursive calls. We speak of this series, of calls to rec( ) as being different invocations of rec( ). These, successive invocations of the same function are possible because, the C compiler keeps track of which invocation calls which. These, recursive invocations end finally when the last invocation gets an, argument value of 1, which the preceding invocation of rec( ) now, uses to calculate its own f value and so on up the ladder. So we, might say what happens is,, rec ( 5 ) returns ( 5 times rec ( 4 ),, which returns ( 4 times rec ( 3 ),, which returns ( 3 times rec ( 2 ),, which returns ( 2 times rec ( 1 ),, which returns ( 1 ) ) ) ) ), , Foxed? Well, that is recursion for you in its simplest garbs. I hope, you agree that it’s difficult to visualize how the control flows from, one function call to another. Possibly Figure 5.4 would make, things a bit clearer., Assume that the number entered through scanf( ) is 3. Using, Figure 5.4 let’s visualize what exactly happens when the recursive, function rec( ) gets called. Go through the figure carefully. The, first time when rec( ) is called from main( ), x collects 3. From, here, since x is not equal to 1, the if block is skipped and rec( ) is, called again with the argument ( x – 1 ), i.e. 2. This is a recursive, call. Since x is still not equal to 1, rec( ) is called yet another time,, with argument (2 - 1). This time as x is 1, control goes back to, previous rec( ) with the value 1, and f is evaluated as 2., Similarly, each rec( ) evaluates its f from the returned value, and, finally 6 is returned to main( ). The sequence would be grasped, better by following the arrows shown in Figure 5.4. Let it be clear, that while executing the program there do not exist so many copies, of the function rec( ). These have been shown in the figure just to
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194, , Let Us C, , help you keep track of how the control flows during successive, recursive calls., , from main( ), , rec ( int x ), {, int f ;, , rec ( int x ), {, int f ;, , rec ( int x ), {, int f ;, , if (x == 1), if (x == 1), return ( 1 ) ;, return ( 1 ) ;, else, else, f = x * rec ( x – 1 ) ;, f = x * rec ( x – 1 ) ;, , if (x == 1), return ( 1 ) ;, else, f = x * rec ( x – 1 ) ;, , return ( f ) ;, , return ( f ) ;, , }, , return ( f ) ;, }, , }, , to main( ), , Figure 5.4, Recursion may seem strange and complicated at first glance, but it, is often the most direct way to code an algorithm, and once you are, familiar with recursion, the clearest way of doing so., , Recursion and Stack, There are different ways in which data can be organized. For, example, if you are to store five numbers then we can store them, in five different variables, an array, a linked list, a binary tree, etc., All these different ways of organizing the data are known as data, structures. The compiler uses one such data structure called stack, for implementing normal as well as recursive function calls.
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Chapter 5: Functions & Pointers, , 195, , A stack is a Last In First Out (LIFO) data structure. This means, that the last item to get stored on the stack (often called Push, operation) is the first one to get out of it (often called as Pop, operation). You can compare this to the stack of plates in a, cafeteria—the last plate that goes on the stack is the first one to get, out of it. Now let us see how the stack works in case of the, following program., main( ), {, int a = 5, b = 2, c ;, c = add ( a, b ) ;, printf ( "sum = %d", c ) ;, }, add ( int i, int j ), {, int sum ;, sum = i + j ;, return sum ;, }, , In this program before transferring the execution control to the, function fun( ) the values of parameters a and b are pushed onto, the stack. Following this the address of the statement printf( ) is, pushed on the stack and the control is transferred to fun( ). It is, necessary to push this address on the stack. In fun( ) the values of, a and b that were pushed on the stack are referred as i and j. In, fun( ) the local variable sum gets pushed on the stack. When, value of sum is returned sum is popped up from the stack. Next, the address of the statement where the control should be returned, is popped up from the stack. Using this address the control returns, to the printf( ) statement in main( ). Before execution of printf( ), begins the two integers that were earlier pushed on the stack are, now popped off., How the values are being pushed and popped even though we, didn’t write any code to do so? Simple—the compiler on
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196, , Let Us C, , encountering the function call would generate code to push, parameters and the address. Similarly, it would generate code to, clear the stack when the control returns back from fun( ). Figure, 5.5 shows the contents of the stack at different stages of execution., , Copy of a, Copy of b, Empty stack, , sum, Address, , Address of, printf( ), Copy of a, , 5, 2, When call to, fun( ) is met, , Copy of b, , xxxx, 5, 2, Before transfering, control to fun( ), , 7, xxxx, , xxxx, , i, , 5, , 5, , j, , 2, , 2, , After control, reaches fun( ), , On returning control, While returning, from fun( ), control from fun( ), , Figure 5.5, Note that in this program popping of sum and address is done by, fun( ), whereas popping of the two integers is done by main( )., When it is done this way it is known as ‘CDecl Calling, Convention’. There are other calling conventions as well where, instead of main( ), fun( ) itself clears the two integers. The calling, convention also decides whether the parameters being passed to, the function are pushed on the stack in left-to-right or right-to-left, order. The standard calling convention always uses the right-to-left
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Chapter 5: Functions & Pointers, , 197, , order. Thus during the call to fun( ) firstly value of b is pushed to, the stack, followed by the value of a., The recursive calls are no different. Whenever we make a, recursive call the parameters and the return address gets pushed on, the stack. The stack gets unwound when the control returns from, the called function. Thus during every recursive function call we, are working with a fresh set of parameters., Also, note that while writing recursive functions you must have an, if statement somewhere in the recursive function to force the, function to return without recursive call being executed. If you, don’t do this and you call the function, you will fall in an, indefinite loop, and the stack will keep on getting filled with, parameters and the return address each time there is a call. Soon, the stack would become full and you would get a run-time error, indicating that the stack has become full. This is a very common, error while writing recursive functions. My advice is to use, printf( ) statement liberally during the development of recursive, function, so that you can watch what is going on and can abort, execution if you see that you have made a mistake., , Adding Functions to the Library, Most of the times we either use the functions present in the, standard library or we define our own functions and use them. Can, we not add our functions to the standard library? And would it, make any sense in doing so? We can add user-defined functions to, the library. It makes sense in doing so as the functions that are to, be added to the library are first compiled and then added. When we, use these functions (by calling them) we save on their compilation, time as they are available in the library in the compiled form., Let us now see how to add user-defined functions to the library., Different compilers provide different utilities to add/delete/modify, functions in the standard library. For example, Turbo C/C++
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198, , Let Us C, , compilers provide a utility called ‘tlib.exe’ (Turbo Librarian). Let, us use this utility to add a function factorial( ) to the library., Given below are the steps to do so:, (a) Write the function definition of factorial( ) in some file, say, ‘fact.c’., int factorial ( int num ), {, int i, f = 1 ;, for ( i = 1 ; i <= num ; i++ ), f=f*i;, return ( f ) ;, }, , (b) Compile the ‘fact.c’ file using Alt F9. A new file called, ‘fact.obj’ would get created containing the compiled code in, machine language., (c) Add the function to the library by issuing the command, C:\>tlib math.lib + c:\fact.obj, , Here, ‘math.lib’ is a library filename, + is a switch, which, means we want to add new function to library and ‘c:\fact.obj’, is the path of the ‘.obj’ file., (d) Declare the prototype of the factorial( ) function in the header, file, say ‘fact.h’. This file should be included while calling the, function., (e) To use the function present inside the library, create a, program as shown below:, #include "c:\\fact.h", main( )
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Chapter 5: Functions & Pointers, , 199, , {, int f ;, f = factorial ( 5 ) ;, printf ( "%d", f ) ;, }, , (f) Compile and execute the program using Ctrl F9., If we wish we can delete the existing functions present in the, library using the minus ( - ) switch., Instead of modifying the existing libraries we can create our own, library. Let’s see how to do this. Let us assume that we wish to, create a library containing the functions factorial( ), prime( ) and, fibonacci( ). As their names suggest, factorial( ) calculates and, returns the factorial value of the integer passed to it, prime( ), reports whether the number passed to it is a prime number or not, and fibonacci( ) prints the first n terms of the Fibonacci series,, where n is the number passed to it. Here are the steps that need to, be carried out to create this library. Note that these steps are, specific to Turbo C/C++ compiler and would vary for other, compilers., (a) Define the functions factorial( ), prime( ) and fibonacci( ) in, a file, say ‘myfuncs.c’. Do not define main( ) in this file., (b) Create a file ‘myfuncs.h’ and declare the prototypes of, factorial( ), prime( ) and fibonacci( ) in it as shown below:, int factorial ( int ) ;, int prime ( int ) ;, void fibonacci ( int ) ;, , (c) From the Options menu select the menu-item ‘Application’., From the dialog that pops us select the option ‘Library’., Select OK.
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200, , Let Us C, , (d) Compile the program using Alt F9. This would create the, library file called ‘myfuncs.lib’., That’s it. The library now stands created. Now we have to use the, functions defined in this library. Here is how it can be done., (a) Create a file, say ‘sample.c’ and type the following code in it., #include "myfuncs.h", main( ), {, int f, result ;, f = factorial ( 5 ) ;, result = prime ( 13 ) ;, fibonacci ( 6 ) ;, printf ( "\n%d %d", f, result ) ;, }, , Note that the file ‘myfuncs.h’ should be in the same directory, as the file ‘sample.c’. If not, then while including ‘myfuncs.h’, mention the appropriate path., (b) Go to the ‘Project’ menu and select ‘Open Project…’ option., On doing so a dialog would pop up. Give the name of the, project, say ‘sample.prj’ and select OK., (c) From the ‘Project’ menu select ‘Add Item’. On doing so a file, dialog would appear. Select the file ‘sample.c’ and then select, ‘Add’. Also add the file ‘myfuncs.lib’ in the same manner., Finally select ‘Done’., (d) Compile and execute the project using Ctrl F9.
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Chapter 5: Functions & Pointers, , 201, , Summary, (a) To avoid repetition of code and bulky programs functionally, related statements are isolated into a function., (b) Function declaration specifies what is the return type of the, function and the types of parameters it accepts., (c) Function definition defines the body of the function., (d) Variables declared in a function are not available to other, functions in a program. So, there won’t be any clash even if, we give same name to the variables declared in different, functions., (e) Pointers are variables which hold addresses of other variables., (f) A function can be called either by value or by reference., (g) Pointers can be used to make a function return more than one, value simultaneously., (h) Recursion is difficult to understand, but in some cases offer a, better solution than loops., (i) Adding too many functions and calling them frequently may, slow down the program execution., , Exercise, Simple functions, Passing values between functions, [A] What would be the output of the following programs:, (a) main( ), {, printf ( "\nOnly stupids use C?" ) ;, display( ) ;, }, display( ), {, printf ( "\nFools too use C!" ) ;, main( ) ;, }
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202, , Let Us C, , (b) main( ), {, printf ( "\nC to it that C survives" ) ;, main( ) ;, }, (c), , main( ), {, int i = 45, c ;, c = check ( i ) ;, printf ( "\n%d", c ) ;, }, check ( int ch ), {, if ( ch >= 45 ), return ( 100 ) ;, else, return ( 10 * 10 ) ;, }, , (d) main( ), {, int i = 45, c ;, c = multiply ( i * 1000 ) ;, printf ( "\n%d", c ) ;, }, check ( int ch ), {, if ( ch >= 40000 ), return ( ch / 10 ) ;, else, return ( 10 ) ;, }, [B] Point out the errors, if any, in the following programs:, (a) main( ), {
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Chapter 5: Functions & Pointers, int i = 3, j = 4, k, l ;, k = addmult ( i, j ) ;, l = addmult ( i, j ) ;, printf ( "\n%d %d", k, l ) ;, }, addmult ( int ii, int jj ), {, int kk, ll ;, kk = ii + jj ;, ll = ii * jj ;, return ( kk, ll ) ;, }, (b) main( ), {, int a ;, a = message( ) ;, }, message( ), {, printf ( "\nViruses are written in C" ) ;, return ;, }, (c), , main( ), {, float a = 15.5 ;, char ch = 'C' ;, printit ( a, ch ) ;, }, printit ( a, ch ), {, printf ( "\n%f %c", a, ch ) ;, }, , (d) main( ), {, message( ) ;, , 203
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204, , Let Us C, , message( ) ;, }, message( ) ;, {, printf ( "\nPraise worthy and C worthy are synonyms" ) ;, }, (e) main( ), {, let_us_c( ), {, printf ( "\nC is a Cimple minded language !" ) ;, printf ( "\nOthers are of course no match !" ) ;, }, }, (f), , main( ), {, message( message ( ) ) ;, }, void message( ), {, printf ( "\nPraise worthy and C worthy are synonyms" ) ;, }, , [C] Answer the following:, , (a) Is this a correctly written function:, sqr ( a ) ;, int a ;, {, return ( a * a ) ;, }, , (b) State whether the following statements are True or False:
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Chapter 5: Functions & Pointers, , 205, , 1. The variables commonly used in C functions are available, to all the functions in a program., 2. To return the control back to the calling function we must, use the keyword return., 3. The same variable names can be used in different, functions without any conflict., 4. Every called function must contain a return statement., 5. A function may contain more than one return statements., 6. Each return statement in a function may return a different, value., 7. A function can still be useful even if you don’t pass any, arguments to it and the function doesn’t return any value, back., 8. Same names can be used for different functions without, any conflict., 9. A function may be called more than once from any other, function., 10. It is necessary for a function to return some value., [D] Answer the following:, , (a) Write a function to calculate the factorial value of any integer, entered through the keyboard., (b) Write a function power ( a, b ), to calculate the value of a, raised to b.
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206, , Let Us C, , (c) Write a general-purpose function to convert any given year, into its roman equivalent. The following table shows the, roman equivalents of decimal numbers:, Decimal, , Roman, , Decimal, , Roman, , 1, , i, , 100, , c, , 5, , v, , 500, , d, , 10, , x, , 1000, , m, , 50, , l, , Example:, Roman equivalent of 1988 is mdcccclxxxviii, Roman equivalent of 1525 is mdxxv, (d) Any year is entered through the keyboard. Write a function to, determine whether the year is a leap year or not., (e) A positive integer is entered through the keyboard. Write a, function to obtain the prime factors of this number., For example, prime factors of 24 are 2, 2, 2 and 3, whereas, prime factors of 35 are 5 and 7., , Function Prototypes, Call by Value/Reference, Pointers, [E] What would be the output of the following programs:, (a) main( ), {, float area ;, int radius = 1 ;, area = circle ( radius ) ;, printf ( "\n%f", area ) ;, }, circle ( int r )
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Chapter 5: Functions & Pointers, , 207, , {, float a ;, a = 3.14 * r * r ;, return ( a ) ;, }, (b) main( ), {, void slogan( ) ;, int c = 5 ;, c = slogan( ) ;, printf ( "\n%d", c ) ;, }, void slogan( ), {, printf ( "\nOnly He men use C!" ) ;, }, [F] Answer the following:, , (a) Write a function which receives a float and an int from, main( ), finds the product of these two and returns the product, which is printed through main( )., (b) Write a function that receives 5 integers and returns the sum,, average and standard deviation of these numbers. Call this, function from main( ) and print the results in main( )., (c) Write a function that receives marks received by a student in 3, subjects and returns the average and percentage of these, marks. Call this function from main( ) and print the results in, main( )., [G] What would be the output of the following programs:, (a) main( ), {, int i = 5, j = 2 ;
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208, , Let Us C, junk ( i, j ) ;, printf ( "\n%d %d", i, j ) ;, , }, junk ( int i, int j ), {, i=i*i;, j=j*j;, }, (b) main( ), {, int i = 5, j = 2 ;, junk ( &i, &j ) ;, printf ( "\n%d %d", i, j ) ;, }, junk ( int *i, int *j ), {, *i = *i * *i ;, *j = *j * *j ;, }, (c), , main( ), {, int i = 4, j = 2 ;, junk ( &i, j ) ;, printf ( "\n%d %d", i, j ) ;, }, junk ( int *i, int j ), {, *i = *i * *i ;, j=j*j;, }, , (d) main( ), {, float a = 13.5 ;, float *b, *c ;, b = &a ; /* suppose address of a is 1006 */
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Chapter 5: Functions & Pointers, c=b;, printf ( "\n%u %u %u", &a, b, c ) ;, printf ( "\n%f %f %f %f %f", a, *(&a), *&a, *b, *c ) ;, }, [H] Point out the errors, if any, in the following programs:, (a) main( ), {, int i = 135, a = 135, k ;, k = pass ( i, a ) ;, printf ( "\n%d", k ) ;, }, pass ( int j, int b ), int c ;, {, c=j+b;, return ( c ) ;, }, (b) main( ), {, int p = 23, f = 24 ;, jiaayjo ( &p, &f ) ;, printf ( "\n%d %d", p, f ) ;, }, jiaayjo ( int q, int g ), {, q=q+q;, g=g+g;, }, (c), , main( ), {, int k = 35, z ;, z = check ( k ) ;, printf ( "\n%d", z ) ;, }, , 209
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210, , Let Us C, , check ( m ), {, int m ;, if ( m > 40 ), return ( 1 ) ;, else, return ( 0 ) ;, }, (d) main( ), {, int i = 35, *z ;, z = function ( &i ) ;, printf ( "\n%d", z ) ;, }, function ( int *m ), {, return ( m + 2 ) ;, }, [I] What would be the output of the following programs:, (a) main( ), {, int i = 0 ;, i++ ;, if ( i <= 5 ), {, printf ( "\nC adds wings to your thoughts" ) ;, exit( ) ;, main( ) ;, }, }, (b) main( ), {, static int i = 0 ;, i++ ;
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Chapter 5: Functions & Pointers, , 211, , if ( i <= 5 ), {, printf ( "\n%d", i ) ;, main( ) ;, }, else, exit( ) ;, }, [J] Attempt the following:, , (a) A 5-digit positive integer is entered through the keyboard,, write a function to calculate sum of digits of the 5-digit, number:, (1) Without using recursion, (2) Using recursion, (b) A positive integer is entered through the keyboard, write a, program to obtain the prime factors of the number. Modify the, function suitably to obtain the prime factors recursively., (c) Write a recursive function to obtain the first 25 numbers of a, Fibonacci sequence. In a Fibonacci sequence the sum of two, successive terms gives the third term. Following are the first, few terms of the Fibonacci sequence:, 1 1 2 3 5 8 13 21 34 55 89..., (d) A positive integer is entered through the keyboard, write a, function to find the binary equivalent of this number using, recursion., (e) Write a recursive function to obtain the running sum of first, 25 natural numbers., (f) Write a C function to evaluate the series, , sin( x) = x − ( x 3 / 3!) + ( x 5 / 5!) − ( x 7 / 7!) + L
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212, , Let Us C, , to five significant digits., (g) Given three variables x, y, z write a function to circularly shift, their values to right. In other words if x = 5, y = 8, z = 10 after, circular shift y = 5, z = 8, x =10 after circular shift y = 5, z = 8, and x = 10. Call the function with variables a, b, c to, circularly shift values., (h) Write a function to find the binary equivalent of a given, decimal integer and display it., (i) If the lengths of the sides of a triangle are denoted by a, b,, and c, then area of triangle is given by, , area = S ( S − a )( S − b)( S − c), where, S = ( a + b + c ) / 2, (j) Write a function to compute the distance between two points, and use it to develop another function that will compute the, area of the triangle whose vertices are A(x1, y1), B(x2, y2),, and C(x3, y3). Use these functions to develop a function, which returns a value 1 if the point (x, y) lines inside the, triangle ABC, otherwise a value 0., (k) Write a function to compute the greatest common divisor, given by Euclid’s algorithm, exemplified for J = 1980, K =, 1617 as follows:, 1980 – 1 * 1617 = 363, 1980 / 1617 = 1, 1617 / 363 = 4, 1617 – 4 * 363 = 165, 363 / 165 = 2, 363 – 2 * 165 = 33, 5 / 33 = 5, 165 – 5 * 33 = 0, Thus, the greatest common divisor is 33.
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6, , Data Types, Revisited, •, •, •, •, •, •, , Integers, long and short, Integers, signed and unsigned, Chars, signed and unsigned, Floats and Doubles, A Few More Issues…, Storage Classes in C, Automatic Storage Class, Register Storage Class, Static Storage Class, External Storage Class, Which to Use When, • Summary, • Exercise, , 213
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214, , Let Us C, , s seen in the first chapter the primary data types could be of, three varieties—char, int, and float. It may seem odd to, many, how C programmers manage with such a tiny set of, data types. Fact is, the C programmers aren’t really deprived. They, can derive many data types from these three types. In fact, the, number of data types that can be derived in C, is in principle,, unlimited. A C programmer can always invent whatever data type, he needs., , A, , Not only this, the primary data types themselves could be of, several types. For example, a char could be an unsigned char or a, signed char. Or an int could be a short int or a long int., Sufficiently confusing? Well, let us take a closer look at these, variations of primary data types in this chapter., To fully define a variable one needs to mention not only its type, but also its storage class. In this chapter we would be exploring the, different storage classes and their relevance in C programming., , Integers, long and short, We had seen earlier that the range of an Integer constant depends, upon the compiler. For a 16-bit compiler like Turbo C or Turbo, C++ the range is –32768 to 32767. For a 32-bit compiler the range, would be –2147483648 to +2147483647. Here a 16-bit compiler, means that when it compiles a C program it generates machine, language code that is targeted towards working on a 16-bit, microprocessor like Intel 8086/8088. As against this, a 32-bit, compiler like VC++ generates machine language code that is, targeted towards a 32-bit microprocessor like Intel Pentium. Note, that this does not mean that a program compiled using Turbo C, would not work on 32-bit processor. It would run successfully but, at that time the 32-bit processor would work as if it were a 16-bit, processor. This happens because a 32-bit processor provides, support for programs compiled using 16-bit compilers. If this, backward compatibility support is not provided the 16-bit program
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215, , Chapter 6: Data Types Revisited, , would not run on it. This is precisely what happens on the new, Intel Itanium processors, which have withdrawn support for 16-bit, code., Remember that out of the two/four bytes used to store an integer,, the highest bit (16th/32nd bit) is used to store the sign of the integer., This bit is 1 if the number is negative, and 0 if the number is, positive., C offers a variation of the integer data type that provides what are, called short and long integer values. The intention of providing, these variations is to provide integers with different ranges, wherever possible. Though not a rule, short and long integers, would usually occupy two and four bytes respectively. Each, compiler can decide appropriate sizes depending on the operating, system and hardware for which it is being written, subject to the, following rules:, (a), (b), (c), (d), , shorts are at least 2 bytes big, longs are at least 4 bytes big, shorts are never bigger than ints, ints are never bigger than longs, , Figure 6.1 shows the sizes of different integers based upon the OS, used., , Compiler, , short, , int, , long, , 16-bit (Turbo C/C++), , 2, , 2, , 4, , 32-bit (Visual C++), , 2, , 4, , 4, , Figure 6.1, long variables which hold long integers are declared using the, keyword long, as in,
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216, , Let Us C, , long int i ;, long int abc ;, , long integers cause the program to run a bit slower, but the range, of values that we can use is expanded tremendously. The value of, a long integer typically can vary from -2147483648 to, +2147483647. More than this you should not need unless you are, taking a world census., If there are such things as longs, symmetry requires shorts as, well—integers that need less space in memory and thus help speed, up program execution. short integer variables are declared as,, short int j ;, short int height ;, , C allows the abbreviation of short int to short and of long int to, long. So the declarations made above can be written as,, long i ;, long abc ;, short j ;, short height ;, , Naturally, most C programmers prefer this short-cut., Sometimes we come across situations where the constant is small, enough to be an int, but still we want to give it as much storage as, a long. In such cases we add the suffix ‘L’ or ‘l’ at the end of the, number, as in 23L., , Integers, signed and unsigned, Sometimes, we know in advance that the value stored in a given, integer variable will always be positive—when it is being used to
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Chapter 6: Data Types Revisited, , 217, , only count things, for example. In such a case we can declare the, variable to be unsigned, as in,, unsigned int num_students ;, , With such a declaration, the range of permissible integer values, (for a 16-bit OS) will shift from the range -32768 to +32767 to the, range 0 to 65535. Thus, declaring an integer as unsigned almost, doubles the size of the largest possible value that it can otherwise, take. This so happens because on declaring the integer as, unsigned, the left-most bit is now free and is not used to store the, sign of the number. Note that an unsigned integer still occupies, two bytes. This is how an unsigned integer can be declared:, unsigned int i ;, unsigned i ;, , Like an unsigned int, there also exists a short unsigned int and a, long unsigned int. By default a short int is a signed short int and, a long int is a signed long int., , Chars, signed and unsigned, Parallel to signed and unsigned ints (either short or long),, similarly there also exist signed and unsigned chars, both, occupying one byte each, but having different ranges. To begin, with it might appear strange as to how a char can have a sign., Consider the statement, char ch = 'A' ;, , Here what gets stored in ch is the binary equivalent of the ASCII, value of ‘A’ (i.e. binary of 65). And if 65’s binary can be stored,, then -54’s binary can also be stored (in a signed char).
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218, , Let Us C, , A signed char is same as an ordinary char and has a range from, -128 to +127; whereas, an unsigned char has a range from 0 to, 255. Let us now see a program that illustrates this range:, main( ), {, char ch = 291 ;, printf ( "\n%d %c", ch, ch ) ;, }, , What output do you expect from this program? Possibly, 291 and, the character corresponding to it. Well, not really. Surprised? The, reason is that ch has been defined as a char, and a char cannot, take a value bigger than +127. Hence when value of ch exceeds, +127, an appropriate value from the other side of the range is, picked up and stored in ch. This value in our case happens to be, 35, hence 35 and its corresponding character #, gets printed out., Here is another program that would make the concept clearer., main( ), {, char ch ;, for ( ch = 0 ; ch <= 255 ; ch++ ), printf ( "\n%d %c", ch, ch ) ;, }, , This program should output ASCII values and their corresponding, characters. Well, No! This is an indefinite loop. The reason is that, ch has been defined as a char, and a char cannot take values, bigger than +127. Hence when value of ch is +127 and we perform, ch++ it becomes -128 instead of +128. -128 is less than 255 hence, the condition is still satisfied. Here onwards ch would take values, like -127, -126, -125, .... -2, -1, 0, +1, +2, ... +127, -128, -127, etc., Thus the value of ch would keep oscillating between -128 to +127,, thereby ensuring that the loop never gets terminated. How do you
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Chapter 6: Data Types Revisited, , 219, , overcome this difficulty? Would declaring ch as an unsigned char, solve the problem? Even this would not serve the purpose since, when ch reaches a value 255, ch++ would try to make it 256, which cannot be stored in an unsigned char. Thus the only, alternative is to declare ch as an int. However, if we are bent upon, writing the program using unsigned char, it can be done as shown, below. The program is definitely less elegant, but workable all the, same., main( ), {, unsigned char ch ;, for ( ch = 0 ; ch <= 254 ; ch++ ), printf ( "\n%d %c", ch, ch ) ;, printf ( "\n%d %c", ch, ch ) ;, }, , Floats and Doubles, A float occupies four bytes in memory and can range from -3.4e38, to +3.4e38. If this is insufficient then C offers a double data type, that occupies 8 bytes in memory and has a range from -1.7e308 to, +1.7e308. A variable of type double can be declared as,, double a, population ;, , If the situation demands usage of real numbers that lie even, beyond the range offered by double data type, then there exists a, long double that can range from -1.7e4932 to +1.7e4932. A long, double occupies 10 bytes in memory., You would see that most of the times in C programming one is, required to use either chars or ints and cases where floats,, doubles or long doubles would be used are indeed rare.
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220, , Let Us C, , Let us now write a program that puts to use all the data types that, we have learnt in this chapter. Go through the following program, carefully, which shows how to use these different data types. Note, the format specifiers used to input and output these data types., main( ), {, char c ;, unsigned char d ;, int i ;, unsigned int j ;, short int k ;, unsigned short int l ;, long int m ;, unsigned long int n ;, float x ;, double y ;, long double z ;, /* char */, scanf ( "%c %c", &c, &d ) ;, printf ( "%c %c", c, d ) ;, /* int */, scanf ( "%d %u", &i, &j ) ;, printf ( "%d %u", i, j ) ;, /* short int */, scanf ( "%d %u", &k, &l ) ;, printf ( "%d %u", k, l ) ;, /* long int */, scanf ( "%ld %lu", &m, &n ) ;, printf ( "%ld %lu", m, n ) ;, /* float, double, long double */, scanf ( "%f %lf %Lf", &x, &y, &z ) ;, printf ( "%f %lf %Lf", x, y, z ) ;
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221, , Chapter 6: Data Types Revisited, }, , The essence of all the data types that we have learnt so far has, been captured in Figure 6.2., , Data Type, , Range, , Bytes, , Format, , signed char, , -128 to + 127, , 1, , %c, , unsigned char, , 0 to 255, , 1, , %c, , short signed int, , -32768 to +32767, , 2, , %d, , short unsigned int, , 0 to 65535, , 2, , %u, , signed int, , -32768 to +32767, , 2, , %d, , unsigned int, , 0 to 65535, , 2, , %u, , long signed int, , -2147483648 to +2147483647, , 4, , %ld, , long unsigned int, , 0 to 4294967295, , 4, , %lu, , float, , -3.4e38 to +3.4e38, , 4, , %f, , double, , -1.7e308 to +1.7e308, , 8, , %lf, , long double, , -1.7e4932 to +1.7e4932, , 10, , %Lf, , Note: The sizes and ranges of int, short and long are compiler, dependent. Sizes in this figure are for 16-bit compiler., , Figure 6.2, , A Few More Issues…, Having seen all the variations of the primary types let us take a, look at some more related issues., (a) We saw earlier that size of an integer is compiler dependent., This is even true in case of chars and floats. Also, depending, upon the microprocessor for which the compiler targets its, code the accuracy of floating point calculations may change., For example, the result of 22.0/7.0 would be reported more
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222, , Let Us C, , accurately by VC++ compiler as compared to TC/TC++, compilers. This is because TC/TC++ targets its compiled code, to 8088/8086 (16-bit) microprocessors. Since these, microprocessors do not offer floating point support, TC/TC++, performs all float operations using a software piece called, Floating Point Emulator. This emulator has limitations and, hence produces less accurate results. Also, this emulator, becomes part of the EXE file, thereby increasing its size. In, addition to this increased size there is a performance penalty, since this bigger code would take more time to execute., (b) If you look at ranges of chars and ints there seems to be one, extra number on the negative side. This is because a negative, number is always stored as 2’s compliment of its binary. For, example, let us see how -128 is stored. Firstly, binary of 128, is calculated (10000000), then its 1’s compliment is obtained, (01111111). A 1’s compliment is obtained by changing all 0s, to 1s and 1s to 0s. Finally, 2’s compliment of this number, i.e., 10000000, gets stored. A 2’s compliment is obtained by, adding 1 to the 1’s compliment. Thus, for -128, 10000000, gets stored. This is an 8-bit number and it can be easily, accommodated in a char. As against this, +128 cannot be, stored in a char because its binary 010000000 (left-most 0 is, for positive sign) is a 9-bit number. However +127 can be, stored as its binary 01111111 turns out to be a 8-bit number., (c) What happens when we attempt to store +128 in a char? The, first number on the negative side, i.e. -128 gets stored. This is, because from the 9-bit binary of +128, 010000000, only the, right-most 8 bits get stored. But when 10000000 is stored the, left-most bit is 1 and it is treated as a sign bit. Thus the value, of the number becomes -128 since it is indeed the binary, of -128, as can be understood from (b) above. Similarly, you, can verify that an attempt to store +129 in a char results in, storing -127 in it. In general, if we exceed the range from, positive side we end up on the negative side. Vice versa is
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Chapter 6: Data Types Revisited, , 223, , also true. If we exceed the range from negative side we end up, on positive side., , Storage Classes in C, We have already said all that needs to be said about constants, but, we are not finished with variables. To fully define a variable one, needs to mention not only its ‘type’ but also its ‘storage class’. In, other words, not only do all variables have a data type, they also, have a ‘storage class’., We have not mentioned storage classes yet, though we have, written several programs in C. We were able to get away with this, because storage classes have defaults. If we don’t specify the, storage class of a variable in its declaration, the compiler will, assume a storage class depending on the context in which the, variable is used. Thus, variables have certain default storage, classes., From C compiler’s point of view, a variable name identifies some, physical location within the computer where the string of bits, representing the variable’s value is stored. There are basically two, kinds of locations in a computer where such a value may be kept—, Memory and CPU registers. It is the variable’s storage class that, determines in which of these two locations the value is stored., Moreover, a variable’s storage class tells us:, (a) Where the variable would be stored., (b) What will be the initial value of the variable, if initial value is, not specifically assigned.(i.e. the default initial value)., (c) What is the scope of the variable; i.e. in which functions the, value of the variable would be available., (d) What is the life of the variable; i.e. how long would the, variable exist.
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224, , Let Us C, , There are four storage classes in C:, (a), (b), (c), (d), , Automatic storage class, Register storage class, Static storage class, External storage class, , Let us examine these storage classes one by one., , Automatic Storage Class, The features of a variable defined to have an automatic storage, class are as under:, Storage, Default initial value, Scope, Life, , −, −, , Memory., An unpredictable value, which is often, called a garbage value., − Local to the block in which the variable, is defined., − Till the control remains within the block, in which the variable is defined., , Following program shows how an automatic storage class variable, is declared, and the fact that if the variable is not initialized it, contains a garbage value., main( ), {, auto int i, j ;, printf ( "\n%d %d", i, j ) ;, }, , The output of the above program could be..., 1211 221, , where, 1211 and 221 are garbage values of i and j. When you run, this program you may get different values, since garbage values
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Chapter 6: Data Types Revisited, , 225, , are unpredictable. So always make it a point that you initialize the, automatic variables properly, otherwise you are likely to get, unexpected results. Note that the keyword for this storage class is, auto, and not automatic., Scope and life of an automatic variable is illustrated in the, following program., main( ), {, auto int i = 1 ;, {, {, {, printf ( "\n%d ", i ) ;, }, printf ( "%d ", i ) ;, }, printf ( "%d", i ) ;, }, }, , The output of the above program is:, 111, , This is because, all printf( ) statements occur within the outermost, block (a block is all statements enclosed within a pair of braces) in, which i has been defined. It means the scope of i is local to the, block in which it is defined. The moment the control comes out of, the block in which the variable is defined, the variable and its, value is irretrievably lost. To catch my point, go through the, following program., main( ), {, auto int i = 1 ;, {
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226, , Let Us C, auto int i = 2 ;, {, auto int i = 3 ;, printf ( "\n%d ", i ) ;, }, printf ( "%d ", i ) ;, , }, printf ( "%d", i ) ;, }, , The output of the above program would be:, 321, , Note that the Compiler treats the three i’s as totally different, variables, since they are defined in different blocks. Once the, control comes out of the innermost block the variable i with value, 3 is lost, and hence the i in the second printf( ) refers to i with, value 2. Similarly, when the control comes out of the next, innermost block, the third printf( ) refers to the i with value 1., Understand the concept of life and scope of an automatic storage, class variable thoroughly before proceeding with the next storage, class., , Register Storage Class, The features of a variable defined to be of register storage class, are as under:, Storage, Default initial value, Scope, , -, , Life, , -, , -, , CPU registers., Garbage value., Local to the block in which the variable, is defined., Till the control remains within the block, in which the variable is defined.
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Chapter 6: Data Types Revisited, , 227, , A value stored in a CPU register can always be accessed faster, than the one that is stored in memory. Therefore, if a variable is, used at many places in a program it is better to declare its storage, class as register. A good example of frequently used variables is, loop counters. We can name their storage class as register., main( ), {, register int i ;, for ( i = 1 ; i <= 10 ; i++ ), printf ( "\n%d", i ) ;, }, , Here, even though we have declared the storage class of i as, register, we cannot say for sure that the value of i would be stored, in a CPU register. Why? Because the number of CPU registers are, limited, and they may be busy doing some other task. What, happens in such an event... the variable works as if its storage class, is auto., Not every type of variable can be stored in a CPU register., For example, if the microprocessor has 16-bit registers then they, cannot hold a float value or a double value, which require 4 and 8, bytes respectively. However, if you use the register storage class, for a float or a double variable you won’t get any error messages., All that would happen is the compiler would treat the variables to, be of auto storage class., , Static Storage Class, The features of a variable defined to have a static storage class are, as under:, Storage, Default initial value, , −, −, , Memory., Zero.
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228, , Let Us C, −, , Scope, , Local to the block in which the variable, is defined., − Value of the variable persists between, different function calls., , Life, , Compare the two programs and their output given in Figure 6.3 to, understand the difference between the automatic and static, storage classes., main( ), {, increment( ) ;, increment( ) ;, increment( ) ;, }, , main( ), {, increment( ) ;, increment( ) ;, increment( ) ;, }, , increment( ), increment( ), {, {, auto int i = 1 ;, static int i = 1 ;, printf ( "%d\n", i ) ;, printf ( "%d\n", i ) ;, i=i+1;, i=i+1;, }, }, The output of the above programs would be:, 1, 1, 1, , 1, 2, 3, , Figure 6.3, The programs above consist of two functions main( ) and, increment( ). The function increment( ) gets called from main( ), thrice. Each time it increments the value of i and prints it. The only, difference in the two programs is that one uses an auto storage, class for variable i, whereas the other uses static storage class.
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Chapter 6: Data Types Revisited, , 229, , Like auto variables, static variables are also local to the block in, which they are declared. The difference between them is that static, variables don’t disappear when the function is no longer active., Their values persist. If the control comes back to the same function, again the static variables have the same values they had last time, around., In the above example, when variable i is auto, each time, increment( ) is called it is re-initialized to one. When the function, terminates, i vanishes and its new value of 2 is lost. The result: no, matter how many times we call increment( ), i is initialized to 1, every time., On the other hand, if i is static, it is initialized to 1 only once. It is, never initialized again. During the first call to increment( ), i is, incremented to 2. Because i is static, this value persists. The next, time increment( ) is called, i is not re-initialized to 1; on the, contrary its old value 2 is still available. This current value of i, (i.e. 2) gets printed and then i = i + 1 adds 1 to i to get a value of 3., When increment( ) is called the third time, the current value of i, (i.e. 3) gets printed and once again i is incremented. In short, if the, storage class is static then the statement static int i = 1 is executed, only once, irrespective of how many times the same function is, called., Consider one more program., main( ), {, int *j ;, int * fun( ) ;, j = fun( ) ;, printf ( "\n%d", *j ) ;, }, int *fun( ), {
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230, , Let Us C, , int k = 35 ;, return ( &k ) ;, }, , Here we are returning an address of k from fun( ) and collecting it, in j. Thus j becomes pointer to k. Then using this pointer we are, printing the value of k. This correctly prints out 35. Now try, calling any function (even printf( ) ) immediately after the call to, fun( ). This time printf( ) prints a garbage value. Why does this, happen? In the first case, when the control returned from fun( ), though k went dead it was still left on the stack. We then accessed, this value using its address that was collected in j. But when we, precede the call to printf( ) by a call to any other function, the, stack is now changed, hence we get the garbage value. If we want, to get the correct value each time then we must declare k as static., By doing this when the control returns from fun( ), k would not, die., All this having been said, a word of advice—avoid using static, variables unless you really need them. Because their values are, kept in memory when the variables are not active, which means, they take up space in memory that could otherwise be used by, other variables., , External Storage Class, The features of a variable whose storage class has been defined as, external are as follows:, Storage, Default initial value, Scope, Life, , −, −, −, −, , Memory., Zero., Global., As long as the program’s execution, doesn’t come to an end.
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Chapter 6: Data Types Revisited, , 231, , External variables differ from those we have already discussed in, that their scope is global, not local. External variables are declared, outside all functions, yet are available to all functions that care to, use them. Here is an example to illustrate this fact., int i ;, main( ), {, printf ( "\ni = %d", i ) ;, increment( ) ;, increment( ) ;, decrement( ) ;, decrement( ) ;, }, increment( ), {, i=i+1;, printf ( "\non incrementing i = %d", i ) ;, }, decrement( ), {, i=i-1;, printf ( "\non decrementing i = %d", i ) ;, }, , The output would be:, i=0, on incrementing i = 1, on incrementing i = 2, on decrementing i = 1, on decrementing i = 0
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232, , Let Us C, , As is obvious from the above output, the value of i is available to, the functions increment( ) and decrement( ) since i has been, declared outside all functions., Look at the following program., int x = 21 ;, main( ), {, extern int y ;, printf ( "\n%d %d", x, y ) ;, }, int y = 31 ;, , Here, x and y both are global variables. Since both of them have, been defined outside all the functions both enjoy external storage, class. Note the difference between the following:, extern int y ;, int y = 31 ;, , Here the first statement is a declaration, whereas the second is the, definition. When we declare a variable no space is reserved for it,, whereas, when we define it space gets reserved for it in memory., We had to declare y since it is being used in printf( ) before it’s, definition is encountered. There was no need to declare x since its, definition is done before its usage. Also remember that a variable, can be declared several times but can be defined only once., Another small issue—what will be the output of the following, program?, int x = 10 ;, main( ), {, int x = 20 ;, printf ( "\n%d", x ) ;
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Chapter 6: Data Types Revisited, , 233, , display( ) ;, }, display( ), {, printf ( "\n%d", x ) ;, }, , Here x is defined at two places, once outside main( ) and once, inside it. When the control reaches the printf( ) in main( ) which x, gets printed? Whenever such a conflict arises, it’s the local, variable that gets preference over the global variable. Hence the, printf( ) outputs 20. When display( ) is called and control reaches, the printf( ) there is no such conflict. Hence this time the value of, the global x, i.e. 10 gets printed., One last thing—a static variable can also be declared outside all, the functions. For all practical purposes it will be treated as an, extern variable. However, the scope of this variable is limited to, the same file in which it is declared. This means that the variable, would not be available to any function that is defined in a file other, than the file in which the variable is defined., , Which to Use When, Dennis Ritchie has made available to the C programmer a number, of storage classes with varying features, believing that the, programmer is in a best position to decide which one of these, storage classes is to be used when. We can make a few ground, rules for usage of different storage classes in different, programming situations with a view to:, (a) economise the memory space consumed by the variables, (b) improve the speed of execution of the program, The rules are as under:
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234, , Let Us C, , −, , Use static storage class only if you want the value of a, variable to persist between different function calls., , −, , Use register storage class for only those variables that are, being used very often in a program. Reason is, there are very, few CPU registers at our disposal and many of them might be, busy doing something else. Make careful utilization of the, scarce resources. A typical application of register storage class, is loop counters, which get used a number of times in a, program., , −, , Use extern storage class for only those variables that are being, used by almost all the functions in the program. This would, avoid unnecessary passing of these variables as arguments, when making a function call. Declaring all the variables as, extern would amount to a lot of wastage of memory space, because these variables would remain active throughout the, life of the program., , −, , If you don’t have any of the express needs mentioned above,, then use the auto storage class. In fact most of the times we, end up using the auto variables, because often it so happens, that once we have used the variables in a function we don’t, mind loosing them., , Summary, (a) We can use different variations of the primary data types,, namely signed and unsigned char, long and short int, float,, double and long double. There are different format, specifications for all these data types when they are used in, scanf( ) and printf( ) functions., (b) The maximum value a variable can hold depends upon the, number of bytes it occupies in memory., (c) By default all the variables are signed. We can declare a, variable as unsigned to accommodate greater value without, increasing the bytes occupied.
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Chapter 6: Data Types Revisited, , 235, , (d) We can make use of proper storage classes like auto,, register, static and extern to control four properties of the, variable—storage, default initial value, scope and life., , Exercise, [A] What would be the output of the following programs:, (a) main( ), {, int i ;, for ( i = 0 ; i <= 50000 ; i++ ), printf ( "\n%d", i ) ;, }, (b) main( ), {, float a = 13.5 ;, double b = 13.5 ;, printf ( "\n%f %lf", a, b ) ;, }, (c), , int i = 0 ;, main( ), {, printf ( "\nmain's i = %d", i ) ;, i++ ;, val( ) ;, printf ( "\nmain's i = %d", i ) ;, val( ) ;, }, val( ), {, i = 100 ;, printf ( "\nval's i = %d", i ) ;, i++ ;, }
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236, (d) main( ), {, int x, y, s = 2 ;, s *= 3 ;, y=f(s);, x=g(s);, printf ( "\n%d %d %d", s, y, x ) ;, }, int t = 8 ;, f ( int a ), {, a += -5 ;, t -= 4 ;, return ( a + t ) ;, }, g ( int a ), {, a=1;, t += a ;, return ( a + t ) ;, }, (e) main( ), {, static int count = 5 ;, printf ( "\ncount = %d", count-- ) ;, if ( count != 0 ), main( ) ;, }, (f), , main( ), {, int i, j ;, for ( i = 1 ; i < 5 ; i++ ), {, j=g(i);, printf ( "\n%d", j ) ;, }, , Let Us C
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Chapter 6: Data Types Revisited, }, g ( int x ), {, static int v = 1 ;, int b = 3 ;, v += x ;, return ( v + x + b ) ;, }, (g) float x = 4.5 ;, main( ), {, float y, float f ( float ) ;, x *= 2.0 ;, y=f(x);, printf ( "\n%f %f", x, y ) ;, }, float f ( float a ), {, a += 1.3 ;, x -= 4.5 ;, return ( a + x ) ;, }, (h) main( ), {, func( ) ;, func( ) ;, }, func( ), {, auto int i = 0 ;, register int j = 0 ;, static int k = 0 ;, i++ ; j++ ; k++ ;, printf ( "\n %d % d %d", i, j, k ) ;, }, , 237
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238, (i), , Let Us C, , int x = 10 ;, main( ), {, int x = 20 ;, {, int x = 30 ;, printf ( "\n%d", x ) ;, }, printf ("\n%d", x ) ;, }, , [B] Point out the errors, if any, in the following programs:, (a) main( ), {, long num ;, num = 2 ;, printf ( "\n%ld", num ) ;, }, (b) main( ), {, char ch = 200 ;, printf ( "\n%d", ch ) ;, }, (c), , main( ), {, unsigned a = 25 ;, long unsigned b = 25l ;, printf ( "\n%lu %u", a, b ) ;, }, , (d) main( ), {, long float a = 25.345e454 ;, unsigned double b = 25 ;, printf ( "\n%lf %d", a, b ) ;
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Chapter 6: Data Types Revisited, , 239, , }, (e) main( ), {, float a = 25.345 ;, float *b ;, b = &a ;, printf ( "\n%f %u", a, b ) ;, }, (f), , static int y ;, main( ), {, static int z ;, printf ("%d %d", y, z ) ;, }, , [C] State whether the following statements are True or False:, , (a) Storage for a register storage class variable is allocated, each time the control reaches the block in which it is, present., (b) An extern storage class variable is not available to the, functions that precede its definition, unless the variable is, explicitly declared in these functions., (c) The value of an automatic storage class variable persists, between various function invocations., (d) If the CPU registers are not available, the register storage, class variables are treated as static storage class variables., (e) The register storage class variables cannot hold float, values., (f) If we try to use register storage class for a float variable, the compiler will flash an error message.
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240, , Let Us C, , (g) If the variable x is defined as extern and a variable x is, also defined as a local variable of some function, then the, global variable gets preference over the local variable., (h) The default value for automatic variable is zero., (i) The life of static variable is till the control remains within, the block in which it is defined., (j) If a global variable is to be defined, then the extern, keyword is necessary in its declaration., (k) The address of register variable is not accessible., [D] Following program calculates the sum of digits of the number, 12345. Go through it and find out why is it necessary to, declare the storage class of the variable sum as static., main( ), {, int a ;, a = sumdig ( 12345 ) ;, printf ( "\n%d", a ) ;, }, sumdig ( int num ), {, static int sum ;, int a, b ;, a = num % 10 ;, b = ( num - a ) / 10 ;, sum = sum + a ;, if ( b != 0 ), sumdig ( b ) ;, else, return ( sum ) ;, }
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7, , The C Preprocessor, • Features of C Preprocessor, • Macro Expansion, Macros with Arguments, Macros versus Functions, • File Inclusion, • Conditional Compilation, • #if and #elif Directives, • Miscellaneous Directives, #undef Directive, #pragma Directive, • Summary, • Exercise, , 241
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242, , Let Us C, , T, , he C preprocessor is exactly what its name implies. It is a, program that processes our source program before it is, passed to the compiler. Preprocessor commands (often, known as directives) form what can almost be considered a, language within C language. We can certainly write C programs, without knowing anything about the preprocessor or its facilities., But preprocessor is such a great convenience that virtually all C, programmers rely on it. This chapter explores the preprocessor, directives and discusses the pros and cons of using them in, programs., , Features of C Preprocessor, There are several steps involved from the stage of writing a C, program to the stage of getting it executed. Figure 7.1 shows these, different steps along with the files created during each stage. You, can observe from the figure that our program passes through, several processors before it is ready to be executed. The input and, output to each of these processors is shown in Figure 7.2., Note that if the source code is stored in a file PR1.C then the, expanded source code gets stored in a file PR1.I. When this, expanded source code is compiled the object code gets stored in, PR1.OBJ. When this object code is linked with the object code of, library functions the resultant executable code gets stored in, PR1.EXE., The preprocessor offers several features called preprocessor, directives. Each of these preprocessor directives begin with a #, symbol. The directives can be placed anywhere in a program but, are most often placed at the beginning of a program, before the, first function definition. We would learn the following, preprocessor directives here:, (a) Macro expansion, (b) File inclusion
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243, , Chapter 7: The C Preprocessor, , Hand written program, Text editor, C Source code (PR1.C), Preprocessor, Expanded source code (PR1.I), Compiler, Object code (PR1.OBJ), Linker, Executable code (PR1.EXE), , Figure 7.1, Processor, , Input, , Output, , Editor, , Program typed from, keyboard, , C source code containing, program and preprocessor, commands, , Preprocessor, , C source code file, , Source code file with the, preprocessing commands, properly sorted out, , Compiler, , Source code file with, preprocessing commands, sorted out, , Relocatable object code, , Linker, , Relocatable object code, and the standard C, library functions, , Executable code in, machine language, , Figure 7.2
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244, , Let Us C, , (c) Conditional Compilation, (d) Miscellaneous directives, Let us understand these features of preprocessor one by one., , Macro Expansion, Have a look at the following program., #define UPPER 25, main( ), {, int i ;, for ( i = 1 ; i <= UPPER ; i++ ), printf ( "\n%d", i ) ;, }, , In this program instead of writing 25 in the for loop we are writing, it in the form of UPPER, which has already been defined before, main( ) through the statement,, #define UPPER 25, , This statement is called ‘macro definition’ or more commonly, just, a ‘macro’. What purpose does it serve? During preprocessing, the, preprocessor replaces every occurrence of UPPER in the program, with 25. Here is another example of macro definition., #define PI 3.1415, main( ), {, float r = 6.25 ;, float area ;, area = PI * r * r ;, printf ( "\nArea of circle = %f", area ) ;, }
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Chapter 7: The C Preprocessor, , 245, , UPPER and PI in the above programs are often called ‘macro, templates’, whereas, 25 and 3.1415 are called their corresponding, ‘macro expansions’., When we compile the program, before the source code passes to, the compiler it is examined by the C preprocessor for any macro, definitions. When it sees the #define directive, it goes through the, entire program in search of the macro templates; wherever it finds, one, it replaces the macro template with the appropriate macro, expansion. Only after this procedure has been completed is the, program handed over to the compiler., In C programming it is customary to use capital letters for macro, template. This makes it easy for programmers to pick out all the, macro templates when reading through the program., Note that a macro template and its macro expansion are separated, by blanks or tabs. A space between # and define is optional., Remember that a macro definition is never to be terminated by a, semicolon., And now a million dollar question... why use #define in the above, programs? What have we gained by substituting PI for 3.1415 in, our program? Probably, we have made the program easier to read., Even though 3.1415 is such a common constant that it is easily, recognizable, there are many instances where a constant doesn’t, reveal its purpose so readily. For example, if the phrase “\x1B[2J”, causes the screen to clear. But which would you find easier to, understand in the middle of your program “\x1B[2J” or, “CLEARSCREEN”? Thus, we would use the macro definition, #define CLEARSCREEN "\x1B[2J", , Then wherever CLEARSCREEN appears in the program it would, automatically be replaced by “\x1B[2J” before compilation begins.
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246, , Let Us C, , There is perhaps a more important reason for using macro, definition than mere readability. Suppose a constant like 3.1415, appears many times in your program. This value may have to be, changed some day to 3.141592. Ordinarily, you would need to go, through the program and manually change each occurrence of the, constant. However, if you have defined PI in a #define directive,, you only need to make one change, in the #define directive itself:, #define PI 3.141592, , Beyond this the change will be made automatically to all, occurrences of PI before the beginning of compilation., In short, it is nice to know that you would be able to change values, of a constant at all the places in the program by just making a, change in the #define directive. This convenience may not matter, for small programs shown above, but with large programs macro, definitions are almost indispensable., But the same purpose could have been served had we used a, variable pi instead of a macro template PI. A variable could also, have provided a meaningful name for a constant and permitted one, change to effect many occurrences of the constant. It’s true that a, variable can be used in this way. Then, why not use it? For three, reasons it’s a bad idea., Firstly, it is inefficient, since the compiler can generate faster and, more compact code for constants than it can for variables., Secondly, using a variable for what is really a constant encourages, sloppy thinking and makes the program more difficult to, understand: if something never changes, it is hard to imagine it as, a variable. And thirdly, there is always a danger that the variable, may inadvertently get altered somewhere in the program. So it’s, no longer a constant that you think it is.
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Chapter 7: The C Preprocessor, , 247, , Thus, using #define can produce more efficient and more easily, understandable programs. This directive is used extensively by C, programmers, as you will see in many programs in this book., Following three examples show places where a #define directive is, popularly used by C programmers., A #define directive is many a times used to define operators as, shown below., #define AND &&, #define OR ||, main( ), {, int f = 1, x = 4, y = 90 ;, if ( ( f < 5 ) AND ( x <= 20 OR y <= 45 ) ), printf ( "\nYour PC will always work fine..." ) ;, else, printf ( "\nIn front of the maintenance man" ) ;, }, , A #define directive could be used even to replace a condition, as, shown below., #define AND &&, #define ARANGE ( a > 25 AND a < 50 ), main( ), {, int a = 30 ;, if ( ARANGE ), printf ( "within range" ) ;, else, printf ( "out of range" ) ;, }
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248, , Let Us C, , A #define directive could be used to replace even an entire C, statement. This is shown below., #define FOUND printf ( "The Yankee Doodle Virus" ) ;, main( ), {, char signature ;, if ( signature == 'Y' ), FOUND, else, printf ( "Safe... as yet !" ) ;, }, , Macros with Arguments, The macros that we have used so far are called simple macros., Macros can have arguments, just as functions can. Here is an, example that illustrates this fact., #define AREA(x) ( 3.14 * x * x ), main( ), {, float r1 = 6.25, r2 = 2.5, a ;, a = AREA ( r1 ) ;, printf ( "\nArea of circle = %f", a ) ;, a = AREA ( r2 ) ;, printf ( "\nArea of circle = %f", a ) ;, }, , Here’s the output of the program..., Area of circle = 122.656250, Area of circle = 19.625000
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Chapter 7: The C Preprocessor, , 249, , In this program wherever the preprocessor finds the phrase, AREA(x) it expands it into the statement ( 3.14 * x * x )., However, that’s not all that it does. The x in the macro template, AREA(x) is an argument that matches the x in the macro, expansion ( 3.14 * x * x ). The statement AREA(r1) in the, program causes the variable r1 to be substituted for x. Thus the, statement AREA(r1) is equivalent to:, ( 3.14 * r1 * r1 ), , After the above source code has passed through the preprocessor,, what the compiler gets to work on will be this:, main( ), {, float r1 = 6.25, r2 = 2.5, a ;, a = 3.14 * r1 *r1 ;, printf ( "Area of circle = %f\n", a ) ;, a = 3.14 *r2 * r2 ;, printf ( "Area of circle = %f", a ) ;, }, , Here is another example of macros with arguments:, #define ISDIGIT(y) ( y >= 48 && y <= 57 ), main( ), {, char ch ;, printf ( "Enter any digit " ) ;, scanf ( "%c", &ch ) ;, if ( ISDIGIT ( ch ) ), printf ( "\nYou entered a digit" ) ;, else, printf ( "\nIllegal input" ) ;
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250, , Let Us C, , }, , Here are some important points to remember while writing macros, with arguments:, (a) Be careful not to leave a blank between the macro template, and its argument while defining the macro. For example, there, should be no blank between AREA and (x) in the definition,, #define AREA(x) ( 3.14 * x * x ), If we were to write AREA (x) instead of AREA(x), the (x), would become a part of macro expansion, which we certainly, don’t want. What would happen is, the template would be, expanded to, ( r1 ) ( 3.14 * r1 * r1 ), , which won’t run. Not at all what we wanted., (b) The entire macro expansion should be enclosed within, parentheses. Here is an example of what would happen if we, fail to enclose the macro expansion within parentheses., #define SQUARE(n) n * n, main( ), {, int j ;, j = 64 / SQUARE ( 4 ) ;, printf ( "j = %d", j ) ;, }, , The output of the above program would be:, j = 64, , whereas, what we expected was j = 4.
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Chapter 7: The C Preprocessor, , 251, , What went wrong? The macro was expanded into, j = 64 / 4 * 4 ;, , which yielded 64., (c) Macros can be split into multiple lines, with a ‘\’ (back slash), present at the end of each line. Following program shows how, we can define and use multiple line macros., #define HLINE for ( i = 0 ; i < 79 ; i++ ) \, printf ( "%c", 196 ) ;, #define VLINE( X, Y ) {\, gotoxy ( X, Y ) ; \, printf ( "%c", 179 ) ; \, }, main( ), {, int i, y ;, clrscr( ) ;, gotoxy ( 1, 12 ) ;, HLINE, for ( y = 1 ; y < 25 ; y++ ), VLINE ( 39, y ) ;, }, , This program draws a vertical and a horizontal line in the, center of the screen., (d) If for any reason you are unable to debug a macro then you, should view the expanded code of the program to see how the, macros are getting expanded. If your source code is present in, the file PR1.C then the expanded source code would be stored
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252, , Let Us C, , in PR1.I. You need to generate this file at the command, prompt by saying:, cpp pr1.c, , Here CPP stands for C PreProcessor. It generates the, expanded source code and stores it in a file called PR1.I. You, can now open this file and see the expanded source code., Note that the file PR1.I gets generated in C:\TC\BIN, directory. The procedure for generating expanded source code, for compilers other than Turbo C/C++ might be a little, different., , Macros versus Functions, In the above example a macro was used to calculate the area of the, circle. As we know, even a function can be written to calculate the, area of the circle. Though macro calls are ‘like’ function calls, they, are not really the same things. Then what is the difference between, the two?, In a macro call the preprocessor replaces the macro template with, its macro expansion, in a stupid, unthinking, literal way. As, against this, in a function call the control is passed to a function, along with certain arguments, some calculations are performed in, the function and a useful value is returned back from the function., This brings us to a question: when is it best to use macros with, arguments and when is it better to use a function? Usually macros, make the program run faster but increase the program size,, whereas functions make the program smaller and compact., If we use a macro hundred times in a program, the macro, expansion goes into our source code at hundred different places,, thus increasing the program size. On the other hand, if a function, is used, then even if it is called from hundred different places in
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Chapter 7: The C Preprocessor, , 253, , the program, it would take the same amount of space in the, program., But passing arguments to a function and getting back the returned, value does take time and would therefore slow down the program., This gets avoided with macros since they have already been, expanded and placed in the source code before compilation., Moral of the story is—if the macro is simple and sweet like in our, examples, it makes nice shorthand and avoids the overheads, associated with function calls. On the other hand, if we have a, fairly large macro and it is used fairly often, perhaps we ought to, replace it with a function., , File Inclusion, The second preprocessor directive we’ll explore in this chapter is, file inclusion. This directive causes one file to be included in, another. The preprocessor command for file inclusion looks like, this:, #include "filename", , and it simply causes the entire contents of filename to be inserted, into the source code at that point in the program. Of course this, presumes that the file being included is existing. When and why, this feature is used? It can be used in two cases:, (a) If we have a very large program, the code is best divided into, several different files, each containing a set of related, functions. It is a good programming practice to keep different, sections of a large program separate. These files are, #included at the beginning of main program file., (b) There are some functions and some macro definitions that we, need almost in all programs that we write. These commonly
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254, , Let Us C, , needed functions and macro definitions can be stored in a file,, and that file can be included in every program we write,, which would add all the statements in this file to our program, as if we have typed them in., It is common for the files that are to be included to have a .h, extension. This extension stands for ‘header file’, possibly because, it contains statements which when included go to the head of your, program. The prototypes of all the library functions are grouped, into different categories and then stored in different header files., For example prototypes of all mathematics related functions are, stored in the header file ‘math.h’, prototypes of console, input/output functions are stored in the header file ‘conio.h’, and, so on., Actually there exist two ways to write #include statement. These, are:, #include "filename", #include <filename>, , The meaning of each of these forms is given below:, #include "goto.c", , This command would look for the file goto.c, in the current directory as well as the, specified list of directories as mentioned in, the include search path that might have been, set up., , #include <goto.c>, , This command would look for the file goto.c, in the specified list of directories only., , The include search path is nothing but a list of directories that, would be searched for the file being included. Different C, compilers let you set the search path in different manners. If you, are using Turbo C/C++ compiler then the search path can be set up, by selecting ‘Directories’ from the ‘Options’ menu. On doing this
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Chapter 7: The C Preprocessor, , 255, , a dialog box appears. In this dialog box against ‘Include, Directories’ we can specify the search path. We can also specify, multiple include paths separated by ‘;’ (semicolon) as shown, below:, c:\tc\lib ; c:\mylib ; d:\libfiles, , The path can contain maximum of 127 characters. Both relative, and absolute paths are valid. For example ‘..\dir\incfiles’ is a valid, path., , Conditional Compilation, We can, if we want, have the compiler skip over part of a source, code by inserting the preprocessing commands #ifdef and #endif,, which have the general form:, #ifdef macroname, statement 1 ;, statement 2 ;, statement 3 ;, #endif, , If macroname has been #defined, the block of code will be, processed as usual; otherwise not., Where would #ifdef be useful? When would you like to compile, only a part of your program? In three cases:, (a) To “comment out” obsolete lines of code. It often happens, that a program is changed at the last minute to satisfy a client., This involves rewriting some part of source code to the, client’s satisfaction and deleting the old code. But veteran, programmers are familiar with the clients who change their, mind and want the old code back again just the way it was.
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256, , Let Us C, , Now you would definitely not like to retype the deleted code, again., One solution in such a situation is to put the old code within a, pair of /*, */ combination. But we might have already, written a comment in the code that we are about to “comment, out”. This would mean we end up with nested comments., Obviously, this solution won’t work since we can’t nest, comments in C., Therefore the solution is to use conditional compilation as, shown below., main( ), {, #ifdef OKAY, statement 1 ;, statement 2 ; /* detects virus */, statement 3 ;, statement 4 ; /* specific to stone virus */, #endif, statement 5 ;, statement 6 ;, statement 7 ;, }, , Here, statements 1, 2, 3 and 4 would get compiled only if the, macro OKAY has been defined, and we have purposefully, omitted the definition of the macro OKAY. At a later date, if, we want that these statements should also get compiled all, that we are required to do is to delete the #ifdef and #endif, statements., (b) A more sophisticated use of #ifdef has to do with making the, programs portable, i.e. to make them work on two totally, different computers. Suppose an organization has two
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Chapter 7: The C Preprocessor, , 257, , different types of computers and you are expected to write a, program that works on both the machines. You can do so by, isolating the lines of code that must be different for each, machine by marking them off with #ifdef. For example:, main( ), {, #ifdef INTEL, code suitable for a Intel PC, #else, code suitable for a Motorola PC, #endif, code common to both the computers, }, , When you compile this program it would compile only the, code suitable for a Intel PC and the common code. This is, because the macro INTEL has not been defined. Note that the, working of #ifdef - #else - #endif is similar to the ordinary if else control instruction of C., If you want to run your program on a Motorola PC, just add a, statement at the top saying,, #define INTEL, , Sometimes, instead of #ifdef the #ifndef directive is used., The #ifndef (which means ‘if not defined’) works exactly, opposite to #ifdef. The above example if written using, #ifndef, would look like this:, main( ), {, #ifndef INTEL, code suitable for a Intel PC, #else, code suitable for a Motorola PC
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258, , Let Us C, #endif, code common to both the computers, , }, , (c) Suppose a function myfunc( ) is defined in a file ‘myfile.h’, which is #included in a file ‘myfile1.h’. Now in your program, file if you #include both ‘myfile.h’ and ‘myfile1.h’ the, compiler flashes an error ‘Multiple declaration for myfunc’., This is because the same file ‘myfile.h’ gets included twice., To avoid this we can write following code in the header file., /* myfile.h */, #ifndef __myfile_h, #define __myfile_h, myfunc( ), {, /* some code */, }, #endif, , First time the file ‘myfile.h’ gets included the preprocessor, checks whether a macro called __myfile_h has been defined, or not. If it has not been then it gets defined and the rest of the, code gets included. Next time we attempt to include the same, file, the inclusion is prevented since __myfile_h already, stands defined. Note that there is nothing special about, __myfile_h. In its place we can use any other macro as well., , #if and #elif Directives, The #if directive can be used to test whether an expression, evaluates to a nonzero value or not. If the result of the expression, is nonzero, then subsequent lines upto a #else, #elif or #endif are, compiled, otherwise they are skipped.
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Chapter 7: The C Preprocessor, , 259, , A simple example of #if directive is shown below:, main( ), {, #if TEST <= 5, statement 1 ;, statement 2 ;, statement 3 ;, #else, statement 4 ;, statement 5 ;, statement 6 ;, #endif, }, , If the expression, TEST <= 5 evaluates to true then statements 1, 2, and 3 are compiled otherwise statements 4, 5 and 6 are compiled., In place of the expression TEST <= 5 other expressions like, ( LEVEL == HIGH || LEVEL == LOW ) or ADAPTER ==, CGA can also be used., If we so desire we can have nested conditional compilation, directives. An example that uses such directives is shown below., #if ADAPTER == VGA, code for video graphics array, #else, #if ADAPTER == SVGA, code for super video graphics array, #else, code for extended graphics adapter, #endif, #endif, , The above program segment can be made more compact by using, another conditional compilation directive called #elif. The same, program using this directive can be rewritten as shown below.
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260, , Let Us C, , Observe that by using the #elif directives the number of #endifs, used in the program get reduced., #if ADAPTER == VGA, code for video graphics array, #elif ADAPTER == SVGA, code for super video graphics array, #else, code for extended graphics adapter, #endif, , Miscellaneous Directives, There are two more preprocessor directives available, though they, are not very commonly used. They are:, (a) #undef, (b) #pragma, , #undef Directive, On some occasions it may be desirable to cause a defined name to, become ‘undefined’. This can be accomplished by means of the, #undef directive. In order to undefine a macro that has been earlier, #defined, the directive,, #undef macro template, , can be used. Thus the statement,, #undef PENTIUM, , would cause the definition of PENTIUM to be removed from the, system. All subsequent #ifdef PENTIUM statements would, evaluate to false. In practice seldom are you required to undefine a, macro, but for some reason if you are required to, then you know, that there is something to fall back upon.
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Chapter 7: The C Preprocessor, , 261, , #pragma Directive, This directive is another special-purpose directive that you can use, to turn on or off certain features. Pragmas vary from one compiler, to another. There are certain pragmas available with Microsoft C, compiler that deal with formatting source listings and placing, comments in the object file generated by the compiler. Turbo, C/C++ compiler has got a pragma that allows you to suppress, warnings generated by the compiler. Some of these pragmas are, discussed below., (a) #pragma startup and #pragma exit: These directives allow, us to specify functions that are called upon program startup, (before main( )) or program exit (just before the program, terminates). Their usage is as follows:, void fun1( ) ;, void fun2( ) ;, #pragma startup fun1, #pragma exit fun2, main( ), {, printf ( "\nInside maim" ) ;, }, void fun1( ), {, printf ( "\nInside fun1" ) ;, }, void fun2( ), {, printf ( "\nInside fun2" ) ;, }
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262, , Let Us C, , And here is the output of the program., Inside fun1, Inside main, Inside fun2, , Note that the functions fun1( ) and fun2( ) should neither, receive nor return any value. If we want two functions to get, executed at startup then their pragmas should be defined in, the reverse order in which you want to get them called., (b) #pragma warn: This directive tells the compiler whether or, not we want to suppress a specific warning. Usage of this, pragma is shown below., #pragma warn –rvl /* return value */, #pragma warn –par /* parameter not used */, #pragma warn –rch /* unreachable code */, int f1( ), {, int a = 5 ;, }, void f2 ( int x ), {, printf ( "\nInside f2" ) ;, }, int f3( ), {, int x = 6 ;, return x ;, x++ ;, }, void main( )
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Chapter 7: The C Preprocessor, , 263, , {, f1( ) ;, f2 ( 7 ) ;, f3( ) ;, }, , If you go through the program you can notice three problems, immediately. These are:, (a) Though promised, f1( ) doesn’t return a value., (b) The parameter x that is passed to f2( ) is not being used, anywhere in f2( )., (c) The control can never reach x++ in f3( )., If we compile the program we should expect warnings, indicating the above problems. However, this does not happen, since we have suppressed the warnings using the #pragma, directives. If we replace the ‘–’ sign with a ‘+’ then these, warnings would be flashed on compilation. Though it is a bad, practice to suppress warnings, at times it becomes useful to, suppress them. For example, if you have written a huge, program and are trying to compile it, then to begin with you, are more interested in locating the errors, rather than the, warnings. At such times you may suppress the warnings., Once you have located all errors, then you may turn on the, warnings and sort them out., , Summary, (a) The preprocessor directives enable the programmer to write, programs that are easy to develop, read, modify and transport, to a different computer system.
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264, , Let Us C, , (b) We can make use of various preprocessor directives such as, #define, #include, #ifdef - #else - #endif, #if and #elif in our, program., (c) The directives like #undef and #pragma are also useful, although they are seldom used., , Exercise, [A] Answer the following:, , (a) What is a preprocessor directive, 1., 2., 3., 4., , a message from compiler to the programmer, a message from compiler to the linker, a message from programmer to the preprocessor, a message from programmer to the microprocessor, , (b) Which of the following are correctly formed #define, statements:, #define, #define, #define, #define, , INCH PER FEET 12, SQR (X) ( X * X ), SQR(X) X * X, SQR(X) ( X * X ), , (c) State True or False:, 1. A macro must always be written in capital letters., 2. A macro should always be accomodated in a single line., 3. After preprocessing when the program is sent for, compilation the macros are removed from the expanded, source code., 4. Macros with arguments are not allowed., 5. Nested macros are allowed., 6. In a macro call the control is passed to the macro.
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Chapter 7: The C Preprocessor, , 265, , (d) How many #include directives can be there in a given, program file?, (e) What is the difference between the following two #include, directives:, #include "conio.h", #include <conio.h>, , (f) A header file is:, 1., 2., 3., 4., , A file that contains standard library functions, A file that contains definitions and macros, A file that contains user - defined functions, A file that is present in current working directory, , (g) Which of the following is not a preprocessor directive, 1., 2., 3., 4., , #if, #elseif, #undef, #pragma, , (h) All macro substitutions in a program are done, 1., 2., 3., 4., , Before compilation of the program, After compilation, During execution, None of the above, , (i) In a program the statement:, #include "filename", , is replaced by the contents of the file “filename”, 1., 2., 3., 4., , Before compilation, After Compilation, During execution, None of the above
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266, , Let Us C, , [B] What would be the output of the following program:, (a) main( ), {, int i = 2 ;, #ifdef DEF, i *= i ;, #else, printf ( "\n%d", i ) ;, #endif, }, (b) #define PRODUCT(x) ( x * x ), main( ), {, int i = 3, j ;, j = PRODUCT( i + 1 ) ;, printf ( "\n%d", j ) ;, }, (c), , #define PRODUCT(x) ( x * x ), main( ), {, int i = 3, j, k ;, j = PRODUCT( i++ ) ;, k = PRODUCT ( ++i ) ;, printf ( "\n%d %d", j, k ) ;, }, , (d) # define SEMI ;, main(), {, int p = 3 SEMI ;, printf ( "%d", p ) SEMI, }
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Chapter 7: The C Preprocessor, , 267, , [C] Attempt the following:, , (a) Write down macro definitions for the following:, 1. To test whether a character entered is a small case letter or, not., 2. To test whether a character entered is a upper case letter or, not., 3. To test whether a character is an alphabet or not. Make, use of the macros you defined in (1) and (2) above., 4. To obtain the bigger of two numbers., (b) Write macro definitions with arguments for calculation of, area and perimeter of a triangle, a square and a circle. Store, these macro definitions in a file called “areaperi.h”. Include, this file in your program, and call the macro definitions for, calculating area and perimeter for different squares, triangles, and circles., (c) Write down macro definitions for the following:, 1., 2., 3., 4., , To find arithmetic mean of two numbers., To find absolute value of a number., To convert a uppercase alphabet to lowercase., To obtain the bigger of two numbers., , (d) Write macro definitions with arguments for calculation of, Simple Interest and Amount. Store these macro definitions in, a file called “interest.h”. Include this file in your program, and, use the macro definitions for calculating simple interest and, amount.
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268, , Let Us C
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8, , Arrays, • What are Arrays, A Simple Program Using Array, • More on Arrays, Array Initialisation, Bounds Checking, Passing Array Elements to a Function, • Pointers and Arrays, Passing an Entire Array to a Function, The Real Thing, • Two Dimensional Arrays, Initialising a 2-Dimensional Array, Memory Map of a 2-Dimensional Array, Pointers and 2-Dimensional Arrays, Pointer to an Array, Passing 2-D Array to a Function, • Array of Pointers, • Three-Dimensional Array, • Summary, • Exercise, , 269
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270, , T, , Let Us C, he C language provides a capability that enables the user to, design a set of similar data types, called array. This chapter, describes how arrays can be created and manipulated in C., , We should note that, in many C books and courses arrays and, pointers are taught separately. I feel it is worthwhile to deal with, these topics together. This is because pointers and arrays are so, closely related that discussing arrays without discussing pointers, would make the discussion incomplete and wanting. In fact all, arrays make use of pointers internally. Hence it is all too relevant, to study them together rather than as isolated topics., , What are Arrays, For understanding the arrays properly, let us consider the, following program:, main( ), {, int x ;, x=5;, x = 10 ;, printf ( "\nx = %d", x ) ;, }, , No doubt, this program will print the value of x as 10. Why so?, Because when a value 10 is assigned to x, the earlier value of x,, i.e. 5, is lost. Thus, ordinary variables (the ones which we have, used so far) are capable of holding only one value at a time (as in, the above example). However, there are situations in which we, would want to store more than one value at a time in a single, variable., For example, suppose we wish to arrange the percentage marks, obtained by 100 students in ascending order. In such a case we, have two options to store these marks in memory:
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Chapter 8: Arrays, , 271, , (a) Construct 100 variables to store percentage marks obtained by, 100 different students, i.e. each variable containing one, student’s marks., (b) Construct one variable (called array or subscripted variable), capable of storing or holding all the hundred values., Obviously, the second alternative is better. A simple reason for, this is, it would be much easier to handle one variable than, handling 100 different variables. Moreover, there are certain logics, that cannot be dealt with, without the use of an array. Now a, formal definition of an array—An array is a collective name given, to a group of ‘similar quantities’. These similar quantities could be, percentage marks of 100 students, or salaries of 300 employees, or, ages of 50 employees. What is important is that the quantities must, be ‘similar’. Each member in the group is referred to by its, position in the group. For example, assume the following group of, numbers, which represent percentage marks obtained by five, students., per = { 48, 88, 34, 23, 96 }, , If we want to refer to the second number of the group, the usual, notation used is per2. Similarly, the fourth number of the group is, referred as per4. However, in C, the fourth number is referred as, per[3]. This is because in C the counting of elements begins with 0, and not with 1. Thus, in this example per[3] refers to 23 and, per[4] refers to 96. In general, the notation would be per[i],, where, i can take a value 0, 1, 2, 3, or 4, depending on the position, of the element being referred. Here per is the subscripted variable, (array), whereas i is its subscript., Thus, an array is a collection of similar elements. These similar, elements could be all ints, or all floats, or all chars, etc. Usually,, the array of characters is called a ‘string’, whereas an array of ints, or floats is called simply an array. Remember that all elements of
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272, , Let Us C, , any given array must be of the same type. i.e. we cannot have an, array of 10 numbers, of which 5 are ints and 5 are floats., , A Simple Program Using Array, Let us try to write a program to find average marks obtained by a, class of 30 students in a test., main( ), {, int avg, sum = 0 ;, int i ;, int marks[30] ; /* array declaration */, for ( i = 0 ; i <= 29 ; i++ ), {, printf ( "\nEnter marks " ) ;, scanf ( "%d", &marks[i] ) ; /* store data in array */, }, for ( i = 0 ; i <= 29 ; i++ ), sum = sum + marks[i] ; /* read data from an array*/, avg = sum / 30 ;, printf ( "\nAverage marks = %d", avg ) ;, }, , There is a lot of new material in this program, so let us take it apart, slowly., Array Declaration, To begin with, like other variables an array needs to be declared so, that the compiler will know what kind of an array and how large, an array we want. In our program we have done this with the, statement:
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Chapter 8: Arrays, , 273, , int marks[30] ;, , Here, int specifies the type of the variable, just as it does with, ordinary variables and the word marks specifies the name of the, variable. The [30] however is new. The number 30 tells how many, elements of the type int will be in our array. This number is often, called the ‘dimension’ of the array. The bracket ( [ ] ) tells the, compiler that we are dealing with an array., Accessing Elements of an Array, Once an array is declared, let us see how individual elements in the, array can be referred. This is done with subscript, the number in, the brackets following the array name. This number specifies the, element’s position in the array. All the array elements are, numbered, starting with 0. Thus, marks[2] is not the second, element of the array, but the third. In our program we are using the, variable i as a subscript to refer to various elements of the array., This variable can take different values and hence can refer to the, different elements in the array in turn. This ability to use variables, as subscripts is what makes arrays so useful., Entering Data into an Array, Here is the section of code that places data into an array:, for ( i = 0 ; i <= 29 ; i++ ), {, printf ( "\nEnter marks " ) ;, scanf ( "%d", &marks[i] ) ;, }, , The for loop causes the process of asking for and receiving a, student’s marks from the user to be repeated 30 times. The first, time through the loop, i has a value 0, so the scanf( ) function will, cause the value typed to be stored in the array element marks[0],, the first element of the array. This process will be repeated until i
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274, , Let Us C, , becomes 29. This is last time through the loop, which is a good, thing, because there is no array element like marks[30]., In scanf( ) function, we have used the “address of” operator (&) on, the element marks[i] of the array, just as we have used it earlier, on other variables (&rate, for example). In so doing, we are, passing the address of this particular array element to the scanf( ), function, rather than its value; which is what scanf( ) requires., Reading Data from an Array, The balance of the program reads the data back out of the array, and uses it to calculate the average. The for loop is much the same,, but now the body of the loop causes each student’s marks to be, added to a running total stored in a variable called sum. When all, the marks have been added up, the result is divided by 30, the, number of students, to get the average., for ( i = 0 ; i <= 29 ; i++ ), sum = sum + marks[i] ;, avg = sum / 30 ;, printf ( "\nAverage marks = %d", avg ) ;, , To fix our ideas, let us revise whatever we have learnt about, arrays:, (a) An array is a collection of similar elements., (b) The first element in the array is numbered 0, so the last, element is 1 less than the size of the array., (c) An array is also known as a subscripted variable., (d) Before using an array its type and dimension must be, declared., (e) However big an array its elements are always stored in, contiguous memory locations. This is a very important point, which we would discuss in more detail later on.
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Chapter 8: Arrays, , 275, , More on Arrays, Array is a very popular data type with C programmers. This is, because of the convenience with which arrays lend themselves to, programming. The features which make arrays so convenient to, program would be discussed below, along with the possible pitfalls, in using them., , Array Initialisation, So far we have used arrays that did not have any values in them to, begin with. We managed to store values in them during program, execution. Let us now see how to initialize an array while, declaring it. Following are a few examples that demonstrate this., int num[6] = { 2, 4, 12, 5, 45, 5 } ;, int n[ ] = { 2, 4, 12, 5, 45, 5 } ;, float press[ ] = { 12.3, 34.2 -23.4, -11.3 } ;, , Note the following points carefully:, (a) Till the array elements are not given any specific values, they, are supposed to contain garbage values., (b) If the array is initialised where it is declared, mentioning the, dimension of the array is optional as in the 2nd example above., Array Elements in Memory, Consider the following array declaration:, int arr[8] ;, , What happens in memory when we make this declaration? 16, bytes get immediately reserved in memory, 2 bytes each for the 8, integers (under Windows/Linux the array would occupy 32 bytes
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276, , Let Us C, , as each integer would occupy 4 bytes). And since the array is not, being initialized, all eight values present in it would be garbage, values. This so happens because the storage class of this array is, assumed to be auto. If the storage class is declared to be static, then all the array elements would have a default initial value as, zero. Whatever be the initial values, all the array elements would, always be present in contiguous memory locations. This, arrangement of array elements in memory is shown in Figure 8.1., , 12, , 34, , 66, , -45, , 23, , 65508 65510 65512 65514 65516, , 346, , 77, , 90, , 65518 65520 65522, , Figure 8.1, , Bounds Checking, In C there is no check to see if the subscript used for an array, exceeds the size of the array. Data entered with a subscript, exceeding the array size will simply be placed in memory outside, the array; probably on top of other data, or on the program itself., This will lead to unpredictable results, to say the least, and there, will be no error message to warn you that you are going beyond, the array size. In some cases the computer may just hang. Thus,, the following program may turn out to be suicidal., main( ), {, int num[40], i ;, for ( i = 0 ; i <= 100 ; i++ ), num[i] = i ;, }
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Chapter 8: Arrays, , 277, , Thus, to see to it that we do not reach beyond the array size is, entirely the programmer’s botheration and not the compiler’s., , Passing Array Elements to a Function, Array elements can be passed to a function by calling the function, by value, or by reference. In the call by value we pass values of, array elements to the function, whereas in the call by reference we, pass addresses of array elements to the function. These two calls, are illustrated below:, /* Demonstration of call by value */, main( ), {, int i ;, int marks[ ] = { 55, 65, 75, 56, 78, 78, 90 } ;, for ( i = 0 ; i <= 6 ; i++ ), display ( marks[i] ) ;, }, display ( int m ), {, printf ( "%d ", m ) ;, }, , And here’s the output..., 55 65 75 56 78 78 90, , Here, we are passing an individual array element at a time to the, function display( ) and getting it printed in the function display( )., Note that since at a time only one element is being passed, this, element is collected in an ordinary integer variable m, in the, function display( )., And now the call by reference.
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278, , Let Us C, , /* Demonstration of call by reference */, main( ), {, int i ;, int marks[ ] = { 55, 65, 75, 56, 78, 78, 90 } ;, for ( i = 0 ; i <= 6 ; i++ ), disp ( &marks[i] ) ;, }, disp ( int *n ), {, printf ( "%d ", *n ) ;, }, , And here’s the output..., 55 65 75 56 78 78 90, , Here, we are passing addresses of individual array elements to the, function display( ). Hence, the variable in which this address is, collected (n) is declared as a pointer variable. And since n contains, the address of array element, to print out the array element we are, using the ‘value at address’ operator (*)., Read the following program carefully. The purpose of the function, disp( ) is just to display the array elements on the screen. The, program is only partly complete. You are required to write the, function show( ) on your own. Try your hand at it., main( ), {, int i ;, int marks[ ] = { 55, 65, 75, 56, 78, 78, 90 } ;, for ( i = 0 ; i <= 6 ; i++ ), disp ( &marks[i] ) ;
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Chapter 8: Arrays, , 279, , }, disp ( int *n ), {, show ( &n ) ;, }, , Pointers and Arrays, To be able to see what pointers have got to do with arrays, let us, first learn some pointer arithmetic. Consider the following, example:, main( ), {, int i = 3, *x ;, float j = 1.5, *y ;, char k = 'c', *z ;, printf ( "\nValue of i = %d", i ) ;, printf ( "\nValue of j = %f", j ) ;, printf ( "\nValue of k = %c", k ) ;, x = &i ;, y = &j ;, z = &k ;, printf ( "\nOriginal address in x = %u", x ) ;, printf ( "\nOriginal address in y = %u", y ) ;, printf ( "\nOriginal address in z = %u", z ) ;, x++ ;, y++ ;, z++ ;, printf ( "\nNew address in x = %u", x ) ;, printf ( "\nNew address in y = %u", y ) ;, printf ( "\nNew address in z = %u", z ) ;, }, , Here is the output of the program.
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280, , Let Us C, , Value of i = 3, Value of j = 1.500000, Value of k = c, Original address in x = 65524, Original address in y = 65520, Original address in z = 65519, New address in x = 65526, New address in y = 65524, New address in z = 65520, , Observe the last three lines of the output. 65526 is original value in, x plus 2, 65524 is original value in y plus 4, and 65520 is original, value in z plus 1. This so happens because every time a pointer is, incremented it points to the immediately next location of its type., That is why, when the integer pointer x is incremented, it points to, an address two locations after the current location, since an int is, always 2 bytes long (under Windows/Linux since int is 4 bytes, long, new value of x would be 65528). Similarly, y points to an, address 4 locations after the current location and z points 1, location after the current location. This is a very important result, and can be effectively used while passing the entire array to a, function., The way a pointer can be incremented, it can be decremented as, well, to point to earlier locations. Thus, the following operations, can be performed on a pointer:, (a) Addition of a number to a pointer. For example,, int i = 4, *j, *k ;, j = &i ;, j=j+1;, j=j+9;, k=j+3;, , (b) Subtraction of a number from a pointer. For example,
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Chapter 8: Arrays, , 281, , int i = 4, *j, *k ;, j = &i ;, j=j-2;, j=j-5;, k=j-6;, , (c) Subtraction of one pointer from another., One pointer variable can be subtracted from another provided, both variables point to elements of the same array. The, resulting value indicates the number of bytes separating the, corresponding array elements. This is illustrated in the, following program., main( ), {, int arr[ ] = { 10, 20, 30, 45, 67, 56, 74 } ;, int *i, *j ;, i = &arr[1] ;, j = &arr[5] ;, printf ( "%d %d", j - i, *j - *i ) ;, }, , Here i and j have been declared as integer pointers holding, addresses of first and fifth element of the array respectively., Suppose the array begins at location 65502, then the elements, arr[1] and arr[5] would be present at locations 65504 and, 65512 respectively, since each integer in the array occupies, two bytes in memory. The expression j - i would print a value, 4 and not 8. This is because j and i are pointing to locations, that are 4 integers apart. What would be the result of the, expression *j - *i? 36, since *j and *i return the values, present at addresses contained in the pointers j and i., (d) Comparison of two pointer variables
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282, , Let Us C, , Pointer variables can be compared provided both variables, point to objects of the same data type. Such comparisons can, be useful when both pointer variables point to elements of the, same array. The comparison can test for either equality or, inequality. Moreover, a pointer variable can be compared with, zero (usually expressed as NULL). The following program, illustrates how the comparison is carried out., main( ), {, int arr[ ] = { 10, 20, 36, 72, 45, 36 } ;, int *j, *k ;, j = &arr [ 4 ] ;, k = ( arr + 4 ) ;, if ( j == k ), printf ( "The two pointers point to the same location" ) ;, else, printf ( "The two pointers do not point to the same location" ) ;, }, , A word of caution! Do not attempt the following operations on, pointers... they would never work out., (a) Addition of two pointers, (b) Multiplication of a pointer with a constant, (c) Division of a pointer with a constant, Now we will try to correlate the following two facts, which we, have learnt above:, (a) Array elements are always stored in contiguous memory, locations., (b) A pointer when incremented always points to an immediately, next location of its type.
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283, , Chapter 8: Arrays, , Suppose we have an array num[ ] = { 24, 34, 12, 44, 56, 17 }. The, following figure shows how this array is located in memory., , 24, , 34, , 65512, , 65514, , 12, , 65516, , 44, , 56, , 17, , 65518, , 65520, , 65522, , Figure 8.2, Here is a program that prints out the memory locations in which, the elements of this array are stored., main( ), {, int num[ ] = { 24, 34, 12, 44, 56, 17 } ;, int i ;, for ( i = 0 ; i <= 5 ; i++ ), {, printf ( "\nelement no. %d ", i ) ;, printf ( "address = %u", &num[i] ) ;, }, }, , The output of this program would look like this:, element no. 0 address = 65512, element no. 1 address = 65514, element no. 2 address = 65516, element no. 3 address = 65518, element no. 4 address = 65520, element no. 5 address = 65522, , Note that the array elements are stored in contiguous memory, locations, each element occupying two bytes, since it is an integer
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284, , Let Us C, , array. When you run this program, you may get different, addresses, but what is certain is that each subsequent address, would be 2 bytes (4 bytes under Windows/Linux) greater than its, immediate predecessor., Our next two programs show ways in which we can access the, elements of this array., main( ), {, int num[ ] = { 24, 34, 12, 44, 56, 17 } ;, int i ;, for ( i = 0 ; i <= 5 ; i++ ), {, printf ( "\naddress = %u ", &num[i] ) ;, printf ( "element = %d", num[i] ) ;, }, }, , The output of this program would be:, address = 65512 element = 24, address = 65514 element = 34, address = 65516 element = 12, address = 65518 element = 44, address = 65520 element = 56, address = 65522 element = 17, , This method of accessing array elements by using subscripted, variables is already known to us. This method has in fact been, given here for easy comparison with the next method, which, accesses the array elements using pointers., main( ), {, int num[ ] = { 24, 34, 12, 44, 56, 17 } ;
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Chapter 8: Arrays, , 285, , int i, *j ;, j = &num[0] ; /* assign address of zeroth element */, for ( i = 0 ; i <= 5 ; i++ ), {, printf ( "\naddress = %u ", j ) ;, printf ( "element = %d", *j ) ;, j++ ; /* increment pointer to point to next location */, }, }, , The output of this program would be:, address = 65512 element = 24, address = 65514 element = 34, address = 65516 element = 12, address = 65518 element = 44, address = 65520 element = 56, address = 65522 element = 17, , In this program, to begin with we have collected the base address, of the array (address of the 0th element) in the variable j using the, statement,, j = &num[0] ; /* assigns address 65512 to j */, , When we are inside the loop for the first time, j contains the, address 65512, and the value at this address is 24. These are, printed using the statements,, printf ( "\naddress = %u ", j ) ;, printf ( "element = %d", *j ) ;, , On incrementing j it points to the next memory location of its type, (that is location no. 65514). But location no. 65514 contains the, second element of the array, therefore when the printf( )
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286, , Let Us C, , statements are executed for the second time they print out the, second element of the array and its address (i.e. 34 and 65514)..., and so on till the last element of the array has been printed., Obviously, a question arises as to which of the above two methods, should be used when? Accessing array elements by pointers is, always faster than accessing them by subscripts. However, from, the point of view of convenience in programming we should, observe the following:, Array elements should be accessed using pointers if the elements, are to be accessed in a fixed order, say from beginning to end, or, from end to beginning, or every alternate element or any such, definite logic., Instead, it would be easier to access the elements using a subscript, if there is no fixed logic in accessing the elements. However, in, this case also, accessing the elements by pointers would work, faster than subscripts., , Passing an Entire Array to a Function, In the previous section we saw two programs—one in which we, passed individual elements of an array to a function, and another in, which we passed addresses of individual elements to a function., Let us now see how to pass an entire array to a function rather than, its individual elements. Consider the following example:, /* Demonstration of passing an entire array to a function */, main( ), {, int num[ ] = { 24, 34, 12, 44, 56, 17 } ;, dislpay ( &num[0], 6 ) ;, }, display ( int *j, int n ), {
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287, , Chapter 8: Arrays, int i ;, for ( i = 0 ; i <= n - 1 ; i++ ), {, printf ( "\nelement = %d", *j ) ;, j++ ; /* increment pointer to point to next element */, }, }, , Here, the display( ) function is used to print out the array, elements. Note that the address of the zeroth element is being, passed to the display( ) function. The for loop is same as the one, used in the earlier program to access the array elements using, pointers. Thus, just passing the address of the zeroth element of the, array to a function is as good as passing the entire array to the, function. It is also necessary to pass the total number of elements, in the array, otherwise the display( ) function would not know, when to terminate the for loop. Note that the address of the zeroth, element (many a times called the base address) can also be passed, by just passing the name of the array. Thus, the following two, function calls are same:, display ( &num[0], 6 ) ;, display ( num, 6 ) ;, , The Real Thing, If you have grasped the concept of storage of array elements in, memory and the arithmetic of pointers, here is some real food for, thought. Once again consider the following array., , 24, , 34, , 12, , 44, , 56, , 17, , 65512, , 65514, , 65516, , 65518, , 65520, , 65522, , Figure 8.3
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288, , Let Us C, , This is how we would declare the above array in C,, int num[ ] = { 24, 34, 12, 44, 56, 17 } ;, , We also know that on mentioning the name of the array we get its, base address. Thus, by saying *num we would be able to refer to, the zeroth element of the array, that is, 24. One can easily see that, *num and *( num + 0 ) both refer to 24., Similarly, by saying *( num + 1 ) we can refer the first element of, the array, that is, 34. In fact, this is what the C compiler does, internally. When we say, num[i], the C compiler internally, converts it to *( num + i ). This means that all the following, notations are same:, num[i], *( num + i ), *( i + num ), i[num], , And here is a program to prove my point., /* Accessing array elements in different ways */, main( ), {, int num[ ] = { 24, 34, 12, 44, 56, 17 } ;, int i ;, for ( i = 0 ; i <= 5 ; i++ ), {, printf ( "\naddress = %u ", &num[i] ) ;, printf ( "element = %d %d ", num[i], *( num + i ) ) ;, printf ( "%d %d", *( i + num ), i[num] ) ;, }, }, , The output of this program would be:
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Chapter 8: Arrays, , 289, , address = 65512 element = 24 24 24 24, address = 65514 element = 34 34 34 34, address = 65516 element = 12 12 12 12, address = 65518 element = 44 44 44 44, address = 65520 element = 56 56 56 56, address = 65522 element = 17 17 17 17, , Two Dimensional Arrays, So far we have explored arrays with only one dimension. It is also, possible for arrays to have two or more dimensions. The twodimensional array is also called a matrix., Here is a sample program that stores roll number and marks, obtained by a student side by side in a matrix., main( ), {, int stud[4][2] ;, int i, j ;, for ( i = 0 ; i <= 3 ; i++ ), {, printf ( "\n Enter roll no. and marks" ) ;, scanf ( "%d %d", &stud[i][0], &stud[i][1] ) ;, }, for ( i = 0 ; i <= 3 ; i++ ), printf ( "\n%d %d", stud[i][0], stud[i][1] ) ;, }, , There are two parts to the program—in the first part through a for, loop we read in the values of roll no. and marks, whereas, in, second part through another for loop we print out these values., Look at the scanf( ) statement used in the first for loop:, scanf ( "%d %d", &stud[i][0], &stud[i][1] ) ;
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290, , Let Us C, , In stud[i][0] and stud[i][1] the first subscript of the variable stud,, is row number which changes for every student. The second, subscript tells which of the two columns are we talking about—the, zeroth column which contains the roll no. or the first column, which contains the marks. Remember the counting of rows and, columns begin with zero. The complete array arrangement is, shown below., , col. no. 0, , col. no. 1, , row no. 0, , 1234, , 56, , row no. 1, , 1212, , 33, , row no. 2, , 1434, , 80, , row no. 3, , 1312, , 78, , Figure 8.4, Thus, 1234 is stored in stud[0][0], 56 is stored in stud[0][1] and, so on. The above arrangement highlights the fact that a twodimensional array is nothing but a collection of a number of onedimensional arrays placed one below the other., In our sample program the array elements have been stored, rowwise and accessed rowwise. However, you can access the array, elements columnwise as well. Traditionally, the array elements are, being stored and accessed rowwise; therefore we would also stick, to the same strategy., , Initialising a 2-Dimensional Array, How do we initialize a two-dimensional array? As simple as this...
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Chapter 8: Arrays, , 291, , int stud[4][2] = {, { 1234, 56 },, { 1212, 33 },, { 1434, 80 },, { 1312, 78 }, };, , or even this would work..., int stud[4][2] = { 1234, 56, 1212, 33, 1434, 80, 1312, 78 } ;, , of course with a corresponding loss in readability., It is important to remember that while initializing a 2-D array it is, necessary to mention the second (column) dimension, whereas the, first dimension (row) is optional., Thus the declarations,, int arr[2][3] = { 12, 34, 23, 45, 56, 45 } ;, int arr[ ][3] = { 12, 34, 23, 45, 56, 45 } ;, , are perfectly acceptable,, whereas,, int arr[2][ ] = { 12, 34, 23, 45, 56, 45 } ;, int arr[ ][ ] = { 12, 34, 23, 45, 56, 45 } ;, , would never work., , Memory Map of a 2-Dimensional Array, Let us reiterate the arrangement of array elements in a twodimensional array of students, which contains roll nos. in one, column and the marks in the other.
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292, , Let Us C, , The array arrangement shown in Figure 8.4 is only conceptually, true. This is because memory doesn’t contain rows and columns., In memory whether it is a one-dimensional or a two-dimensional, array the array elements are stored in one continuous chain. The, arrangement of array elements of a two-dimensional array in, memory is shown below:, , s[0][0] s[0][1] s[1][0] s[1][1] s[2][0] s[2][1] s[3][0] s[3][1], 1234, , 56, , 1212, , 33, , 1434, , 80, , 1312, , 78, , 65508, , 65510, , 65512, , 65514, , 65516, , 65518, , 65520, , 65522, , Figure 8.5, We can easily refer to the marks obtained by the third student, using the subscript notation as shown below:, printf ( "Marks of third student = %d", stud[2][1] ) ;, , Can we not refer the same element using pointer notation, the way, we did in one-dimensional arrays? Answer is yes. Only the, procedure is slightly difficult to understand. So, read on..., , Pointers and 2-Dimensional Arrays, The C language embodies an unusual but powerful capability—it, can treat parts of arrays as arrays. More specifically, each row of a, two-dimensional array can be thought of as a one-dimensional, array. This is a very important fact if we wish to access array, elements of a two-dimensional array using pointers., Thus, the declaration,, int s[5][2] ;
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Chapter 8: Arrays, , 293, , can be thought of as setting up an array of 5 elements, each of, which is a one-dimensional array containing 2 integers. We refer, to an element of a one-dimensional array using a single subscript., Similarly, if we can imagine s to be a one-dimensional array then, we can refer to its zeroth element as s[0], the next element as s[1], and so on. More specifically, s[0] gives the address of the zeroth, one-dimensional array, s[1] gives the address of the first onedimensional array and so on. This fact can be demonstrated by the, following program., /* Demo: 2-D array is an array of arrays */, main( ), {, int s[4][2] = {, { 1234, 56 },, { 1212, 33 },, { 1434, 80 },, { 1312, 78 }, };, int i ;, for ( i = 0 ; i <= 3 ; i++ ), printf ( "\nAddress of %d th 1-D array = %u", i, s[i] ) ;, }, , And here is the output..., Address of 0 th 1-D array = 65508, Address of 1 th 1-D array = 65512, Address of 2 th 1-D array = 65516, Address of 3 th 1-D array = 65520, , Let’s figure out how the program works. The compiler knows that, s is an array containing 4 one-dimensional arrays, each containing, 2 integers. Each one-dimensional array occupies 4 bytes (two, bytes for each integer). These one-dimensional arrays are placed, linearly (zeroth 1-D array followed by first 1-D array, etc.). Hence
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294, , Let Us C, , each one-dimensional arrays starts 4 bytes further along than the, last one, as can be seen in the memory map of the array shown, below., , s[0][0] s[0][1] s[1][0] s[1][1] s[2][0] s[2][1] s[3][0] s[3][1], 1234, , 56, , 1212, , 33, , 1434, , 80, , 1312, , 78, , 65508, , 65510, , 65512, , 65514, , 65516, , 65518, , 65520, , 65522, , Figure 8.6, We know that the expressions s[0] and s[1] would yield the, addresses of the zeroth and first one-dimensional array, respectively. From Figure 8.6 these addresses turn out to be 65508, and 65512., Now, we have been able to reach each one-dimensional array., What remains is to be able to refer to individual elements of a onedimensional array. Suppose we want to refer to the element s[2][1], using pointers. We know (from the above program) that s[2] would, give the address 65516, the address of the second one-dimensional, array. Obviously ( 65516 + 1 ) would give the address 65518. Or, ( s[2] + 1 ) would give the address 65518. And the value at this, address can be obtained by using the value at address operator,, saying *( s[2] + 1 ). But, we have already studied while learning, one-dimensional arrays that num[i] is same as *( num + i )., Similarly, *( s[2] + 1 ) is same as, *( *( s + 2 ) + 1 ). Thus, all the, following expressions refer to the same element,, s[2][1], * ( s[2] + 1 ), *(*(s+2)+1)
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Chapter 8: Arrays, , 295, , Using these concepts the following program prints out each, element of a two-dimensional array using pointer notation., /* Pointer notation to access 2-D array elements */, main( ), {, int s[4][2] = {, { 1234, 56 },, { 1212, 33 },, { 1434, 80 },, { 1312, 78 }, };, int i, j ;, for ( i = 0 ; i <= 3 ; i++ ), {, printf ( "\n" ) ;, for ( j = 0 ; j <= 1 ; j++ ), printf ( "%d ", *( *( s + i ) + j ) ) ;, }, }, , And here is the output..., 1234, 1212, 1434, 1312, , 56, 33, 80, 78, , Pointer to an Array, If we can have a pointer to an integer, a pointer to a float, a pointer, to a char, then can we not have a pointer to an array? We certainly, can. The following program shows how to build and use it.
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296, , Let Us C, , /* Usage of pointer to an array */, main( ), {, int s[5][2] = {, { 1234, 56 },, { 1212, 33 },, { 1434, 80 },, { 1312, 78 }, };, int ( *p )[2] ;, int i, j, *pint ;, for ( i = 0 ; i <= 3 ; i++ ), {, p = &s[i] ;, pint = p ;, printf ( "\n" ) ;, for ( j = 0 ; j <= 1 ; j++ ), printf ( "%d ", *( pint + j ) ) ;, }, }, , And here is the output..., 1234, 1212, 1434, 1312, , 56, 33, 80, 78, , Here p is a pointer to an array of two integers. Note that the, parentheses in the declaration of p are necessary. Absence of them, would make p an array of 2 integer pointers. Array of pointers is, covered in a later section in this chapter. In the outer for loop each, time we store the address of a new one-dimensional array. Thus, first time through this loop p would contain the address of the, zeroth 1-D array. This address is then assigned to an integer, pointer pint. Lastly, in the inner for loop using the pointer pint we
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Chapter 8: Arrays, , 297, , have printed the individual elements of the 1-D array to which p is, pointing., But why should we use a pointer to an array to print elements of a, 2-D array. Is there any situation where we can appreciate its usage, better? The entity pointer to an array is immensely useful when we, need to pass a 2-D array to a function. This is discussed in the next, section., , Passing 2-D Array to a Function, There are three ways in which we can pass a 2-D array to a, function. These are illustrated in the following program., /* Three ways of accessing a 2-D array */, main( ), {, int a[3][4] = {, 1, 2, 3, 4,, 5, 6, 7, 8,, 9, 0, 1, 6, };, clrscr( ) ;, display ( a, 3, 4 ) ;, show ( a, 3, 4 ) ;, print ( a, 3, 4 ) ;, }, display ( int *q, int row, int col ), {, int i, j ;, for ( i = 0 ; i < row ; i++ ), {, for ( j = 0 ; j < col ; j++ ), printf ( "%d ", * ( q + i * col + j ) ) ;
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298, , Let Us C, , printf ( "\n" ) ;, }, printf ("\n" ) ;, }, show ( int ( *q )[4], int row, int col ), {, int i, j ;, int *p ;, for ( i = 0 ; i < row ; i++ ), {, p=q+i;, for ( j = 0 ; j < col ; j++ ), printf ( "%d ", * ( p + j ) ) ;, printf ( "\n" ) ;, }, printf ( "\n" ) ;, }, print ( int q[ ][4], int row, int col ), {, int i, j ;, for ( i = 0 ; i < row ; i++ ), {, for ( j = 0 ; j < col ; j++ ), printf ( "%d ", q[i][j] ) ;, printf ( "\n" ) ;, }, printf ( "\n" ) ;, }, , And here is the output…, 1234, 5678
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299, , Chapter 8: Arrays, 9016, 1234, 5678, 9016, 1234, 5678, 9016, , In the display( ) function we have collected the base address of the, 2-D array being passed to it in an ordinary int pointer. Then, through the two for loops using the expression * ( q + i * col + j ), we have reached the appropriate element in the array. Suppose i is, equal to 2 and j is equal to 3, then we wish to reach the element, a[2][3]. Let us see whether the expression * ( q + i * col + j ) does, give this element or not. Refer Figure 8.7 to understand this., , 1, , 2, , 3, , 4, , 5, , 6, , 7, , 8, , 9, , 0, , 1, , 6, , 65502 …04 …06 …08 …10 …12 …14 …16 …18 …20 …22 …24, , Figure 8.7, The expression * ( q + i * col + j ) becomes * ( 65502 + 2 * 4 + 3)., This turns out to be * (65502 + 11 ). Since 65502 is address of an, integer, * ( 65502 + 11 ) turns out to be * (65524). Value at this, address is 6. This is indeed same as a[2][3]. A more general, formula for accessing each array element would be:, * ( base address + row no. * no. of columns + column no. ), , In the show( ) function we have defined q to be a pointer to an, array of 4 integers through the declaration:
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300, , Let Us C, , int ( *q )[4] ;, , To begin with, q holds the base address of the zeroth 1-D array,, i.e. 4001 (refer Figure 8.7). This address is then assigned to p, an, int pointer, and then using this pointer all elements of the zeroth 1D array are accessed. Next time through the loop when i takes a, value 1, the expression q + i fetches the address of the first 1-D, array. This is because, q is a pointer to zeroth 1-D array and, adding 1 to it would give us the address of the next 1-D array. This, address is once again assigned to p, and using it all elements of the, next 1-D array are accessed., In the third function print( ), the declaration of q looks like this:, int q[ ][4] ;, , This is same as int ( *q )[4], where q is pointer to an array of 4, integers. The only advantage is that we can now use the more, familiar expression q[i][j] to access array elements. We could have, used the same expression in show( ) as well., , Array of Pointers, The way there can be an array of ints or an array of floats,, similarly there can be an array of pointers. Since a pointer variable, always contains an address, an array of pointers would be nothing, but a collection of addresses. The addresses present in the array of, pointers can be addresses of isolated variables or addresses of, array elements or any other addresses. All rules that apply to an, ordinary array apply to the array of pointers as well. I think a, program would clarify the concept., main( ), {, int *arr[4] ; /* array of integer pointers */
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301, , Chapter 8: Arrays, int i = 31, j = 5, k = 19, l = 71, m ;, arr[0] = &i ;, arr[1] = &j ;, arr[2] = &k ;, arr[3] = &l ;, for ( m = 0 ; m <= 3 ; m++ ), printf ( "%d ", * ( arr[m] ) ) ;, }, , Figure 8.8 shows the contents and the arrangement of the array of, pointers in memory. As you can observe, arr contains addresses of, isolated int variables i, j, k and l. The for loop in the program, picks up the addresses present in arr and prints the values present, at these addresses., , i, , j, , k, , l`, , 31, , 5, , 19, , 71, , 65516, , 65514, , 65512, , 65510, , arr[0], , arr[1], , arr[2], , arr[3], , 65516, , 65514, , 65512, , 65510, , 65518, , 65520, , 65522, , 65524, , Figure 8.8, An array of pointers can even contain the addresses of other, arrays. The following program would justify this., main( ), {, static int a[ ] = { 0, 1, 2, 3, 4 } ;
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302, , Let Us C, , int *p[ ] = { a, a + 1, a + 2, a + 3, a + 4 } ;, printf ( "\n%u %u %d", p, *p, * ( *p ) ) ;, }, , I would leave it for you to figure out the output of this program., , Three-Dimensional Array, We aren’t going to show a programming example that uses a threedimensional array. This is because, in practice, one rarely uses this, array. However, an example of initializing a three-dimensional, array will consolidate your understanding of subscripts:, int arr[3][4][2] = {, {, { 2, 4 },, { 7, 8 },, { 3, 4 },, { 5, 6 }, },, {, { 7, 6 },, { 3, 4 },, { 5, 3 },, { 2, 3 }, },, {, { 8, 9 },, { 7, 2 },, { 3, 4 },, { 5, 1 },, }, };, , A three-dimensional array can be thought of as an array of arrays, of arrays. The outer array has three elements, each of which is a
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303, , Chapter 8: Arrays, , two-dimensional array of four one-dimensional arrays, each of, which contains two integers. In other words, a one-dimensional, array of two elements is constructed first. Then four such onedimensional arrays are placed one below the other to give a twodimensional array containing four rows. Then, three such twodimensional arrays are placed one behind the other to yield a threedimensional array containing three 2-dimensional arrays. In the, array declaration note how the commas have been given. Figure, 8.9 would possibly help you in visualising the situation better., , 2nd 2-D Array, , 8, , 1st 2-D Array, , 9, , 7, , th, , 0 2-D Array, , 2, , 4, , 6, 4, , 7, , 8, , 3, , 3, , 4, , 3, , 5, , 6, , 2, 1, , 4, , Figure 8.9, Again remember that the arrangement shown above is only, conceptually true. In memory the same array elements are stored, linearly as shown in Figure 8.10., , 0th 2-D Array, , 1st 2-D Array, , 2nd 2-D Array, , 2 4 7 8 3 4 5 6 7 6 3 4 5 3 2 3 8 9 7 2 3 4 5 1, 65478, , Figure 8.10, , 65494, , 65510
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304, , Let Us C, , How would you refer to the array element 1 in the above array?, The first subscript should be [2], since the element is in third twodimensional array; the second subscript should be [3] since the, element is in fourth row of the two-dimensional array; and the, third subscript should be [1] since the element is in second position, in the one-dimensional array. We can therefore say that the, element 1 can be referred as arr[2][3][1]. It may be noted here that, the counting of array elements even for a 3-D array begins with, zero. Can we not refer to this element using pointer notation? Of, course, yes. For example, the following two expressions refer to, the same element in the 3-D array:, arr[2][3][1], *( *( *( arr + 2 ) + 3 ) + 1 ), , Summary, (a) An array is similar to an ordinary variable except that it can, store multiple elements of similar type., (b) Compiler doesn’t perform bounds checking on an array., (c) The array variable acts as a pointer to the zeroth element of, the array. In a 1-D array, zeroth element is a single value,, whereas, in a 2-D array this element is a 1-D array., (d) On incrementing a pointer it points to the next location of its, type., (e) Array elements are stored in contiguous memory locations, and so they can be accessed using pointers., (f) Only limited arithmetic can be done on pointers., , Exercise, Simple arrays, [A] What would be the output of the following programs:, (a) main( )
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Chapter 8: Arrays, , 305, , {, int num[26], temp ;, num[0] = 100 ;, num[25] = 200 ;, temp = num[25] ;, num[25] = num[0] ;, num[0] = temp ;, printf ( "\n%d %d", num[0], num[25] ) ;, }, (b) main( ), {, int array[26], i ;, for ( i = 0 ; i <= 25 ; i++ ), {, array[i] = 'A' + i ;, printf ( "\n%d %c", array[i], array[i] ) ;, }, }, (c), , main( ), {, int sub[50], i ;, for ( i = 0 ; i <= 48 ; i++ ) ;, {, sub[i] = i ;, printf ( "\n%d", sub[i] ) ;, }, }, , [B] Point out the errors, if any, in the following program, segments:, (a) /* mixed has some char and some int values */, int char mixed[100] ;, main( ), {, int a[10], i ;
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306, , Let Us C, for ( i = 1 ; i <= 10 ; i++ ), {, scanf ( "%d", a[i] ) ;, printf ( "%d", a[i] ) ;, }, , }, (b) main( ), {, int size ;, scanf ( "%d", &size ) ;, int arr[size] ;, for ( i = 1 ; i <= size ; i++ ), {, scanf ( "%d", arr[i] ) ;, printf ( "%d", arr[i] ) ;, }, }, (c) main( ), {, int i, a = 2, b = 3 ;, int arr[ 2 + 3 ] ;, for ( i = 0 ; i < a+b ; i++ ), {, scanf ( "%d", &arr[i] ) ;, printf ( "\n%d", arr[i] ) ;, }, }, [C] Answer the following:, , (a) An array is a collection of, 1., 2., 3., 4., , different data types scattered throughout memory, the same data type scattered throughout memory, the same data type placed next to each other in memory, different data types placed next to each other in memory
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Chapter 8: Arrays, , 307, , (b) Are the following array declarations correct?, int a (25) ;, int size = 10, b[size] ;, int c = {0,1,2} ;, , (c) Which element of the array does this expression reference?, num[4], , (d) What is the difference between the 5’s in these two, expressions? (Select the correct answer), int num[5] ;, num[5] = 11 ;, , 1., 2., 3., 4., , first is particular element, second is type, first is array size, second is particular element, first is particular element, second is array size, both specify array size, , (e) State whether the following statements are True or False:, 1. The array int num[26] has twenty-six elements., 2. The expression num[1] designates the first element in the, array, 3. It is necessary to initialize the array at the time of, declaration., 4. The expression num[27] designates the twenty-eighth, element in the array., [D] Attempt the following:, , (a) Twenty-five numbers are entered from the keyboard into an, array. The number to be searched is entered through the, keyboard by the user. Write a program to find if the number to, be searched is present in the array and if it is present, display, the number of times it appears in the array.
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308, , Let Us C, , (b) Twenty-five numbers are entered from the keyboard into an, array. Write a program to find out how many of them are, positive, how many are negative, how many are even and how, many odd., (c) Implement the Selection Sort, Bubble Sort and Insertion sort, algorithms on a set of 25 numbers. (Refer Figure 8.11 for the, logic of the algorithms), −, −, −, , Selection sort, Bubble Sort, Insertion Sort, , Selection Sort, Iteration 1, , Iteration 2, , 0, , 44, , 33, , 33, , 22, , 0 11, , 0, , 11, , 0 11, , 1, , 33, , 44, , 44, , 44, , 1 44, , 1, , 44, , 1 33, , 2, , 55, , 55, , 55, , 55, , 2 55, , 2, , 55, , 2 55, , 3, , 22, , 22, , 22, , 33, , 3 33, , 3, , 33, , 3 44, , 4, , 11, , 11, , 11, , 11, , 4 22, , 4, , 22, , 4 22, , Iteration 3, , Iteration 4, Result, , 0 11, , 0, , 11, , 0, , 11, , 0, , 11, , 1 22, , 1, , 22, , 1, , 22, , 1, , 22, , 2 55, , 2, , 44, , 2, , 33, , 2, , 33, , 3 44, , 3, , 55, , 3, , 55, , 3, , 44, , 4 33, , 4, , 33, , 4, , 44, , 4, , 55, , Figure 8.11 (a)
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309, , Chapter 8: Arrays, , Bubble Sort, Iteration 1, , Iteration 2, , 0 44, , 33, , 33, , 33, , 0, , 33, , 0 33, , 0 33, , 1 33, , 44, , 44, , 44, , 1, , 44, , 1 44, , 1 22, , 2 55, , 55, , 55, , 22, , 2, , 22, , 2 22, , 2 44, , 3 22, , 22, , 22, , 55, , 3, , 11, , 3 11, , 3 11, , 11, , 4, , 55, , 4 55, , 4 55, , 4 11, , 11, , 11, , Iteration 3, , Iteration 4, Result, , 0 33, , 0 22, , 0, , 22, , 0 11, , 1 22, , 1 33, , 1, , 11, , 1 22, , 2 11, , 2 11, , 2, , 33, , 2 33, , 3 44, , 3 44, , 3, , 44, , 3 44, , 4 55, , 4 55, , 4, , 55, , 4 55, , Figure 8.11 (b), Insertion Sort, Iteration 1, , Iteration 2, , Iteration 3, , Iteration 4, , 44, , 33, , 33, , 22, , 0, , 11, , 33, , 44, , 44, , 33, , 1, , 22, , 55, , 55, , 55, , 44, , 2, , 33, , 22, , 22, , 22, , 55, , 3, , 44, , 11, , 11, , 11, , 11, , 4, , 55, , Figure 8.11 (c), , Result
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310, , Let Us C, , (d) Implement the following procedure to generate prime, numbers from 1 to 100 into a program. This procedure is, called sieve of Eratosthenes., step 1, step 2, step 3, step 4, step 5, , Fill an array num[100] with numbers from 1 to 100, Starting with the second entry in the array, set all its, multiples to zero., Proceed to the next non-zero element and set all its, multiples to zero., Repeat step 3 till you have set up the multiples of, all the non-zero elements to zero, At the conclusion of step 4, all the non-zero entries, left in the array would be prime numbers, so print, out these numbers., , More on arrays, Arrays and pointers, [E] What would be the output of the following programs:, (a) main( ), {, int b[ ] = { 10, 20, 30, 40, 50 } ;, int i ;, for ( i = 0 ; i <= 4 ; i++ ), printf ( "\n%d" *( b + i ) ) ;, }, (b) main( ), {, int b[ ] = { 0, 20, 0, 40, 5 } ;, int i, *k ;, k=b;, for ( i = 0 ; i <= 4 ; i++ ), {, printf ( "\n%d" *k ) ;
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Chapter 8: Arrays, k++ ;, }, }, (c), , main( ), {, int a[ ] = { 2, 4, 6, 8, 10 } ;, int i ;, change ( a, 5 ) ;, for ( i = 0 ; i <= 4 ; i++ ), printf( "\n%d", a[i] ) ;, }, change ( int *b, int n ), {, int i ;, for ( i = 0 ; i < n ; i++ ), *( b + i ) = *( b + i ) + 5 ;, }, , (d) main( ), {, int a[5], i, b = 16 ;, for ( i = 0 ; i < 5 ; i++ ), a[i] = 2 * i ;, f ( a, b ) ;, for ( i = 0 ; i < 5 ; i++ ), printf ( "\n%d", a[i] ) ;, printf( "\n%d", b ) ;, }, f ( int *x, int y ), {, int i ;, for ( i = 0 ; i < 5 ; i++ ), *( x + i ) += 2 ;, y += 2 ;, }, , 311
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312, , Let Us C, , (e) main( ), {, static int a[5] ;, int i ;, for ( i = 0 ; i <= 4 ; i++ ), printf ( "\n%d", a[i] ) ;, }, (f), , main( ), {, int a[5] = { 5, 1, 15, 20, 25 } ;, int i, j, k = 1, m ;, i = ++a[1] ;, j = a[1]++ ;, m = a[i++] ;, printf ( "\n%d %d %d", i, j, m ) ;, }, , [F] Point out the errors, if any, in the following programs:, (a) main( ), {, int array[6] = { 1, 2, 3, 4, 5, 6 } ;, int i ;, for ( i = 0 ; i <= 25 ; i++ ), printf ( "\n%d", array[i] ) ;, }, (b) main( ), {, int sub[50], i ;, for ( i = 1 ; i <= 50 ; i++ ), {, sub[i] = i ;, printf ( "\n%d" , sub[i] ) ;, }, }
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Chapter 8: Arrays, (c), , main( ), {, int a[ ] = { 10, 20, 30, 40, 50 } ;, int j ;, j = a ; /* store the address of zeroth element */, j=j+3;, printf ( "\n%d" *j ) ;, }, , (d) main( ), {, float a[ ] = { 13.24, 1.5, 1.5, 5.4, 3.5 } ;, float *j ;, j=a;, j=j+4;, printf ( "\n%d %d %d", j, *j, a[4] ) ;, }, (e) main( ), {, float a[ ] = { 13.24, 1.5, 1.5, 5.4, 3.5 } ;, float *j, *k ;, j=a;, k=a+4;, j=j*2;, k=k/2;, printf ( "\n%d %d", *j, *k ) ;, }, (f), , main( ), {, int max = 5 ;, float arr[max] ;, for ( i = 0 ; i < max ; i++ ), scanf ( "%f", &arr[i] ) ;, }, , 313
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314, , Let Us C, , [G] Answer the following:, , (a) What would happen if you try to put so many values into an, array when you initialize it that the size of the array is, exceeded?, 1., 2., 3., 4., , nothing, possible system malfunction, error message from the compiler, other data may be overwritten, , (b) In an array int arr[12] the word arr represents the, a_________ of the array, (c) What would happen if you put too few elements in an array, when you initialize it?, 1., 2., 3., 4., , nothing, possible system malfunction, error message from the compiler, unused elements will be filled with 0’s or garbage, , (d) What would happen if you assign a value to an element of an, array whose subscript exceeds the size of the array?, 1., 2., 3., 4., , the element will be set to 0, nothing, it’s done all the time, other data may be overwritten, error message from the compiler, , (e) When you pass an array as an argument to a function, what, actually gets passed?, 1., 2., 3., 4., , address of the array, values of the elements of the array, address of the first element of the array, number of elements of the array
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Chapter 8: Arrays, , 315, , (f) Which of these are reasons for using pointers?, 1., 2., 3., 4., , To manipulate parts of an array, To refer to keywords such as for and if, To return more than one value from a function, To refer to particular programs more conveniently, , (g) If you don’t initialize a static array, what would be the, elements set to?, 1., 2., 3., 4., , 0, an undetermined value, a floating point number, the character constant '\0', , [H] State True or False:, , (a) Address of a floating-point variable is always a whole, number., (b) Which of the following is the correct way of declaring a float, pointer:, 5., 6., 7., 8., , float ptr ;, float *ptr ;, *float ptr ;, None of the above, , (c) Add the missing statement for the following program to print, 35., main( ), {, int j, *ptr ;, *ptr = 35 ;, printf ( "\n%d", j ) ;, }
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316, , Let Us C, , (d) if int s[5] is a one-dimensional array of integers, which of the, following refers to the third element in the array?, 9., 10., 11., 12., , *( s + 2 ), *( s + 3 ), s+3, s+2, , [I] Attempt the following:, , (a) Write a program to copy the contents of one array into another, in the reverse order., (b) If an array arr contains n elements, then write a program to, check if arr[0] = arr[n-1], arr[1] = arr[n-2] and so on., (c) Find the smallest number in an array using pointers., (d) Write a program which performs the following tasks:, −, −, −, −, , initialize an integer array of 10 elements in main( ), pass the entire array to a function modify( ), in modify( ) multiply each element of array by 3, return the control to main( ) and print the new array, elements in main( ), , (e) The screen is divided into 25 rows and 80 columns. The, characters that are displayed on the screen are stored in a, special memory called VDU memory (not to be confused with, ordinary memory). Each character displayed on the screen, occupies two bytes in VDU memory. The first of these bytes, contains the ASCII value of the character being displayed,, whereas, the second byte contains the colour in which the, character is displayed., For example, the ASCII value of the character present on, zeroth row and zeroth column on the screen is stored at
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317, , Chapter 8: Arrays, , location number 0xB8000000. Therefore the colour of this, character would be present at location number 0xB8000000 +, 1. Similarly ASCII value of character in row 0, col 1 will be at, location 0xB8000000 + 2, and its colour at 0xB8000000 + 3., With this knowledge write a program which when executed, would keep converting every capital letter on the screen to, small case letter and every small case letter to capital letter., The procedure should stop the moment the user hits a key, from the keyboard., This is an activity of a rampant Virus called Dancing Dolls., (For monochrome adapter, use 0xB0000000 instead of, 0xB8000000)., , More than one dimension, [J] What would be the output of the following programs:, (a) main( ), {, int n[3][3] = {, 2, 4, 3,, 6, 8, 5,, 3, 5, 1, };, printf ( "\n%d %d %d", *n, n[3][3], n[2][2] ) ;, }, (b) main( ), {, int n[3][3] = {, 2, 4, 3,, 6, 8, 5,, 3, 5, 1, };, int i, *ptr ;
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318, , Let Us C, ptr = n ;, for ( i = 0 ; i <= 8 ; i++ ), printf ( "\n%d", *( ptr + i ) ) ;, , }, (c), , main( ), {, int n[3][3] = {, 2, 4, 3,, 6, 8, 5,, 3, 5, 1, };, int i, j ;, for ( i = 0 ; i <= 2 ; i++ ), for ( j = 0 ; j <= 2 ; j++ ), printf ( "\n%d %d", n[i][j], *( *( n + i ) + j ) ) ;, }, , [K] Point out the errors, if any, in the following programs:, (a) main( ), {, int twod[ ][ ] = {, 2, 4,, 6, 8, };, printf ( "\n%d", twod ) ;, }, (b) main( ), {, int three[3][ ] = {, 2, 4, 3,, 6, 8, 2,, 2, 3 ,1, };, printf ( "\n%d", three[1][1] ) ;
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319, , Chapter 8: Arrays, }, [L] Attempt the following:, , (a) How will you initialize a three-dimensional array, threed[3][2][3]? How will you refer the first and last element, in this array?, (b) Write a program to pick up the largest number from any 5 row, by 5 column matrix., (c) Write a program to obtain transpose of a 4 x 4 matrix. The, transpose of a matrix is obtained by exchanging the elements, of each row with the elements of the corresponding column., (d) Very often in fairs we come across a puzzle that contains 15, numbered square pieces mounted on a frame. These pieces, can be moved horizontally or vertically. A possible, arrangement of these pieces is shown below:, , 1, , 4, , 15, , 7, , 8, , 10, , 2, , 11, , 14, , 3, , 6, , 13, , 12, , 9, , 5, , Figure 8.12, As you can see there is a blank at bottom right corner., Implement the following procedure through a program:
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320, , Let Us C, , Draw the boxes as shown above. Display the numbers in the, above order. Allow the user to hit any of the arrow keys (up,, down, left, or right)., If user hits say, right arrow key then the piece with a number, 5 should move to the right and blank should replace the, original position of 5. Similarly, if down arrow key is hit, then, 13 should move down and blank should replace the original, position of 13. If left arrow key or up arrow key is hit then no, action should be taken., The user would continue hitting the arrow keys till the, numbers aren’t arranged in ascending order., Keep track of the number of moves in which the user manages, to arrange the numbers in ascending order. The user who, manages it in minimum number of moves is the one who, wins., How do we tackle the arrow keys? We cannot receive them, using scanf( ) function. Arrow keys are special keys which, are identified by their ‘scan codes’. Use the following, function in your program. It would return the scan code of the, arrow key being hit. Don’t worry about how this function is, written. We are going to deal with it later. The scan codes for, the arrow keys are:, up arrow key – 72 down arrow key – 80, left arrow key – 75 right arrow key – 77, /* Returns scan code of the key that has been hit */, #include "dos.h", getkey( ), {, union REGS i, o ;
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321, , Chapter 8: Arrays, while ( !kbhit( ) ), ;, i.h.ah = 0 ;, int86 ( 22, &i, &o ) ;, return ( o.h.ah ) ;, }, , (e) Those readers who are from an Engineering/Science, background may try writing programs for following problems., (1), (2), (3), (4), , Write a program to add two 6 x 6 matrices., Write a program to multiply any two 3 x 3 matrices., Write a program to sort all the elements of a 4 x 4 matrix., Write a program to obtain the determinant value of a 5 x, 5 matrix., , (f) Match the following with reference to the following program, segment:, int i, j, = 25;, int *pi, *pj = & j;, ……., ……. /* more lines of program */, ……., *pj = j + 5;, j = *pj + 5 ;, pj = pj ;, *pi = i + j, , Each integer quantity occupies 2 bytes of memory. The value, assigned to i begin at (hexadecimal) address F9C and the, value assigned to j begins at address F9E. Match the value, represented by left hand side quantities with the right., 1., 2., 3., 4., , &i, &j, pj, *pj, , a., b., c., d., , 30, F9E, 35, FA2
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322, , Let Us C, 5., 6., 7., 8., 9., 10., , i, pi, *pi, ( pi + 2 ), (*pi + 2), * ( pi + 2 ), , e., f., g., h., i., j., k., l., , F9C, 67, unspecified, 65, F9E, F9E, FAO, F9D, , (g) Match the following with reference to the following segment:, int x[3][5] = {, { 1, 2, 3, 4, 5 },, { 6, 7, 8, 9, 10 },, { 11, 12, 13, 14, 15 }, }, *n = &x ;, 1., 2., 3., 4., 5., 6., 7., 8., 9., 10., , *( *( x + 2 ) + 1), *( *x + 2 ) + 5, *( *( x + 1) ), *( *( x ) + 2 ) + 1, * ( *( x + 1 ) + 3 ), *n, *( n +2 ), (*(n + 3 ) + 1, *(n + 5)+1, ++*n, , a., 9, b., 13, c., 4, d., 3, e., 2, f., 12, g., 14, h., 7, i., 1, j., 8, k., 5, l., 10, m., 6, (h) Match the following with reference to the following program, segment:, struct, {, int x, y;, } s[ ] = { 10, 20, 15, 25, 8, 75, 6, 2 };, int *i ;, i=s;
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323, , Chapter 8: Arrays, 1., 2., 3., 4., 5., 6., 7., 8., 9., 10., , *( i + 3 ), s[i[7]].x, s[ (s + 2)->y / 3[I]].y, i[i[1]-i[2]], i[s[3].y], ( s + 1 )->x + 5, *( 1 +i )**( i + 4 ) / *i, s[i[0] – i[4]].y + 10, ( *(s + *( i + 1) / *i ) ).x + 2, ++i[i[6]], , a., b., c., d., e., f., g., h., i., j., k., l., , 85, 2, 6, 7, 16, 15, 25, 8, 1, 100, 10, 20, , (i) Match the following with reference to the following program, segment:, unsigned int arr[3][3] = {, 2, 4, 6,, 9, 1, 10,, 16, 64, 5, };, 1., 2., 3., 4., 5., 6., 7., 8., 9., 10., , **arr, **arr < *( *arr + 2 ), *( arr + 2 ) / ( *( *arr + 1 ) > **arr ), *( arr[1] + 1 ) | arr[1][2], *( arr[0] ) | *( arr[2] ), arr[1][1] < arr[0][1], arr[2][[1] & arr[2][0], arr[2][2] | arr[0][1], arr[0][1] ^ arr[0][2], ++**arr + --arr[1][1], , a., b., c., d., e., f., g., h., i., j., k., l., , 64, 18, 6, 3, 0, 16, 1, 11, 20, 2, 5, 4, , (j) Write a program that interchanges the odd and even, components of an array., (k) Write a program to find if a square matrix is symmetric.
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324, , Let Us C, , (l) Write a function to find the norm of a matrix. The norm is, defined as the square root of the sum of squares of all, elements in the matrix., (m) Given an array p[5], write a function to shift it circularly left, by two positions. Thus, if p[0] = 15, p[1]= 30, p[2] = 28,, p[3]= 19 and p[4] = 61 then after the shift p[0] = 28, p[1] =, 19, p[2] = 61, p[3] = 15 and p[4] = 30. Call this function for a, (4 x 5 ) matrix and get its rows left shifted., (n) A 6 x 6 matrix is entered through the keyboard and stored in a, 2-dimensional array mat[7][7]. Write a program to obtain the, Determinant values of this matrix., (o) For the following set of sample data, compute the standard, deviation and the mean., -6, -12, 8, 13, 11, 6, 7, 2, -6, -9, -10, 11, 10, 9, 2, , The formula for standard deviation is, , ( xi − x ) 2, n, where xi is the data item and x is the mean., (p) The area of a triangle can be computed by the sine law when 2, sides of the triangle and the angle between them are known., Area = (1 / 2 ) ab sin ( angle ), , Given the following 6 triangular pieces of land, write a, program to find their area and determine which is largest,, Plot No., , a, , b, , angle, , 1, 2, 3, , 137.4, 155.2, 149.3, , 80.9, 92.62, 97.93, , 0.78, 0.89, 1.35
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325, , Chapter 8: Arrays, 4, 5, 6, , 160.0, 155.6, 149.7, , 100.25, 68.95, 120.0, , 9.00, 1.25, 1.75, , (q) For the following set of n data points (x, y), compute the, correlation coefficient r, given by, r=, , ∑ xy − ∑ x∑ y, [n∑ x − ( ∑ x ) ] [n∑ y − ( ∑ y ), 2, , x, 34.22, 39.87, 41.85, 43.23, 40.06, 53.29, 53.29, 54.14, 49.12, 40.71, 55.15, , 2, , 2, , 2, , ], , y, 102.43, 100.93, 97.43, 97.81, 98.32, 98.32, 100.07, 97.08, 91.59, 94.85, 94.65, , (r) For the following set of point given by (x, y) fit a straight line, given by, y = a + bx, , where,, __, , __, , a = y − bx, b=, x, 3.0, , and, , n∑ yx − ∑ x ∑ y, , [ n ∑ x 2 − (∑ x ) 2 ], y, 1.5
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326, 4.5, 5.5, 6.5, 7.5, 8.5, 8.0, 9.0, 9.5, 10.0, , Let Us C, 2.0, 3.5, 5.0, 6.0, 7.5, 9.0, 10.5, 12.0, 14.0, , (s) The X and Y coordinates of 10 different points are entered, through the keyboard. Write a program to find the distance of, last point from the first point (sum of distance between, consecutive points).
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9, , Puppetting On, Strings, •, •, •, •, , •, •, •, •, •, , What are Strings, More about Strings, Pointers and Strings, Standard Library String Functions, strlen( ), strcpy( ), strcat( ), strcmp( ), Two-Dimensional Array of Characters, Array of Pointers to Strings, Limitation of Array of Pointers to Strings, Solution, Summary, Exercise, , 327
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328, , Let Us C, , n the last chapter you learnt how to define arrays of differing, sizes and dimensions, how to initialize arrays, how to pass, arrays to a function, etc. With this knowledge under your belt,, you should be ready to handle strings, which are, simply put, a, special kind of array. And strings, the ways to manipulate them,, and how pointers are related to strings are going to be the topics of, discussion in this chapter., , I, , What are Strings, The way a group of integers can be stored in an integer array,, similarly a group of characters can be stored in a character array., Character arrays are many a time also called strings. Many, languages internally treat strings as character arrays, but somehow, conceal this fact from the programmer. Character arrays or strings, are used by programming languages to manipulate text such as, words and sentences., A string constant is a one-dimensional array of characters, terminated by a null ( ‘\0’ ). For example,, char name[ ] = { 'H', 'A', 'E', 'S', 'L', 'E', 'R', '\0' } ;, , Each character in the array occupies one byte of memory and the, last character is always ‘\0’. What character is this? It looks like, two characters, but it is actually only one character, with the \, indicating that what follows it is something special. ‘\0’ is called, null character. Note that ‘\0’ and ‘0’ are not same. ASCII value of, ‘\0’ is 0, whereas ASCII value of ‘0’ is 48. Figure 9.1 shows the, way a character array is stored in memory. Note that the elements, of the character array are stored in contiguous memory locations., The terminating null (‘\0’) is important, because it is the only way, the functions that work with a string can know where the string, ends. In fact, a string not terminated by a ‘\0’ is not really a string,, but merely a collection of characters.
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329, , Chapter 9: Puppetting On Strings, , H, , A, , E, , S, , L, , E, , R, , \0, , 65518 65519 65520 65521 65522 65523 65524 65525, , Figure 9.1, C concedes the fact that you would use strings very often and, hence provides a shortcut for initializing strings. For example, the, string used above can also be initialized as,, char name[ ] = "HAESLER" ;, , Note that, in this declaration ‘\0’ is not necessary. C inserts the, null character automatically., , More about Strings, In what way are character arrays different than numeric arrays?, Can elements in a character array be accessed in the same way as, the elements of a numeric array? Do I need to take any special care, of ‘\0’? Why numeric arrays don’t end with a ‘\0’? Declaring, strings is okay, but how do I manipulate them? Questions galore!!, Well, let us settle some of these issues right away with the help of, sample programs., /* Program to demonstrate printing of a string */, main( ), {, char name[ ] = "Klinsman" ;, int i = 0 ;, while ( i <= 7 ), {, printf ( "%c", name[i] ) ;, i++ ;, }
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330, , Let Us C, , }, , And here is the output..., Klinsman, , No big deal. We have initialized a character array, and then printed, out the elements of this array within a while loop. Can we write, the while loop without using the final value 7? We can; because, we know that each character array always ends with a ‘\0’., Following program illustrates this., main( ), {, char name[ ] = "Klinsman" ;, int i = 0 ;, while ( name[i] != `\0' ), {, printf ( "%c", name[i] ) ;, i++ ;, }, }, , And here is the output..., Klinsman, , This program doesn’t rely on the length of the string (number of, characters in it) to print out its contents and hence is definitely, more general than the earlier one. Here is another version of the, same program; this one uses a pointer to access the array elements., main( ), {, char name[ ] = "Klinsman" ;, char *ptr ;
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Chapter 9: Puppetting On Strings, , 331, , ptr = name ; /* store base address of string */, while ( *ptr != `\0' ), {, printf ( "%c", *ptr ) ;, ptr++ ;, }, }, , As with the integer array, by mentioning the name of the array we, get the base address (address of the zeroth element) of the array., This base address is stored in the variable ptr using,, ptr = name ;, , Once the base address is obtained in ptr, *ptr would yield the, value at this address, which gets printed promptly through,, printf ( "%c", *ptr ) ;, , Then, ptr is incremented to point to the next character in the, string. This derives from two facts: array elements are stored in, contiguous memory locations and on incrementing a pointer it, points to the immediately next location of its type. This process is, carried out till ptr doesn’t point to the last character in the string,, that is, ‘\0’., In fact, the character array elements can be accessed exactly in the, same way as the elements of an integer array. Thus, all the, following notations refer to the same element:, name[i], *( name + i ), *( i + name ), i[name]
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332, , Let Us C, , Even though there are so many ways (as shown above) to refer to, the elements of a character array, rarely is any one of them used., This is because printf( ) function has got a sweet and simple way, of doing it, as shown below. Note that printf( ) doesn’t print the, ‘\0’., main( ), {, char name[ ] = "Klinsman" ;, printf ( "%s", name ) ;, }, , The %s used in printf( ) is a format specification for printing out a, string. The same specification can be used to receive a string from, the keyboard, as shown below., main( ), {, char name[25] ;, printf ( "Enter your name " ) ;, scanf ( "%s", name ) ;, printf ( "Hello %s!", name ) ;, }, , And here is a sample run of the program..., Enter your name Debashish, Hello Debashish!, , Note that the declaration char name[25] sets aside 25 bytes under, the array name[ ], whereas the scanf( ) function fills in the, characters typed at keyboard into this array until the enter key is, hit. Once enter is hit, scanf( ) places a ‘\0’ in the array. Naturally,, we should pass the base address of the array to the scanf( ), function.
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Chapter 9: Puppetting On Strings, , 333, , While entering the string using scanf( ) we must be cautious about, two things:, (a) The length of the string should not exceed the dimension of, the character array. This is because the C compiler doesn’t, perform bounds checking on character arrays. Hence, if you, carelessly exceed the bounds there is always a danger of, overwriting something important, and in that event, you, would have nobody to blame but yourselves., (b) scanf( ) is not capable of receiving multi-word strings., Therefore names such as ‘Debashish Roy’ would be, unacceptable. The way to get around this limitation is by, using the function gets( ). The usage of functions gets( ) and, its counterpart puts( ) is shown below., main( ), {, char name[25] ;, printf ( "Enter your full name " ) ;, gets ( name ) ;, puts ( "Hello!" ) ;, puts ( name ) ;, }, , And here is the output..., Enter your name Debashish Roy, Hello!, Debashish Roy, , The program and the output are self-explanatory except for, the fact that, puts( ) can display only one string at a time, (hence the use of two puts( ) in the program above). Also, on, displaying a string, unlike printf( ), puts( ) places the cursor, on the next line. Though gets( ) is capable of receiving only
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334, , Let Us C, , one string at a time, the plus point with gets( ) is that it can, receive a multi-word string., If we are prepared to take the trouble we can make scanf( ), accept multi-word strings by writing it in this manner:, char name[25] ;, printf ( "Enter your full name " ) ;, scanf ( "%[^\n]s", name ) ;, , Though workable this is the best of the ways to call a, function, you would agree., , Pointers and Strings, Suppose we wish to store “Hello”. We may either store it in a, string or we may ask the C compiler to store it at some location in, memory and assign the address of the string in a char pointer. This, is shown below:, char str[ ] = "Hello" ;, char *p = "Hello" ;, , There is a subtle difference in usage of these two forms. For, example, we cannot assign a string to another, whereas, we can, assign a char pointer to another char pointer. This is shown in the, following program., main( ), {, char str1[ ] = "Hello" ;, char str2[10] ;, char *s = "Good Morning" ;, char *q ;
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Chapter 9: Puppetting On Strings, , 335, , str2 = str1 ; /* error */, q = s ; /* works */, }, , Also, once a string has been defined it cannot be initialized to, another set of characters. Unlike strings, such an operation is, perfectly valid with char pointers., main( ), {, char str1[ ] = "Hello" ;, char *p = "Hello" ;, str1 = "Bye" ; /* error */, p = "Bye" ; /* works */, }, , Standard Library String Functions, With every C compiler a large set of useful string handling library, functions are provided. Figure 9.2 lists the more commonly used, functions along with their purpose., , Function, , Use, , strlen, strlwr, strupr, strcat, strncat, , Finds length of a string, Converts a string to lowercase, Converts a string to uppercase, Appends one string at the end of another, Appends first n characters of a string at the end of, another
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336, strcpy, strncpy, strcmp, strncmp, strcmpi, stricmp, strnicmp, strdup, strchr, strrchr, strstr, strset, strnset, strrev, , Let Us C, Copies a string into another, Copies first n characters of one string into another, Compares two strings, Compares first n characters of two strings, Compares two strings without regard to case ("i" denotes, that this function ignores case), Compares two strings without regard to case (identical to, strcmpi), Compares first n characters of two strings without regard, to case, Duplicates a string, Finds first occurrence of a given character in a string, Finds last occurrence of a given character in a string, Finds first occurrence of a given string in another string, Sets all characters of string to a given character, Sets first n characters of a string to a given character, Reverses string, , Figure 9.2, Out of the above list we shall discuss the functions strlen( ),, strcpy( ), strcat( ) and strcmp( ), since these are the most, commonly used functions. This will also illustrate how the library, functions in general handle strings. Let us study these functions, one by one., , strlen( ), This function counts the number of characters present in a string., Its usage is illustrated in the following program., main( ), {, char arr[ ] = "Bamboozled" ;, int len1, len2 ;
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338, , Let Us C, , xstrlen ( char *s ), {, int length = 0 ;, while ( *s != '\0' ), {, length++ ;, s++ ;, }, return ( length ) ;, }, , The output would be..., string = Bamboozled length = 10, string = Humpty Dumpty length = 13, , The function xstrlen( ) is fairly simple. All that it does is keep, counting the characters till the end of string is not met. Or in other, words keep counting characters till the pointer s doesn’t point to, ‘\0’., , strcpy( ), This function copies the contents of one string into another. The, base addresses of the source and target strings should be supplied, to this function. Here is an example of strcpy( ) in action..., main( ), {, char source[ ] = "Sayonara" ;
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Chapter 9: Puppetting On Strings, , 339, , char target[20] ;, strcpy ( target, source ) ;, printf ( "\nsource string = %s", source ) ;, printf ( "\ntarget string = %s", target ) ;, }, , And here is the output..., source string = Sayonara, target string = Sayonara, , On supplying the base addresses, strcpy( ) goes on copying the, characters in source string into the target string till it doesn't, encounter the end of source string (‘\0’). It is our responsibility to, see to it that the target string’s dimension is big enough to hold the, string being copied into it. Thus, a string gets copied into another,, piece-meal, character by character. There is no short cut for this., Let us now attempt to mimic strcpy( ), via our own string copy, function, which we will call xstrcpy( )., main( ), {, char source[ ] = "Sayonara" ;, char target[20] ;, xstrcpy ( target, source ) ;, printf ( "\nsource string = %s", source ) ;, printf ( "\ntarget string = %s", target ) ;, }, xstrcpy ( char *t, char *s ), {, while ( *s != '\0' ), {, *t = *s ;, s++ ;
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340, , Let Us C, , t++ ;, }, *t = '\0' ;, }, , The output of the program would be..., source string = Sayonara, target string = Sayonara, , Note that having copied the entire source string into the target, string, it is necessary to place a ‘\0’ into the target string, to mark, its end., If you look at the prototype of strcpy( ) standard library function,, it looks like this…, strcpy ( char *t, const char *s ) ;, , We didn’t use the keyword const in our version of xstrcpy( ) and, still our function worked correctly. So what is the need of the, const qualifier?, What would happen if we add the following lines beyond the last, statement of xstrcpy( )?., s=s-8;, *s = 'K' ;, , This would change the source string to “Kayonara”. Can we not, ensure that the source string doesn’t change even accidentally in, xstrcpy( )? We can, by changing the definition as follows:, void xstrcpy ( char *t, const char *s ), {, while ( *s != '\0' ), {
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Chapter 9: Puppetting On Strings, , 341, , *t = *s ;, s++ ;, t++ ;, }, *t = '\0' ;, }, , By declaring char *s as const we are declaring that the source, string should remain constant (should not change). Thus the const, qualifier ensures that your program does not inadvertently alter a, variable that you intended to be a constant. It also reminds, anybody reading the program listing that the variable is not, intended to change., We can use const in several situations. The following code, fragment would help you to fix your ideas about const further., char *p = "Hello" ; /* pointer is variable, so is string */, *p = 'M' ; /* works */, p = "Bye" ; /* works */, const char *q = "Hello" ; /* string is fixed pointer is not */, *q = 'M' ; /* error */, q = "Bye" ; /* works */, char const *s = "Hello" ; /* string is fixed pointer is not */, *s = 'M' ; /* error */, s = "Bye" ; /* works */, char * const t = "Hello" ; /* pointer is fixed string is not */, *t = 'M' ; /* works */, t = "Bye" ; /* error */, const char * const u = "Hello" ; /* string is fixed so is pointer */, *u = 'M' ; /* error */, u = "Bye" ; /* error */
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342, , Let Us C, , The keyword const can be used in context of ordinary variables, like int, float, etc. The following program shows how this can be, done., main( ), {, float r, a ;, const float pi = 3.14 ;, printf ( "\nEnter radius of circle " ) ;, scanf ( "%f", &r ) ;, a = pi * r * r ;, printf ( "\nArea of circle = %f", a ) ;, }, , strcat( ), This function concatenates the source string at the end of the target, string. For example, “Bombay” and “Nagpur” on concatenation, would result into a string “BombayNagpur”. Here is an example of, strcat( ) at work., main( ), {, char source[ ] = "Folks!" ;, char target[30] = "Hello" ;, strcat ( target, source ) ;, printf ( "\nsource string = %s", source ) ;, printf ( "\ntarget string = %s", target ) ;, }, , And here is the output..., source string = Folks!, target string = HelloFolks!
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Chapter 9: Puppetting On Strings, , 343, , Note that the target string has been made big enough to hold the, final string. I leave it to you to develop your own xstrcat( ) on, lines of xstrlen( ) and xstrcpy( )., , strcmp( ), This is a function which compares two strings to find out whether, they are same or different. The two strings are compared character, by character until there is a mismatch or end of one of the strings, is reached, whichever occurs first. If the two strings are identical,, strcmp( ) returns a value zero. If they’re not, it returns the numeric, difference between the ASCII values of the first non-matching, pairs of characters. Here is a program which puts strcmp( ) in, action., main( ), {, char string1[ ] = "Jerry" ;, char string2[ ] = "Ferry" ;, int i, j, k ;, i = strcmp ( string1, "Jerry" ) ;, j = strcmp ( string1, string2 ) ;, k = strcmp ( string1, "Jerry boy" ) ;, printf ( "\n%d %d %d", i, j, k ) ;, }, , And here is the output..., 0 4 -32, , In the first call to strcmp( ), the two strings are identical—“Jerry”, and “Jerry”—and the value returned by strcmp( ) is zero. In the, second call, the first character of “Jerry” doesn't match with the, first character of “Ferry” and the result is 4, which is the numeric
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344, , Let Us C, , difference between ASCII value of ‘J’ and ASCII value of ‘F’. In, the third call to strcmp( ) “Jerry” doesn’t match with “Jerry boy”,, because the null character at the end of “Jerry” doesn’t match the, blank in “Jerry boy”. The value returned is -32, which is the value, of null character minus the ASCII value of space, i.e., ‘\0’ minus, ‘ ’, which is equal to -32., The exact value of mismatch will rarely concern us. All we usually, want to know is whether or not the first string is alphabetically, before the second string. If it is, a negative value is returned; if it, isn’t, a positive value is returned. Any non-zero value means there, is a mismatch. Try to implement this procedure into a function, xstrcmp( )., , Two-Dimensional Array of Characters, In the last chapter we saw several examples of 2-dimensional, integer arrays. Let’s now look at a similar entity, but one dealing, with characters. Our example program asks you to type your name., When you do so, it checks your name against a master list to see if, you are worthy of entry to the palace. Here’s the program..., #define FOUND 1, #define NOTFOUND 0, main( ), {, char masterlist[6][10] = {, "akshay",, "parag",, "raman",, "srinivas",, "gopal",, "rajesh", };, int i, flag, a ;, char yourname[10] ;
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Chapter 9: Puppetting On Strings, , 345, , printf ( "\nEnter your name " ) ;, scanf ( "%s", yourname ) ;, flag = NOTFOUND ;, for ( i = 0 ; i <= 5 ; i++ ), {, a = strcmp ( &masterlist[i][0], yourname ) ;, if ( a == 0 ), {, printf ( "Welcome, you can enter the palace" ) ;, flag = FOUND ;, break ;, }, }, if ( flag == NOTFOUND ), printf ( "Sorry, you are a trespasser" ) ;, }, , And here is the output for two sample runs of this program..., Enter your name dinesh, Sorry, you are a trespasser, Enter your name raman, Welcome, you can enter the palace, , Notice how the two-dimensional character array has been, initialized. The order of the subscripts in the array declaration is, important. The first subscript gives the number of names in the, array, while the second subscript gives the length of each item in, the array., Instead of initializing names, had these names been supplied from, the keyboard, the program segment would have looked like this..., for ( i = 0 ; i <= 5 ; i++ ), scanf ( "%s", &masterlist[i][0] ) ;
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346, , Let Us C, , While comparing the strings through strcmp( ), note that the, addresses of the strings are being passed to strcmp( ). As seen in, the last section, if the two strings match, strcmp( ) would return a, value 0, otherwise it would return a non-zero value., The variable flag is used to keep a record of whether the control, did reach inside the if or not. To begin with, we set flag to, NOTFOUND. Later through the loop if the names match, flag is, set to FOUND. When the control reaches beyond the for loop, if, flag is still set to NOTFOUND, it means none of the names in the, masterlist[ ][ ] matched with the one supplied from the keyboard., The names would be stored in the memory as shown in Figure 9.3., Note that each string ends with a ‘\0’. The arrangement as you can, appreciate is similar to that of a two-dimensional numeric array., , 65454, , a, , k, , s, , h, , a, , y, , 65464, , p, , a, , r, , a, , g, , \0, , 65474, , r, , a, , m a, , n, , \0, , 65484, , s, , r, , i, , n, , i, , v, , 65494, , g, , o, , p, , a, , l, , \0, , 65504, , r, , a, , j, , e, , s, , h, , Figure 9.3, , \0, , a, \0, , s, , \0, 65513, (last location)
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Chapter 9: Puppetting On Strings, , 347, , Here, 65454, 65464, 65474, etc. are the base addresses of, successive names. As seen from the above pattern some of the, names do not occupy all the bytes reserved for them. For example,, even though 10 bytes are reserved for storing the name “akshay”,, it occupies only 7 bytes. Thus, 3 bytes go waste. Similarly, for, each name there is some amount of wastage. In fact, more the, number of names, more would be the wastage. Can this not be, avoided? Yes, it can be... by using what is called an ‘array of, pointers’, which is our next topic of discussion., , Array of Pointers to Strings, As we know, a pointer variable always contains an address., Therefore, if we construct an array of pointers it would contain a, number of addresses. Let us see how the names in the earlier, example can be stored in the array of pointers., char *names[ ] = {, "akshay",, "parag",, "raman",, "srinivas",, "gopal",, "rajesh", };, , In this declaration names[ ] is an array of pointers. It contains base, addresses of respective names. That is, base address of “akshay” is, stored in names[0], base address of “parag” is stored in names[1], and so on. This is depicted in Figure 9.4.
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348, , Let Us C, , akshay\0, , raman\0, , 182, , srinivas\0, , 195, , 201, , gopal\0, , rajesh\0, , 210, , parag\0, , 216, , 189, , names[ ], 182, , 189, , 195, , 201, , 210, , 216, , 65514, , 65516, , 65518, , 65520, , 65522, , 65524, , Figure 9.4, In the two-dimensional array of characters, the strings occupied 60, bytes. As against this, in array of pointers, the strings occupy only, 41 bytes—a net saving of 19 bytes. A substantial saving, you, would agree. But realize that actually 19 bytes are not saved, since, 12 bytes are sacrificed for storing the addresses in the array, names[ ]. Thus, one reason to store strings in an array of pointers, is to make a more efficient use of available memory., Another reason to use an array of pointers to store strings is to, obtain greater ease in manipulation of the strings. This is shown by, the following programs. The first one uses a two-dimensional, array of characters to store the names, whereas the second uses an, array of pointers to strings. The purpose of both the programs is, very simple. We want to exchange the position of the names, “raman” and “srinivas”., /* Exchange names using 2-D array of characters */, main( ), {, char names[ ][10] = {
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Chapter 9: Puppetting On Strings, , 349, , "akshay",, "parag",, "raman",, "srinivas",, "gopal",, "rajesh", };, int i ;, char t ;, printf ( "\nOriginal: %s %s", &names[2][0], &names[3][0] ) ;, for ( i = 0 ; i <= 9 ; i++ ), {, t = names[2][i] ;, names[2][i] = names[3][i] ;, names[3][i] = t ;, }, printf ( "\nNew: %s %s", &names[2][0], &names[3][0] ) ;, }, , And here is the output..., Original: raman srinivas, New: srinivas raman, , Note that in this program to exchange the names we are required to, exchange corresponding characters of the two names. In effect, 10, exchanges are needed to interchange two names., Let us see, if the number of exchanges can be reduced by using an, array of pointers to strings. Here is the program..., main( ), {, char *names[ ] = {
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350, , Let Us C, "akshay",, "parag",, "raman",, "srinivas",, "gopal",, "rajesh", };, , char *temp ;, printf ( "Original: %s %s", names[2], names[3] ) ;, temp = names[2] ;, names[2] = names[3] ;, names[3] = temp ;, printf ( "\nNew: %s %s", names[2], names[3] ) ;, }, , And here is the output..., Original: raman srinivas, New: srinivas raman, , The output is same as the earlier program. In this program all that, we are required to do is exchange the addresses (of the names), stored in the array of pointers, rather than the names themselves., Thus, by effecting just one exchange we are able to interchange, names. This makes handling strings very convenient., Thus, from the point of view of efficient memory usage and ease, of programming, an array of pointers to strings definitely scores, over a two-dimensional character array. That is why, even though, in principle strings can be stored and handled through a twodimensional array of characters, in actual practice it is the array of, pointers to strings, which is more commonly used.
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Chapter 9: Puppetting On Strings, , 351, , Limitation of Array of Pointers to Strings, When we are using a two-dimensional array of characters we are, at liberty to either initialize the strings where we are declaring the, array, or receive the strings using scanf( ) function. However,, when we are using an array of pointers to strings we can initialize, the strings at the place where we are declaring the array, but we, cannot receive the strings from keyboard using scanf( ). Thus, the, following program would never work out., main( ), {, char *names[6] ;, int i ;, for ( i = 0 ; i <= 5 ; i++ ), {, printf ( "\nEnter name " ) ;, scanf ( "%s", names[i] ) ;, }, }, , The program doesn’t work because; when we are declaring the, array it is containing garbage values. And it would be definitely, wrong to send these garbage values to scanf( ) as the addresses, where it should keep the strings received from the keyboard., , Solution, If we are bent upon receiving the strings from keyboard using, scanf( ) and then storing their addresses in an array of pointers to, strings we can do it in a slightly round about manner as shown, below., #include "alloc.h", main( )
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352, , Let Us C, , {, char *names[6] ;, char n[50] ;, int len, i ;, char *p ;, for ( i = 0 ; i <= 5 ; i++ ), {, printf ( "\nEnter name " ) ;, scanf ( "%s", n ) ;, len = strlen ( n ) ;, p = malloc ( len + 1 ) ;, strcpy ( p, n ) ;, names[i] = p ;, }, for ( i = 0 ; i <= 5 ; i++ ), printf ( "\n%s", names[i] ) ;, }, , Here we have first received a name using scanf( ) in a string n[ ]., Then we have found out its length using strlen( ) and allocated, space for making a copy of this name. This memory allocation has, been done using a standard library function called malloc( ). This, function requires the number of bytes to be allocated and returns, the base address of the chunk of memory that it allocates. The, address returned by this function is always of the type void *., Hence it has been converted into char * using a feature called, typecasting. Typecasting is discussed in detail in Chapter 15. The, prototype of this function has been declared in the file ‘alloc.h’., Hence we have #included this file., But why did we not use array to allocate memory? This is because, with arrays we have to commit to the size of the array at the time, of writing the program. Moreover, there is no way to increase or, decrease the array size during execution of the program. In other, words, when we use arrays static memory allocation takes place.
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Chapter 9: Puppetting On Strings, , 353, , Unlike this, using malloc( ) we can allocate memory dynamically,, during execution. The argument that we pass to malloc( ) can be a, variable whose value can change during execution., Once we have allocated the memory using malloc( ) we have, copied the name received through the keyboard into this allocated, space and finally stored the address of the allocated chunk in the, appropriate element of names[ ], the array of pointers to strings., This solution suffers in performance because we need to allocate, memory and then do the copying of string for each name received, through the keyboard., , Summary, (a) A string is nothing but an array of characters terminated by, ‘\0’., (b) Being an array, all the characters of a string are stored in, contiguous memory locations., (c) Though scanf( ) can be used to receive multi-word strings,, gets( ) can do the same job in a cleaner way., (d) Both printf( ) and puts( ) can handle multi-word strings., (e) Strings can be operated upon using several standard library, functions like strlen( ), strcpy( ), strcat( ) and strcmp( ), which can manipulate strings. More importantly we imitated, some of these functions to learn how these standard library, functions are written., (f) Though in principle a 2-D array can be used to handle several, strings, in practice an array of pointers to strings is preferred, since it takes less space and is efficient in processing strings., (g) malloc( ) function can be used to allocate space in memory on, the fly during execution of the program., , Exercise, Simple strings
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354, , Let Us C, , [A] What would be the output of the following programs:, , (a) main( ), {, char c[2] = "A" ;, printf ( "\n%c", c[0] ) ;, printf ( "\n%s", c ) ;, }, (b) main( ), {, char s[ ] = "Get organised! learn C!!" ;, printf ( "\n%s", &s[2] ) ;, printf ( "\n%s", s ) ;, printf ( "\n%s", &s ) ;, printf ( "\n%c", s[2] ) ;, }, (c) main( ), {, char s[ ] = "No two viruses work similarly" ;, int i = 0 ;, while ( s[i] != 0 ), {, printf ( "\n%c %c", s[i], *( s + i ) ) ;, printf ( "\n%c %c", i[s], *( i + s ) ) ;, i++ ;, }, }, (d) main( ), {, char s[ ] = "Churchgate: no church no gate" ;, char t[25] ;, char *ss, *tt ;, ss = s ;, while ( *ss != '\0' ), *ss++ = *tt++ ;
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356, , Let Us C, int arr[ ] = { ‘A’, ‘B’, ‘C’, ‘D’ } ;, int i ;, for ( i = 0 ; i <= 3 ; i++ ), printf ( "\n%d", arr[i] ) ;, , }, , (c) main( ), {, char arr[8] = "Rhombus" ;, int i ;, for ( i = 0 ; i <= 7 ; i++ ), printf ( "\n%d", *arr ) ;, arr++ ;, }, [C] Fill in the blanks:, , (a) "A" is a ___________ while ’A’ is a ____________., (b) A string is terminated by a ______ character, which is written, as ______., (c) The array char name[10] can consist of a maximum of, ______ characters., (d) The array elements are always stored in _________ memory, locations., [D] Attempt the following:, , (a) Which is more appropriate for reading in a multi-word string?, gets( ), , printf( ), , scanf( ), , puts( ), , (b) If the string "Alice in wonder land" is fed to the following, scanf( ) statement, what will be the contents of the arrays, str1, str2, str3 and str4?, scanf ( "%s%s%s%s%s", str1, str2, str3, str4 ) ;
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357, , Chapter 9: Puppetting On Strings, , (c) Write a program that converts all lowercase characters in a, given string to its equivalent uppercase character., (d) Write a program that extracts part of the given string from the, specified position. For example, if the sting is "Working with, strings is fun", then if from position 4, 4 characters are to be, extracted then the program should return string as "king"., Moreover, if the position from where the string is to be, extracted is given and the number of characters to be, extracted is 0 then the program should extract entire string, from the specified position., (e) Write a program that converts a string like "124" to an integer, 124., (f) Write a program that replaces two or more consecutive blanks, in a string by a single blank. For example, if the input is, Grim, , return, , to the, , planet, , of, , apes!!, , the output should be, Grim return to the planet of apes!!, , Two-dimensional array, Array of pointers to strings, [E] Answer the following:, , (a) How many bytes in memory would be occupied by the, following array of pointers to strings? How many bytes would, be required to store the same strings, if they are stored in a, two-dimensional character array?, char *mess[ ] = {, "Hammer and tongs",, "Tooth and nail",
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358, , Let Us C, "Spit and polish",, "You and C", };, , (b) Can an array of pointers to strings be used to collect strings, from the keyboard? If not, why not?, [F] Attempt the following:, , (a) Write a program that uses an array of pointers to strings str[ ]., Receive two strings str1 and str2 and check if str1 is, embedded in any of the strings in str[ ]. If str1 is found, then, replace it with str2., char *str[ ] = {, "We will teach you how to...",, "Move a mountain",, "Level a building",, "Erase the past",, "Make a million",, "...all through C!", , };, For example if str1 contains "mountain" and str2 contains, "car", then the second string in str should get changed to, "Move a car"., (b) Write a program to sort a set of names stored in an array in, alphabetical order., (c) Write a program to reverse the strings stored in the following, array of pointers to strings:, char *s[ ] = {, "To err is human...",, "But to really mess things up...",, "One needs to know C!!", };
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359, , Chapter 9: Puppetting On Strings, , Hint: Write a function xstrrev ( string ) which should reverse, the contents of one string. Call this function for reversing each, string stored in s., (d) Develop a program that receives the month and year from the, keyboard as integers and prints the calendar in the following, format., , September 2004, Mon, , Tue, , 6, 13, 20, 27, , 7, 14, 21, 28, , Wed, 1, 8, 15, 22, 29, , Thu, 2, 9, 16, 23, 30, , Fri, 3, 10, 17, 24, , Sat, 4, 11, 18, 25, , Sun, 5, 12, 19, 26, , Note that according to the Gregorian calendar 01/01/1900 was, Monday. With this as the base the calendar should be, generated., (e) Modify the above program suitably so that once the calendar, for a particular month and year has been displayed on the
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360, , Let Us C, , screen, then using arrow keys the user must be able to change, the calendar in the following manner:, Up arrow key, Down arrow key, Right arrow key, Left arrow key, , : Next year, same month, : Previous year, same month, : Same year, next month, : Same year, previous month, , If the escape key is hit then the procedure should stop., Hint: Use the getkey( ) function discussed in Chapter 8,, problem number [L](c)., (f) A factory has 3 division and stocks 4 categories of products., An inventory table is updated for each division and for each, product as they are received. There are three independent, suppliers of products to the factory:, (a) Design a data format to represent each transaction., (b) Write a program to take a transaction and update the, inventory., (c) If the cost per item is also given write a program to, calculate the total inventory values., (g) A dequeue is an ordered set of elements in which elements, may be inserted or retrieved from either end. Using an array, simulate a dequeue of characters and the operations retrieve, left, retrieve right, insert left, insert right. Exceptional, conditions such as dequeue full or empty should be indicated., Two pointers (namely, left and right) are needed in this, simulation., (h) Write a program to delete all vowels from a sentence. Assume, that the sentence is not more than 80 characters long., (i) Write a program that will read a line and delete from it all, occurrences of the word ‘the’.
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Chapter 9: Puppetting On Strings, , 361, , (j) Write a program that takes a set of names of individuals and, abbreviates the first, middle and other names except the last, name by their first letter., (k) Write a program to count the number of occurrences of any, two vowels in succession in a line of text. For example, in the, sentence, “Pleases read this application and give me gratuity”, such occurrences are ea, ea, ui.
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10, , Structures, • Why Use Structures, Declaring a Structure, Accessing Structure Elements, How Structure Elements are Stored, • Array of Structures, • Additional Features of Structures, • Uses of Structures, • Summary, • Exercise, , 363
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364, , Let Us C, , W, , hich mechanic is good enough who knows how to repair, only one type of vehicle? None. Same thing is true about, C language. It wouldn’t have been so popular had it been, able to handle only all ints, or all floats or all chars at a time. In, fact when we handle real world data, we don’t usually deal with, little atoms of information by themselves—things like integers,, characters and such. Instead we deal with entities that are, collections of things, each thing having its own attributes, just as, the entity we call a ‘book’ is a collection of things such as title,, author, call number, publisher, number of pages, date of, publication, etc. As you can see all this data is dissimilar, for, example author is a string, whereas number of pages is an integer., For dealing with such collections, C provides a data type called, ‘structure’. A structure gathers together, different atoms of, information that comprise a given entity. And structure is the topic, of this chapter., , Why Use Structures, We have seen earlier how ordinary variables can hold one piece of, information and how arrays can hold a number of pieces of, information of the same data type. These two data types can, handle a great variety of situations. But quite often we deal with, entities that are collection of dissimilar data types., For example, suppose you want to store data about a book. You, might want to store its name (a string), its price (a float) and, number of pages in it (an int). If data about say 3 such books is to, be stored, then we can follow two approaches:, (a) Construct individual arrays, one for storing names, another for, storing prices and still another for storing number of pages., (b) Use a structure variable., Let us examine these two approaches one by one. For the sake of, programming convenience assume that the names of books would
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Chapter 10: Structures, , 365, , be single character long. Let us begin with a program that uses, arrays., main( ), {, char name[3] ;, float price[3] ;, int pages[3], i ;, printf ( "\nEnter names, prices and no. of pages of 3 books\n" ) ;, for ( i = 0 ; i <= 2 ; i++ ), scanf ( "%c %f %d", &name[i], &price[i], &pages[i] );, printf ( "\nAnd this is what you entered\n" ) ;, for ( i = 0 ; i <= 2 ; i++ ), printf ( "%c %f %d\n", name[i], price[i], pages[i] );, }, , And here is the sample run..., Enter names, prices and no. of pages of 3 books, A 100.00 354, C 256.50 682, F 233.70 512, And this is what you entered, A 100.000000 354, C 256.500000 682, F 233.700000 512, , This approach no doubt allows you to store names, prices and, number of pages. But as you must have realized, it is an unwieldy, approach that obscures the fact that you are dealing with a group, of characteristics related to a single entity—the book.
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366, , Let Us C, , The program becomes more difficult to handle as the number of, items relating to the book go on increasing. For example, we, would be required to use a number of arrays, if we also decide to, store name of the publisher, date of purchase of book, etc. To solve, this problem, C provides a special data type—the structure., A structure contains a number of data types grouped together., These data types may or may not be of the same type. The, following example illustrates the use of this data type., main( ), {, struct book, {, char name ;, float price ;, int pages ;, };, struct book b1, b2, b3 ;, printf ( "\nEnter names, prices & no. of pages of 3 books\n" ) ;, scanf ( "%c %f %d", &b1.name, &b1.price, &b1.pages ) ;, scanf ( "%c %f %d", &b2.name, &b2.price, &b2.pages ) ;, scanf ( "%c %f %d", &b3.name, &b3.price, &b3.pages ) ;, printf ( "\nAnd this is what you entered" ) ;, printf ( "\n%c %f %d", b1.name, b1.price, b1.pages ) ;, printf ( "\n%c %f %d", b2.name, b2.price, b2.pages ) ;, printf ( "\n%c %f %d", b3.name, b3.price, b3.pages ) ;, }, , And here is the output..., Enter names, prices and no. of pages of 3 books, A 100.00 354, C 256.50 682, F 233.70 512
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Chapter 10: Structures, , 367, , And this is what you entered, A 100.000000 354, C 256.500000 682, F 233.700000 512, , This program demonstrates two fundamental aspects of structures:, (a) declaration of a structure, (b) accessing of structure elements, Let us now look at these concepts one by one., , Declaring a Structure, In our example program, the following statement declares the, structure type:, struct book, {, char name ;, float price ;, int pages ;, };, , This statement defines a new data type called struct book. Each, variable of this data type will consist of a character variable called, name, a float variable called price and an integer variable called, pages. The general form of a structure declaration statement is, given below:, struct <structure name>, {, structure element 1 ;, structure element 2 ;, structure element 3 ;, ......, ......
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368, , Let Us C, , };, , Once the new structure data type has been defined one or more, variables can be declared to be of that type. For example the, variables b1, b2, b3 can be declared to be of the type struct book,, as,, struct book b1, b2, b3 ;, , This statement sets aside space in memory. It makes available, space to hold all the elements in the structure—in this case, 7, bytes—one for name, four for price and two for pages. These, bytes are always in adjacent memory locations., If we so desire, we can combine the declaration of the structure, type and the structure variables in one statement., For example,, struct book, {, char name ;, float price ;, int pages ;, };, struct book b1, b2, b3 ;, , is same as..., struct book, {, char name ;, float price ;, int pages ;, } b1, b2, b3 ;, or even..., struct
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Chapter 10: Structures, , 369, , {, char name ;, float price ;, int pages ;, } b1, b2, b3 ;, , Like primary variables and arrays, structure variables can also be, initialized where they are declared. The format used is quite, similar to that used to initiate arrays., struct book, {, char name[10] ;, float price ;, int pages ;, };, struct book b1 = { "Basic", 130.00, 550 } ;, struct book b2 = { "Physics", 150.80, 800 } ;, , Note the following points while declaring a structure type:, (a) The closing brace in the structure type declaration must be, followed by a semicolon., (b) It is important to understand that a structure type declaration, does not tell the compiler to reserve any space in memory. All, a structure declaration does is, it defines the ‘form’ of the, structure., (c) Usually structure type declaration appears at the top of the, source code file, before any variables or functions are defined., In very large programs they are usually put in a separate, header file, and the file is included (using the preprocessor, directive #include) in whichever program we want to use this, structure type.
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370, , Let Us C, , Accessing Structure Elements, Having declared the structure type and the structure variables, let, us see how the elements of the structure can be accessed., In arrays we can access individual elements of an array using a, subscript. Structures use a different scheme. They use a dot (.), operator. So to refer to pages of the structure defined in our, sample program we have to use,, b1.pages, , Similarly, to refer to price we would use,, b1.price, , Note that before the dot there must always be a structure variable, and after the dot there must always be a structure element., , How Structure Elements are Stored, Whatever be the elements of a structure, they are always stored in, contiguous memory locations. The following program would, illustrate this:, /* Memory map of structure elements */, main( ), {, struct book, {, char name ;, float price ;, int pages ;, };, struct book b1 = { 'B', 130.00, 550 } ;, printf ( "\nAddress of name = %u", &b1.name ) ;
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371, , Chapter 10: Structures, printf ( "\nAddress of price = %u", &b1.price ) ;, printf ( "\nAddress of pages = %u", &b1.pages ) ;, }, , Here is the output of the program..., Address of name = 65518, Address of price = 65519, Address of pages = 65523, , Actually the structure elements are stored in memory as shown in, the Figure 10.1., , b1.name, , b1.price, , b1.pages, , ‘B’, , 130.00, , 550, , 65518, , 65519, , 65523, , Figure 10.1, , Array of Structures, Our sample program showing usage of structure is rather simple, minded. All it does is, it receives values into various structure, elements and output these values. But that’s all we intended to do, anyway... show how structure types are created, how structure, variables are declared and how individual elements of a structure, variable are referenced., In our sample program, to store data of 100 books we would be, required to use 100 different structure variables from b1 to b100,, which is definitely not very convenient. A better approach would, be to use an array of structures. Following program shows how to, use an array of structures.
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372, , Let Us C, , /* Usage of an array of structures */, main( ), {, struct book, {, char name ;, float price ;, int pages ;, };, struct book b[100] ;, int i ;, for ( i = 0 ; i <= 99 ; i++ ), {, printf ( "\nEnter name, price and pages " ) ;, scanf ( "%c %f %d", &b[i].name, &b[i].price, &b[i].pages ) ;, }, for ( i = 0 ; i <= 99 ; i++ ), printf ( "\n%c %f %d", b[i].name, b[i].price, b[i].pages ) ;, }, linkfloat( ), {, float a = 0, *b ;, b = &a ; /* cause emulator to be linked */, a = *b ; /* suppress the warning - variable not used */, }, , Now a few comments about the program:, (a) Notice how the array of structures is declared..., struct book b[100] ;
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Chapter 10: Structures, , 373, , This provides space in memory for 100 structures of the type, struct book., (b) The syntax we use to reference each element of the array b is, similar to the syntax used for arrays of ints and chars. For, example, we refer to zeroth book’s price as b[0].price., Similarly, we refer first book’s pages as b[1].pages., (c) It should be appreciated what careful thought Dennis Ritchie, has put into C language. He first defined array as a collection, of similar elements; then realized that dissimilar data types, that are often found in real life cannot be handled using, arrays, therefore created a new data type called structure. But, even using structures programming convenience could not be, achieved, because a lot of variables (b1 to b100 for storing, data about hundred books) needed to be handled. Therefore he, allowed us to create an array of structures; an array of similar, data types which themselves are a collection of dissimilar data, types. Hats off to the genius!, (d) In an array of structures all elements of the array are stored in, adjacent memory locations. Since each element of this array is, a structure, and since all structure elements are always stored, in adjacent locations you can very well visualise the, arrangement of array of structures in memory. In our example,, b[0]’s name, price and pages in memory would be, immediately followed by b[1]’s name, price and pages, and, so on., (e) What is the function linkfloat( ) doing here? If you don’t, define it you are bound to get the error "Floating Point, Formats Not Linked" with majority of C Compilers. What, causes this error to occur? When parsing our source file, if the, compiler encounters a reference to the address of a float, it, sets a flag to have the linker link in the floating-point, emulator. A floating point emulator is used to manipulate, floating point numbers in runtime library functions like
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374, , Let Us C, , scanf( ) and atof( ). There are some cases in which the, reference to the float is a bit obscure and the compiler does, not detect the need for the emulator. The most common is, using scanf( ) to read a float in an array of structures as, shown in our program., How can we force the formats to be linked? That’s where the, linkfloat( ) function comes in. It forces linking of the, floating-point emulator into an application. There is no need, to call this function, just define it anywhere in your program., , Additional Features of Structures, Let us now explore the intricacies of structures with a view of, programming convenience. We would highlight these intricacies, with suitable examples:, (a) The values of a structure variable can be assigned to another, structure variable of the same type using the assignment, operator. It is not necessary to copy the structure elements, piece-meal. Obviously, programmers prefer assignment to, piece-meal copying. This is shown in the following example., main( ), {, struct employee, {, char name[10] ;, int age ;, float salary ;, };, struct employee e1 = { "Sanjay", 30, 5500.50 } ;, struct employee e2, e3 ;, /* piece-meal copying */, strcpy ( e2.name, e1.name ) ;, e2.age = e1.age ;
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Chapter 10: Structures, , 375, , e2.salary = e1.salary ;, /* copying all elements at one go */, e3 = e2 ;, printf ( "\n%s %d %f", e1.name, e1.age, e1.salary ) ;, printf ( "\n%s %d %f", e2.name, e2.age, e2.salary ) ;, printf ( "\n%s %d %f", e3.name, e3.age, e3.salary ) ;, }, , The output of the program would be..., Sanjay 30 5500.500000, Sanjay 30 5500.500000, Sanjay 30 5500.500000, , Ability to copy the contents of all structure elements of one, variable into the corresponding elements of another structure, variable is rather surprising, since C does not allow assigning, the contents of one array to another just by equating the two., As we saw earlier, for copying arrays we have to copy the, contents of the array element by element., This copying of all structure elements at one go has been, possible only because the structure elements are stored in, contiguous memory locations. Had this not been so, we would, have been required to copy structure variables element by, element. And who knows, had this been so, structures would, not have become popular at all., (b) One structure can be nested within another structure. Using, this facility complex data types can be created. The following, program shows nested structures at work., main( ), {, struct address
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376, , Let Us C, {, char phone[15] ;, char city[25] ;, int pin ;, };, struct emp, {, char name[25] ;, struct address a ;, };, struct emp e = { "jeru", "531046", "nagpur", 10 };, printf ( "\nname = %s phone = %s", e.name, e.a.phone ) ;, printf ( "\ncity = %s pin = %d", e.a.city, e.a.pin ) ;, , }, , And here is the output..., name = jeru phone = 531046, city = nagpur pin = 10, , Notice the method used to access the element of a structure, that is part of another structure. For this the dot operator is, used twice, as in the expression,, e.a.pin or e.a.city, , Of course, the nesting process need not stop at this level. We, can nest a structure within a structure, within another, structure, which is in still another structure and so on... till the, time we can comprehend the structure ourselves. Such, construction however gives rise to variable names that can be, surprisingly self descriptive, for example:, maruti.engine.bolt.large.qty
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Chapter 10: Structures, , 377, , This clearly signifies that we are referring to the quantity of, large sized bolts that fit on an engine of a maruti car., (c) Like an ordinary variable, a structure variable can also be, passed to a function. We may either pass individual structure, elements or the entire structure variable at one go. Let us, examine both the approaches one by one using suitable, programs., /* Passing individual structure elements */, main( ), {, struct book, {, char name[25] ;, char author[25] ;, int callno ;, };, struct book b1 = { "Let us C", "YPK", 101 } ;, display ( b1.name, b1.author, b1.callno ) ;, }, display ( char *s, char *t, int n ), {, printf ( "\n%s %s %d", s, t, n ) ;, }, , And here is the output..., Let us C YPK 101, , Observe that in the declaration of the structure, name and, author have been declared as arrays. Therefore, when we call, the function display( ) using,, display ( b1.name, b1.author, b1.callno ) ;
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378, , Let Us C, , we are passing the base addresses of the arrays name and, author, but the value stored in callno. Thus, this is a mixed, call—a call by reference as well as a call by value., It can be immediately realized that to pass individual elements, would become more tedious as the number of structure, elements go on increasing. A better way would be to pass the, entire structure variable at a time. This method is shown in the, following program., struct book, {, char name[25] ;, char author[25] ;, int callno ;, };, main( ), {, struct book b1 = { "Let us C", "YPK", 101 } ;, display ( b1 ) ;, }, display ( struct book b ), {, printf ( "\n%s %s %d", b.name, b.author, b.callno ) ;, }, , And here is the output..., Let us C YPK 101, , Note that here the calling of function display( ) becomes quite, compact,, display ( b1 ) ;
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Chapter 10: Structures, , 379, , Having collected what is being passed to the display( ), function, the question comes, how do we define the formal, arguments in the function. We cannot say,, struct book b1 ;, , because the data type struct book is not known to the, function display( ). Therefore, it becomes necessary to define, the structure type struct book outside main( ), so that it, becomes known to all functions in the program., (d) The way we can have a pointer pointing to an int, or a pointer, pointing to a char, similarly we can have a pointer pointing to, a struct. Such pointers are known as ‘structure pointers’., Let us look at a program that demonstrates the usage of a, structure pointer., main( ), {, struct book, {, char name[25] ;, char author[25] ;, int callno ;, };, struct book b1 = { "Let us C", "YPK", 101 } ;, struct book *ptr ;, ptr = &b1 ;, printf ( "\n%s %s %d", b1.name, b1.author, b1.callno ) ;, printf ( "\n%s %s %d", ptr->name, ptr->author, ptr->callno ) ;, }, , The first printf( ) is as usual. The second printf( ) however is, peculiar. We can’t use ptr.name or ptr.callno because ptr is, not a structure variable but a pointer to a structure, and the dot
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380, , Let Us C, , operator requires a structure variable on its left. In such cases, C provides an operator ->, called an arrow operator to refer to, the structure elements. Remember that on the left hand side of, the ‘.’ structure operator, there must always be a structure, variable, whereas on the left hand side of the ‘->’ operator, there must always be a pointer to a structure. The arrangement, of the structure variable and pointer to structure in memory is, shown in the Figure 10.2., , b1.name, , b1.author, , b1.callno, , Let Us C, , YPK, , 101, , 65472, , 65497, , 65522, , ptr, 65472, 65524, , Figure 10.2, Can we not pass the address of a structure variable to a, function? We can. The following program demonstrates this., /* Passing address of a structure variable */, struct book, {, char name[25] ;, char author[25] ;, int callno ;, };, main( ), {, struct book b1 = { "Let us C", "YPK", 101 } ;, display ( &b1 ) ;
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Chapter 10: Structures, , 381, , }, display ( struct book *b ), {, printf ( "\n%s %s %d", b->name, b->author, b->callno ) ;, }, , And here is the output..., Let us C YPK 101, , Again note that to access the structure elements using pointer, to a structure we have to use the ‘->’ operator., Also, the structure struct book should be declared outside, main( ) such that this data type is available to display( ) while, declaring pointer to the structure., (e) Consider the following code snippet:, struct emp, {, int a ;, char ch ;, float s ;, };, struct emp e ;, printf ( "%u %u %u", &e.a, &e.ch, &e.s ) ;, , If we execute this program using TC/TC++ compiler we get, the addresses as:, 65518 65520 65521, , As expected, in memory the char begins immediately after, the int and float begins immediately after the char.
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382, , Let Us C, , However, if we run the same program using VC++ compiler, then the output turns out to be:, 1245044 1245048 1245052, , It can be observed from this output that the float doesn’t get, stored immediately after the char. In fact there is a hole of, three bytes after the char. Let us understand the reason for, this. VC++ is a 32-bit compiler targeted to generate code for a, 32-bit microprocessor. The architecture of this microprocessor, is such that it is able to fetch the data that is present at an, address, which is a multiple of four much faster than the data, present at any other address. Hence the VC++ compiler aligns, every element of a structure at an address that is multiple of, four. That’s the reason why there were three holes created, between the char and the float., However, some programs need to exercise precise control, over the memory areas where data is placed. For example,, suppose we wish to read the contents of the boot sector (first, sector on the floppy/hard disk) into a structure. For this the, byte arrangement of the structure elements must match the, arrangement of various fields in the boot sector of the disk., The #pragma pack directive offers a way to fulfill this, requirement. This directive specifies packing alignment for, structure members. The pragma takes effect at the first, structure declaration after the pragma is seen. Turbo C/C++, compiler doesn’t support this feature, VC++ compiler does., The following code shows how to use this directive., #pragma pack(1), struct emp, {, int a ;, char ch ;, float s ;, };
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Chapter 10: Structures, , 383, , #pragma pack( ), struct emp e ;, printf ( "%u %u %u", &e.a, &e.ch, &e.s ) ;, , Here, #pragma pack ( 1 ) lets each structure element to begin, on a 1-byte boundary as justified by the output of the program, given below:, 1245044 1245048 1245049, , Uses of Structures, Where are structures useful? The immediate application that, comes to the mind is Database Management. That is, to maintain, data about employees in an organization, books in a library, items, in a store, financial accounting transactions in a company etc. But, mind you, use of structures stretches much beyond database, management. They can be used for a variety of purposes like:, (a), (b), (c), (d), (e), (f), (g), (h), (i), (j), (k), (l), , Changing the size of the cursor, Clearing the contents of the screen, Placing the cursor at an appropriate position on screen, Drawing any graphics shape on the screen, Receiving a key from the keyboard, Checking the memory size of the computer, Finding out the list of equipment attached to the computer, Formatting a floppy, Hiding a file from the directory, Displaying the directory of a disk, Sending the output to printer, Interacting with the mouse, , And that is certainly a very impressive list! At least impressive, enough to make you realize how important a data type a structure, is and to be thorough with it if you intend to program any of the
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384, , Let Us C, , above applications. Some of these applications would be discussed, in Chapters 16 to 19., , Summary, (a) A structure is usually used when we wish to store dissimilar, data together., (b) Structure elements can be accessed through a structure, variable using a dot (.) operator., (c) Structure elements can be accessed through a pointer to a, structure using the arrow (->) operator., (d) All elements of one structure variable can be assigned to, another structure variable using the assignment (=) operator., (e) It is possible to pass a structure variable to a function either, by value or by address., (f) It is possible to create an array of structures., , Exercise, [A] What would be the output of the following programs:, (a) main( ), {, struct gospel, {, int num ;, char mess1[50] ;, char mess2[50] ;, }m;, m.num = 1 ;, strcpy ( m.mess1, "If all that you have is hammer" ) ;, strcpy ( m.mess2, "Everything looks like a nail" ) ;, /* assume that the strucure is located at address 1004 */, printf ( "\n%u %u %u", &m.num, m.mess1, m.mess2 ) ;, }
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386, , Let Us C, char language[10] ;, };, struct employee e = { "Hacker", "C" } ;, printf ( "\n%s %d", e.name, e.language ) ;, , }, (c), , struct virus, {, char signature[25] ;, char status[20] ;, int size ;, } v[2] = {, "Yankee Doodle", "Deadly", 1813,, "Dark Avenger", "Killer", 1795, };, main( ), {, int i ;, for ( i = 0 ; i <=1 ; i++ ), printf ( "\n%s %s", v.signature, v.status ) ;, }, , (d) struct s, {, char ch ;, int i ;, float a ;, };, main( ), {, struct s var = { 'C', 100, 12.55 } ;, f ( var ) ;, g ( &var ) ;, }, f ( struct s v ), {, printf ( "\n%c %d %f", v -> ch, v -> i, v -> a ) ;, }
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Chapter 10: Structures, , 387, , g ( struct s *v ), {, printf ( "\n%c %d %f", v.ch, v.i, v.a ) ;, }, (e) struct s, {, int i ;, struct s *p ;, };, main( ), {, struct s var1, var2 ;, var1.i = 100 ;, var2.i = 200 ;, var1.p = &var2 ;, var2.p = &var1 ;, printf ( "\n%d %d", var1.p -> i, var2.p -> i ) ;, }, [C] Answer the following:, , (a) Ten floats are to be stored in memory. What would you, prefer, an array or a structure?, (b) Given the statement,, maruti.engine.bolts = 25 ;, , which of the following is True?, 1., 2., 3., 4., , structure bolts is nested within structure engine, structure engine is nested within structure maruti, structure maruti is nested within structure engine, structure maruti is nested within structure bolts, , (c) State True or False:, 1. All structure elements are stored in contiguous memory, locations.
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388, , Let Us C, , 2. An array should be used to store dissimilar elements, and, a structure to store similar elements., 3. In an array of structures, not only are all structures stored, in contiguous memory locations, but the elements of, individual structures are also stored in contiguous, locations., (d) struct time, {, int hours ;, int minutes ;, int seconds ;, }t;, struct time *tt ;, tt = &t ;, Looking at the above declarations, which of the following, refers to seconds correctly:, 1. tt.seconds, 2. ( *tt ).seconds, 3. time.t, 4. tt -> seconds, [D] Attempt the following:, , (a) Create a structure to specify data on students given below:, Roll number, Name, Department, Course, Year of joining, Assume that there are not more than 450 students in the, collage., (a) Write a function to print names of all students who joined, in a particular year., (b) Write a function to print the data of a student whose roll, number is given.
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Chapter 10: Structures, , 389, , (b) Create a structure to specify data of customers in a bank. The, data to be stored is: Account number, Name, Balance in, account. Assume maximum of 200 customers in the bank., (a) Write a function to print the Account number and name, of each customer with balance below Rs. 100., (b) If a customer request for withdrawal or deposit, it is, given in the form:, Acct. no, amount, code (1 for deposit, 0 for withdrawal), Write a program to give a message, “The balance is, insufficient for the specified withdrawal”., (c) An automobile company has serial number for engine parts, starting from AA0 to FF9. The other characteristics of parts to, be specified in a structure are: Year of manufacture, material, and quantity manufactured., (a) Specify a structure to store information corresponding to, a part., (b) Write a program to retrieve information on parts with, serial numbers between BB1 and CC6., (d) A record contains name of cricketer, his age, number of test, matches that he has played and the average runs that he has, scored in each test match. Create an array of structure to hold, records of 20 such cricketer and then write a program to read, these records and arrange them in ascending order by average, runs. Use the qusort( ) standard library function., (e) There is a structure called employee that holds information, like employee code, name, date of joining. Write a program to, create an array of the structure and enter some data into it., Then ask the user to enter current date. Display the names of, those employees whose tenure is 3 or more than 3 years, according to the given current date., (f) Write a menu driven program that depicts the working of a, library. The menu options should be:
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390, 1., 2., 3., 4., 5., 6., 7., , Let Us C, Add book information, Display book information, List all books of given author, List the title of specified book, List the count of books in the library, List the books in the order of accession number, Exit, , Create a structure called library to hold accession number,, title of the book, author name, price of the book, and flag, indicating whether book is issued or not., (g) Write a program that compares two given dates. To store date, use structure say date that contains three members namely, date, month and year. If the dates are equal then display, message as "Equal" otherwise "Unequal"., (h) Linked list is a very common data structure often used to store, similar data in memory. While the elements of an array, occupy contiguous memory locations, those of a linked list, are not constrained to be stored in adjacent location. The, individual elements are stored “somewhere” in memory,, rather like a family dispersed, but still bound together. The, order of the elements is maintained by explicit links between, them. Thus, a linked list is a collection of elements called, nodes, each of which stores two item of information—an, element of the list, and a link, i.e., a pointer or an address that, indicates explicitly the location of the node containing the, successor of this list element., Write a program to build a linked list by adding new nodes at, the beginning, at the end or in the middle of the linked list., Also write a function display( ) which display all the nodes, present in the linked list., (i) A stack is a data structure in which addition of new element, or deletion of existing element always takes place at the same
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Chapter 10: Structures, , 391, , end. This end is often known as ‘top’ of stack. This situation, can be compared to a stack of plates in a cafeteria where every, new plate taken off the stack is also from the ‘top’ of the, stack. There are several application where stack can be put to, use. For example, recursion, keeping track of function calls,, evaluation of expressions, etc. Write a program to implement, a stack using a linked list., (j) Unlike a stack, in a queue the addition of new element takes, place at the end (called ‘rear’ of queue) whereas deletion takes, place at the other end (called ‘front’ of queue). Write a, program to implement a queue using a linked list.
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392, , Let Us C
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11, , Console, Input/Output, • Types of I/O, • Console I/O Functions, Formatted Console I/O Functions, sprintf( ) and sscanf( ) Functions, Unformatted Console I/O Functions, • Summary, • Exercise, , 393
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394, , Let Us C, , A, , s mentioned in the first chapter, Dennis Ritchie wanted C, to remain compact. In keeping with this intention he, deliberately omitted everything related with Input/Output, (I/O) from his definition of the language. Thus, C simply has no, provision for receiving data from any of the input devices (like say, keyboard, disk, etc.), or for sending data to the output devices (like, say VDU, disk, etc.). Then how do we manage I/O, and how is it, that we were we able to use printf( ) and scanf( ) if C has nothing, to offer for I/O? This is what we intend to explore in this chapter., , Types of I/O, Though C has no provision for I/O, it of course has to be dealt with, at some point or the other. There is not much use writing a, program that spends all its time telling itself a secret. Each, Operating System has its own facility for inputting and outputting, data from and to the files and devices. It’s a simple matter for a, system programmer to write a few small programs that would link, the C compiler for particular Operating system’s I/O facilities., The developers of C Compilers do just that. They write several, standard I/O functions and put them in libraries. These libraries are, available with all C compilers. Whichever C compiler you are, using it’s almost certain that you have access to a library of I/O, functions., Do understand that the I/O facilities with different operating, systems would be different. Thus, the way one OS displays output, on screen may be different than the way another OS does it. For, example, the standard library function printf( ) for DOS-based C, compiler has been written keeping in mind the way DOS outputs, characters to screen. Similarly, the printf( ) function for a Unixbased compiler has been written keeping in mind the way Unix, outputs characters to screen. We as programmers do not have to, bother about which printf( ) has been written in what manner. We, should just use printf( ) and it would take care of the rest of the
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Chapter 11: Console Input/Output, , 395, , details that are OS dependent. Same is true about all other standard, library functions available for I/O., There are numerous library functions available for I/O. These can, be classified into three broad categories:, (a), , Console I/O functions, , - Functions to receive input, from keyboard and write, output to VDU., , (b), , File I/O functions, , - Functions to perform I/O, operations on a floppy disk or, hard disk., , In this chapter we would be discussing only Console I/O functions., File I/O functions would be discussed in Chapter 12., , Console I/O Functions, The screen and keyboard together are called a console. Console, I/O functions can be further classified into two categories—, formatted and unformatted console I/O functions. The basic, difference between them is that the formatted functions allow the, input read from the keyboard or the output displayed on the VDU, to be formatted as per our requirements. For example, if values of, average marks and percentage marks are to be displayed on the, screen, then the details like where this output would appear on the, screen, how many spaces would be present between the two, values, the number of places after the decimal points, etc. can be, controlled using formatted functions. The functions available, under each of these two categories are shown in Figure 11.1. Now, let us discuss these console I/O functions in detail.
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396, , Let Us C, , Console Input/Output functions, , Formatted functions, , Unformatted functions, , Type, , Input, , Output, , Type, , Input, , Output, , char, , scanf( ), , printf( ), , char, , putch( ), putchar( ), , int, float, , scanf( ), scanf( ), , printf( ), printf( ), , int, float, , getch( ), getche( ), getchar( ), -, , string, , scanf( ), , printf( ), , string, , gets( ), , puts( ), , -, , Figure 11.1, , Formatted Console I/O Functions, As can be seen from Figure 11.1 the functions printf( ), and, scanf( ) fall under the category of formatted console I/O functions., These functions allow us to supply the input in a fixed format and, let us obtain the output in the specified form. Let us discuss these, functions one by one., We have talked a lot about printf( ), used it regularly, but without, having introduced it formally. Well, better late than never. Its, general form looks like this..., printf ( "format string", list of variables ) ;, , The format string can contain:, (a) Characters that are simply printed as they are, (b) Conversion specifications that begin with a % sign
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Chapter 11: Console Input/Output, , 397, , (c) Escape sequences that begin with a \ sign, For example, look at the following program:, main( ), {, int avg = 346 ;, float per = 69.2 ;, printf ( "Average = %d\nPercentage = %f", avg, per ) ;, }, , The output of the program would be..., Average = 346, Percentage = 69.200000, , How does printf( ) function interpret the contents of the format, string. For this it examines the format string from left to right. So, long as it doesn’t come across either a % or a \ it continues to, dump the characters that it encounters, on to the screen. In this, example Average = is dumped on the screen. The moment it, comes across a conversion specification in the format string it, picks up the first variable in the list of variables and prints its value, in the specified format. In this example, the moment %d is met the, variable avg is picked up and its value is printed. Similarly, when, an escape sequence is met it takes the appropriate action. In this, example, the moment \n is met it places the cursor at the beginning, of the next line. This process continues till the end of format string, is not reached., , Format Specifications, The %d and %f used in the printf( ) are called format specifiers., They tell printf( ) to print the value of avg as a decimal integer, and the value of per as a float. Following is the list of format, specifiers that can be used with the printf( ) function.
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398, , Let Us C, , Data type, Integer, , Real, Character, , Format specifier, short signed, , %d or %I, , short unsigned, , %u, , long singed, , %ld, , long unsigned, , %lu, , unsigned hexadecimal, , %x, , unsigned octal, , %o, , float, , %f, , double, , %lf, , signed character, , %c, , unsigned character, , %c, , String, , %s, , Figure 11.2, We can provide following optional specifiers in the format, specifications., Specifier, , Description, , dd, , Digits specifying field width, , ., , Decimal point separating field width from precision, (precision stands for the number of places after the, decimal point), , dd, , Digits specifying precision, , -, , Minus sign for left justifying the output in the, specified field width, , Figure 11.3
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Chapter 11: Console Input/Output, , 399, , Now a short explanation about these optional format specifiers., The field-width specifier tells printf( ) how many columns on, screen should be used while printing a value. For example, %10d, says, “print the variable as a decimal integer in a field of 10, columns”. If the value to be printed happens not to fill up the, entire field, the value is right justified and is padded with blanks, on the left. If we include the minus sign in format specifier (as in, %-10d), this means left justification is desired and the value will, be padded with blanks on the right. Here is an example that should, make this point clear., main( ), {, int weight = 63 ;, printf ( "\nweight is %d kg", weight ) ;, printf ( "\nweight is %2d kg", weight ) ;, printf ( "\nweight is %4d kg", weight ) ;, printf ( "\nweight is %6d kg", weight ) ;, printf ( "\nweight is %-6d kg", weight ) ;, }, , The output of the program would look like this ..., Columns, , 0123456789012345678901234567890, weight is 63 kg, weight is 63 kg, weight is 63 kg, weight is 63 kg, weight is 63 kg, , Specifying the field width can be useful in creating tables of, numeric values, as the following program demonstrates., main( ), {, printf ( "\n%f %f %f", 5.0, 13.5, 133.9 ) ;
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400, , Let Us C, , printf ( "\n%f %f %f", 305.0, 1200.9, 3005.3 ) ;, }, , And here is the output..., 5.000000 13.500000 133.900000, 305.000000 1200.900000 3005.300000, , Even though the numbers have been printed, the numbers have not, been lined up properly and hence are hard to read. A better way, would be something like this..., main( ), {, printf ( "\n%10.1f %10.1f %10.1f", 5.0, 13.5, 133.9 ) ;, printf ( "\n%10.1f %10.1f %10.1f", 305.0, 1200.9, 3005.3 );, }, , This results into a much better output..., 01234567890123456789012345678901, 5.0, 13.5, 133.9, 305.0, 1200.9, 3005.3, , The format specifiers could be used even while displaying a string, of characters. The following program would clarify this point:, /* Formatting strings with printf( ) */, main( ), {, char firstname1[ ] = "Sandy" ;, char surname1[ ] = "Malya" ;, char firstname2[ ] = "AjayKumar" ;, char surname2[ ] = "Gurubaxani" ;, printf ( "\n%20s%20s", firstname1, surname1 ) ;, printf ( "\n%20s%20s", firstname2, surname2 ) ;
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Chapter 11: Console Input/Output, , 401, , }, , And here’s the output..., 012345678901234567890123456789012345678901234567890, Sandy, Malya, AjayKumar, Gurubaxani, , The format specifier %20s reserves 20 columns for printing a, string and then prints the string in these 20 columns with right, justification. This helps lining up names of different lengths, properly. Obviously, the format %-20s would have left justified, the string., , Escape Sequences, We saw earlier how the newline character, \n, when inserted in a, printf( )’s format string, takes the cursor to the beginning of the, next line. The newline character is an ‘escape sequence’, so called, because the backslash symbol (\) is considered as an ‘escape’, character—it causes an escape from the normal interpretation of a, string, so that the next character is recognized as one having a, special meaning., The following example shows usage of \n and a new escape, sequence \t, called ‘tab’. A \t moves the cursor to the next tab stop., A 80-column screen usually has 10 tab stops. In other words, the, screen is divided into 10 zones of 8 columns each. Printing a tab, takes the cursor to the beginning of next printing zone. For, example, if cursor is positioned in column 5, then printing a tab, takes it to column 8., main( ), {, printf ( "You\tmust\tbe\tcrazy\nto\thate\tthis\tbook" ) ;, }
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402, , Let Us C, , And here’s the output..., 1, 2, 3, 4, 01234567890123456789012345678901234567890, You, must, be, crazy, to, hate, this, book, , The \n character causes a new line to begin following ‘crazy’. The, tab and newline are probably the most commonly used escape, sequences, but there are others as well. Figure 11.4 shows a, complete list of these escape sequences., , Esc. Seq., , Purpose, , Esc. Seq., , Purpose, , \n, , New line, , \t, , Tab, , \b, , Backspace, , \r, , Carriage return, , \f, , Form feed, , \a, , Alert, , \’, , Single quote, , \”, , Double quote, , \\, , Backslash, , Figure 11.4, The first few of these escape sequences are more or less selfexplanatory. \b moves the cursor one position to the left of its, current position. \r takes the cursor to the beginning of the line in, which it is currently placed. \a alerts the user by sounding the, speaker inside the computer. Form feed advances the computer, stationery attached to the printer to the top of the next page., Characters that are ordinarily used as delimiters... the single quote,, double quote, and the backslash can be printed by preceding them, with the backslash. Thus, the statement,, printf ( "He said, \"Let's do it!\"" ) ;
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Chapter 11: Console Input/Output, , 403, , will print..., He said, "Let's do it!", , So far we have been describing printf( )’s specification as if we, are forced to use only %d for an integer, only %c for a char, only, %s for a string and so on. This is not true at all. In fact, printf( ), uses the specification that we mention and attempts to perform the, specified conversion, and does its best to produce a proper result., Sometimes the result is nonsensical, as in case when we ask it to, print a string using %d. Sometimes the result is useful, as in the, case we ask printf( ) to print ASCII value of a character using, %d. Sometimes the result is disastrous and the entire program, blows up., The following program shows a few of these conversions, some, sensible, some weird., main( ), {, char ch = 'z' ;, int i = 125 ;, float a = 12.55 ;, char s[ ] = "hello there !" ;, printf ( "\n%c %d %f", ch, ch, ch ) ;, printf ( "\n%s %d %f", s, s, s ) ;, printf ( "\n%c %d %f",i ,i, i ) ;, printf ( "\n%f %d\n", a, a ) ;, }, , And here’s the output ..., z 122 -9362831782501783000000000000000000000000000.000000, hello there ! 3280 9362831782501783000000000000000000000000000.000000, } 125 -9362831782501783000000000000000000000000000.000000
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404, , Let Us C, , 12.550000 0, , I would leave it to you to analyze the results by yourselves. Some, of the conversions you would find are quite sensible., Let us now turn our attention to scanf( ). scanf( ) allows us to, enter data from keyboard that will be formatted in a certain way., The general form of scanf( ) statement is as follows:, scanf ( "format string", list of addresses of variables ) ;, , For example:, scanf ( "%d %f %c", &c, &a, &ch ) ;, , Note that we are sending addresses of variables (addresses are, obtained by using ‘&’ the ‘address of’ operator) to scanf( ), function. This is necessary because the values received from, keyboard must be dropped into variables corresponding to these, addresses. The values that are supplied through the keyboard must, be separated by either blank(s), tab(s), or newline(s). Do not, include these escape sequences in the format string., All the format specifications that we learnt in printf( ) function are, applicable to scanf( ) function as well., , sprintf( ) and sscanf( ) Functions, The sprintf( ) function works similar to the printf( ) function, except for one small difference. Instead of sending the output to, the screen as printf( ) does, this function writes the output to an, array of characters. The following program illustrates this., main( ), {
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Chapter 11: Console Input/Output, , 405, , int i = 10 ;, char ch = 'A' ;, float a = 3.14 ;, char str[20] ;, printf ( "\n%d %c %f", i, ch, a ) ;, sprintf ( str, "%d %c %f", i, ch, a ) ;, printf ( "\n%s", str ) ;, }, , In this program the printf( ) prints out the values of i, ch and a on, the screen, whereas sprintf( ) stores these values in the character, array str. Since the string str is present in memory what is written, into str using sprintf( ) doesn’t get displayed on the screen. Once, str has been built, its contents can be displayed on the screen. In, our program this was achieved by the second printf( ) statement., The counterpart of sprintf( ) is the sscanf( ) function. It allows us, to read characters from a string and to convert and store them in C, variables according to specified formats. The sscanf( ) function, comes in handy for in-memory conversion of characters to values., You may find it convenient to read in strings from a file and then, extract values from a string by using sscanf( ). The usage of, sscanf( ) is same as scanf( ), except that the first argument is the, string from which reading is to take place., , Unformatted Console I/O Functions, There are several standard library functions available under this, category—those that can deal with a single character and those, that can deal with a string of characters. For openers let us look at, those which handle one character at a time., So far for input we have consistently used the scanf( ) function., However, for some situations the scanf( ) function has one glaring, weakness... you need to hit the Enter key before the function can
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406, , Let Us C, , digest what you have typed. However, we often want a function, that will read a single character the instant it is typed without, waiting for the Enter key to be hit. getch( ) and getche( ) are two, functions which serve this purpose. These functions return the, character that has been most recently typed. The ‘e’ in getche( ), function means it echoes (displays) the character that you typed to, the screen. As against this getch( ) just returns the character that, you typed without echoing it on the screen. getchar( ) works, similarly and echo’s the character that you typed on the screen, but, unfortunately requires Enter key to be typed following the, character that you typed. The difference between getchar( ) and, fgetchar( ) is that the former is a macro whereas the latter is a, function. Here is a sample program that illustrates the use of these, functions., main( ), {, char ch ;, printf ( "\nPress any key to continue" ) ;, getch( ) ; /* will not echo the character */, printf ( "\nType any character" ) ;, ch = getche( ) ; /* will echo the character typed */, printf ( "\nType any character" ) ;, getchar( ) ; /* will echo character, must be followed by enter key */, printf ( "\nContinue Y/N" ) ;, fgetchar( ) ; /* will echo character, must be followed by enter key */, }, , And here is a sample run of this program..., Press any key to continue, Type any character B, Type any character W, Continue Y/N Y
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Chapter 11: Console Input/Output, , 407, , putch( ) and putchar( ) form the other side of the coin. They print, a character on the screen. As far as the working of putch( ), putchar( ) and fputchar( ) is concerned it’s exactly same. The, following program illustrates this., main( ), {, char ch = 'A' ;, putch ( ch ) ;, putchar ( ch ) ;, fputchar ( ch ) ;, putch ( 'Z' ) ;, putchar ( 'Z' ) ;, fputchar ( 'Z' ) ;, }, , And here is the output..., AAAZZZ, , The limitation of putch( ), putchar( ) and fputchar( ) is that they, can output only one character at a time., , gets( ) and puts( ), gets( ) receives a string from the keyboard. Why is it needed?, Because scanf( ) function has some limitations while receiving, string of characters, as the following example illustrates..., main( ), {, char name[50] ;, printf ( "\nEnter name " ) ;, scanf ( "%s", name ) ;, printf ( "%s", name ) ;
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408, , Let Us C, , }, , And here is the output..., Enter name Jonty Rhodes, Jonty, , Surprised? Where did “Rhodes” go? It never got stored in the array, name[ ], because the moment the blank was typed after “Jonty”, scanf( ) assumed that the name being entered has ended. The result, is that there is no way (at least not without a lot of trouble on the, programmer’s part) to enter a multi-word string into a single, variable (name in this case) using scanf( ). The solution to this, problem is to use gets( ) function. As said earlier, it gets a string, from the keyboard. It is terminated when an Enter key is hit. Thus,, spaces and tabs are perfectly acceptable as part of the input string., More exactly, gets( ) gets a newline (\n) terminated string of, characters from the keyboard and replaces the \n with a \0., The puts( ) function works exactly opposite to gets( ) function. It, outputs a string to the screen., Here is a program which illustrates the usage of these functions:, main( ), {, char footballer[40] ;, puts ( "Enter name" ) ;, gets ( footballer ) ; /* sends base address of array */, puts ( "Happy footballing!" ) ;, puts ( footballer ) ;, }, , Following is the sample output:, Enter name
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Chapter 11: Console Input/Output, , 409, , Jonty Rhodes, Happy footballing!, Jonty Rhodes, , Why did we use two puts( ) functions to print “Happy, footballing!” and “Jonty Rhodes”? Because, unlike printf( ),, puts( ) can output only one string at a time. If we attempt to print, two strings using puts( ), only the first one gets printed. Similarly,, unlike scanf( ), gets( ) can be used to read only one string at a, time., , Summary, (a) There is no keyword available in C for doing input/output., (b) All I/O in C is done using standard library functions., (c) There are several functions available for performing console, input/output., (d) The formatted console I/O functions can force the user to, receive the input in a fixed format and display the output in a, fixed format., (e) There are several format specifiers and escape sequences, available to format input and output., (f) Unformatted console I/O functions work faster since they do, not have the overheads of formatting the input or output., , Exercise, [A] What would be the output of the following programs:, (a) main( ), {, char ch ;, ch = getchar( ) ;, if ( islower ( ch ) ), putchar ( toupper ( ch ) ) ;, else, putchar ( tolower ( ch ) ) ;
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410, , Let Us C, , }, (b) main( ), {, int i = 2 ;, float f = 2.5367 ;, char str[ ] = "Life is like that" ;, printf ( "\n%4d\t%3.3f\t%4s", i, f, str ) ;, }, (c), , main( ), {, printf ( "More often than \b\b not \rthe person who \, wins is the one who thinks he can!" ) ;, }, , (d) char p[ ] = "The sixth sick sheikh's sixth ship is sick" ;, main( ), {, int i = 0 ;, while ( p[i] != '\0' ), {, putch ( p[i] ) ;, i++ ;, }, }, [B] Point out the errors, if any, in the following programs:, (a) main( ), {, int i ;, char a[ ] = "Hello" ;, while ( a != '\0' ), {, printf ( "%c", *a ) ;, a++ ;, }, }
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412, , Let Us C, int no = 97;, double val = 2.34174 ;, char name[10] = "Shweta" ;, sprintf ( buffer, "%d %lf %s", no, val, name ) ;, printf ( "\n%s", buffer ) ;, sscanf ( buffer, "%4d %2.2lf %s", &no, &val, name ) ;, printf ( "\n%s", buffer ) ;, printf ( "\n%d %lf %s", no, val, name ) ;, , }, [C] Answer the following:, , (a) To receive the string "We have got the guts, you get the, glory!!" in an array char str[100] which of the following, functions would you use?, 1., 2., 3., 4., , scanf ( "%s", str ) ;, gets ( str ) ;, getche ( str ) ;, fgetchar ( str ) ;, , (b) Which function would you use if a single key were to be, received through the keyboard?, 1., 2., 3., 4., , scanf( ), gets( ), getche( ), getchar( ), , (c) If an integer is to be entered through the keyboard, which, function would you use?, 1., 2., 3., 4., , scanf( ), gets( ), getche( ), getchar( )
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Chapter 11: Console Input/Output, , 413, , (d) If a character string is to be received through the keyboard, which function would work faster?, 1. scanf( ), 2. gets( ), (e) What is the difference between getchar( ), fgetchar( ),, getch( ) and getche( )?, (f) The format string of a printf( ) function can contain:, 1., 2., 3., 4., , Characters, format specifications and escape sequences, Character, integers and floats, Strings, integers and escape sequences, Inverted commas, percentage sign and backslash character, , (g) A field-width specifier in a printf( ) function:, 1., 2., 3., 4., , Controls the margins of the program listing, Specifies the maximum value of a number, Controls the size of type used to print numbers, Specifies how many columns will be used to print the, number, , [D] Answer the following:, , (a) Write down two functions xgets( ) and xputs( ) which work, similar to the standard library functions gets( ) and puts( )., (b) Write down a function getint( ), which would receive a, numeric string from the keyboard, convert it to an integer, number and return the integer to the calling function. A, sample usage of getint( ) is shown below:, main( ), {, int a ;
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414, , Let Us C, a = getint( ) ;, printf ( "you entered %d", a ), , }
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12, , File Input/Output, • Data Organization, • File Operations, Opening a File, Reading from a File, Trouble in Opening a File, Closing the File, • Counting Characters, Tabs, Spaces, …, • A File-copy Program, Writing to a File, • File Opening Modes, • String (line) I/O in Files, The Awkward Newline, • Record I/O in Files, • Text Files and Binary Files, • Record I/O Revisited, • Database Management, • Low Level Disk I/O, A Low Level File-copy Program, • I/O Under Windows, • Summary, • Exercise, , 415
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416, , Let Us C, , O, , ften it is not enough to just display the data on the screen., This is because if the data is large, only a limited amount, of it can be stored in memory and only a limited amount, of it can be displayed on the screen. It would be inappropriate to, store this data in memory for one more reason. Memory is volatile, and its contents would be lost once the program is terminated. So, if we need the same data again it would have to be either entered, through the keyboard again or would have to be regenerated, programmatically. Obviously both these operations would be, tedious. At such times it becomes necessary to store the data in a, manner that can be later retrieved and displayed either in part or in, whole. This medium is usually a ‘file’ on the disk. This chapter, discusses how file I/O operations can be performed., , Data Organization, Before we start doing file input/output let us first find out how data, is organized on the disk. All data stored on the disk is in binary, form. How this binary data is stored on the disk varies from one, OS to another. However, this does not affect the C programmer, since he has to use only the library functions written for the, particular OS to be able to perform input/output. It is the compiler, vendor’s responsibility to correctly implement these library, functions by taking the help of OS. This is illustrated in Figure, 12.1., , Our program, , Figure 12.1, , C Library, functions, , OS, , Disk
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Chapter 12: File Input/Output, , 417, , File Operations, There are different operations that can be carried out on a file., These are:, (a), (b), (c), (d), (e), (f), , Creation of a new file, Opening an existing file, Reading from a file, Writing to a file, Moving to a specific location in a file (seeking), Closing a file, , Let us now write a program to read a file and display its contents, on the screen. We will first list the program and show what it does,, and then dissect it line by line. Here is the listing…, /* Display contents of a file on screen. */, # include "stdio.h", main( ), {, FILE *fp ;, char ch ;, fp = fopen ( "PR1.C", "r" ) ;, while ( 1 ), {, ch = fgetc ( fp ) ;, if ( ch == EOF ), break ;, printf ( "%c", ch ) ;, }, fclose ( fp ) ;, }
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418, , Let Us C, , On execution of this program it displays the contents of the file, ‘PR1.C’ on the screen. Let us now understand how it does the, same., , Opening a File, Before we can read (or write) information from (to) a file on a disk, we must open the file. To open the file we have called the function, fopen( ). It would open a file “PR1.C” in ‘read’ mode, which tells, the C compiler that we would be reading the contents of the file., Note that “r” is a string and not a character; hence the double, quotes and not single quotes. In fact fopen( ) performs three, important tasks when you open the file in “r” mode:, (a) Firstly it searches on the disk the file to be opened., (b) Then it loads the file from the disk into a place in memory, called buffer., (c) It sets up a character pointer that points to the first character, of the buffer., Why do we need a buffer at all? Imagine how inefficient it would, be to actually access the disk every time we want to read a, character from it. Every time we read something from a disk, it, takes some time for the disk drive to position the read/write head, correctly. On a floppy disk system, the drive motor has to actually, start rotating the disk from a standstill position every time the disk, is accessed. If this were to be done for every character we read, from the disk, it would take a long time to complete the reading, operation. This is where a buffer comes in. It would be more, sensible to read the contents of the file into the buffer while, opening the file and then read the file character by character from, the buffer rather than from the disk. This is shown in Figure 12.2.
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419, , Chapter 12: File Input/Output, , PR1.C, Memory, 40, DISK, Buffer, fp, 40, , Figure 12.2, Same argument also applies to writing information in a file., Instead of writing characters in the file on the disk one character at, a time it would be more efficient to write characters in a buffer and, then finally transfer the contents from the buffer to the disk., To be able to successfully read from a file information like mode, of opening, size of file, place in the file from where the next read, operation would be performed, etc. has to be maintained. Since all, this information is inter-related, all of it is gathered together by, fopen( ) in a structure called FILE. fopen( ) returns the address of, this structure, which we have collected in the structure pointer, called fp. We have declared fp as, FILE *fp ;
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420, , Let Us C, , The FILE structure has been defined in the header file “stdio.h”, (standing for standard input/output header file). Therefore, it is, necessary to #include this file., , Reading from a File, Once the file has been opened for reading using fopen( ), as we, have seen, the file’s contents are brought into buffer (partly or, wholly) and a pointer is set up that points to the first character in, the buffer. This pointer is one of the elements of the structure to, which fp is pointing (refer Figure 12.2)., To read the file’s contents from memory there exists a function, called fgetc( ). This has been used in our program as,, ch = fgetc ( fp ) ;, , fgetc( ) reads the character from the current pointer position,, advances the pointer position so that it now points to the next, character, and returns the character that is read, which we collected, in the variable ch. Note that once the file has been opened, we no, longer refer to the file by its name, but through the file pointer fp., We have used the function fgetc( ) within an indefinite while loop., There has to be a way to break out of this while. When shall we, break out... the moment we reach the end of file. But what is end, of file? A special character, whose ASCII value is 26, signifies end, of file. This character is inserted beyond the last character in the, file, when it is created., While reading from the file, when fgetc( ) encounters this special, character, instead of returning the character that it has read, it, returns the macro EOF. The EOF macro has been defined in the, file “stdio.h”. In place of the function fgetc( ) we could have as, well used the macro getc( ) with the same effect.
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Chapter 12: File Input/Output, , 421, , In our program we go on reading each character from the file till, end of file is not met. As each character is read we display it on the, screen. Once out of the loop, we close the file., , Trouble in Opening a File, There is a possibility that when we try to open a file using the, function fopen( ), the file may not be opened. While opening the, file in “r” mode, this may happen because the file being opened, may not be present on the disk at all. And you obviously cannot, read a file that doesn’t exist. Similarly, while opening the file for, writing, fopen( ) may fail due to a number of reasons, like, disk, space may be insufficient to open a new file, or the disk may be, write protected or the disk is damaged and so on., Crux of the matter is that it is important for any program that, accesses disk files to check whether a file has been opened, successfully before trying to read or write to the file. If the file, opening fails due to any of the several reasons mentioned above,, the fopen( ) function returns a value NULL (defined in “stdio.h”, as #define NULL 0). Here is how this can be handled in a, program..., #include "stdio.h", main( ), {, FILE *fp ;, fp = fopen ( "PR1.C", "r" ) ;, if ( fp == NULL ), {, puts ( "cannot open file" ) ;, exit( ) ;, }, }
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422, , Let Us C, , Closing the File, When we have finished reading from the file, we need to close it., This is done using the function fclose( ) through the statement,, fclose ( fp ) ;, , Once we close the file we can no longer read from it using getc( ), unless we reopen the file. Note that to close the file we don’t use, the filename but the file pointer fp. On closing the file the buffer, associated with the file is removed from memory., In this program we have opened the file for reading. Suppose we, open a file with an intention to write characters into it. This time, too a buffer would get associated with it. When we attempt to, write characters into this file using fputc( ) the characters would, get written to the buffer. When we close this file using fclose( ), three operations would be performed:, (a) The characters in the buffer would be written to the file on the, disk., (b) At the end of file a character with ASCII value 26 would get, written., (c) The buffer would be eliminated from memory., You can imagine a possibility when the buffer may become full, before we close the file. In such a case the buffer’s contents would, be written to the disk the moment it becomes full. All this buffer, management is done for us by the library functions., , Counting Characters, Tabs, Spaces, …, Having understood the first file I/O program in detail let us now, try our hand at one more. Let us write a program that will read a, file and count how many characters, spaces, tabs and newlines are, present in it. Here is the program…
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424, , Let Us C, , Here is a sample run..., Number of characters = 125, Number of blanks = 25, Number of tabs = 13, Number of lines = 22, , The above statistics are true for a file “PR1.C”, which I had on my, disk. You may give any other filename and obtain different results., I believe the program is self-explanatory., In this program too we have opened the file for reading and then, read it character by character. Let us now try a program that needs, to open a file for writing., , A File-copy Program, We have already used the function fgetc( ) which reads characters, from a file. Its counterpart is a function called fputc( ) which, writes characters to a file. As a practical use of these character I/O, functions we can copy the contents of one file into another, as, demonstrated in the following program. This program takes the, contents of a file and copies them into another file, character by, character., #include "stdio.h", main( ), {, FILE *fs, *ft ;, char ch ;, fs = fopen ( "pr1.c", "r" ) ;, if ( fs == NULL ), {, puts ( "Cannot open source file" ) ;, exit( ) ;
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Chapter 12: File Input/Output, , 425, , }, ft = fopen ( "pr2.c", "w" ) ;, if ( ft == NULL ), {, puts ( "Cannot open target file" ) ;, fclose ( fs ) ;, exit( ) ;, }, while ( 1 ), {, ch = fgetc ( fs ) ;, if ( ch == EOF ), break ;, else, fputc ( ch, ft ) ;, }, fclose ( fs ) ;, fclose ( ft ) ;, }, , I hope most of the stuff in the program can be easily understood,, since it has already been dealt with in the earlier section. What is, new is only the function fputc( ). Let us see how it works., , Writing to a File, The fputc( ) function is similar to the putch( ) function, in the, sense that both output characters. However, putch( ) function, always writes to the VDU, whereas, fputc( ) writes to the file., Which file? The file signified by ft. The writing process continues, till all characters from the source file have been written to the, target file, following which the while loop terminates.
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426, , Let Us C, , Note that our sample file-copy program is capable of copying only, text files. To copy files with extension .EXE or .COM, we need to, open the files in binary mode, a topic that would be dealt with in, sufficient detail in a later section., , File Opening Modes, In our first program on disk I/O we have opened the file in read, (“r”) mode. However, “r” is but one of the several modes in which, we can open a file. Following is a list of all possible modes in, which a file can be opened. The tasks performed by fopen( ) when, a file is opened in each of these modes are also mentioned., "r", , Searches file. If the file is opened successfully fopen( ), loads it into memory and sets up a pointer which points to, the first character in it. If the file cannot be opened fopen( ), returns NULL., Operations possible – reading from the file., , "w", , Searches file. If the file exists, its contents are overwritten., If the file doesn’t exist, a new file is created. Returns, NULL, if unable to open file., Operations possible – writing to the file., , "a", , Searches file. If the file is opened successfully fopen( ), loads it into memory and sets up a pointer that points to the, last character in it. If the file doesn’t exist, a new file is, created. Returns NULL, if unable to open file., Operations possible - adding new contents at the end of file., , "r+", , Searches file. If is opened successfully fopen( ) loads it into, memory and sets up a pointer which points to the first, character in it. Returns NULL, if unable to open the file.
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Chapter 12: File Input/Output, , 427, , Operations possible - reading existing contents, writing new, contents, modifying existing contents of the file., "w+", , Searches file. If the file exists, its contents are overwritten., If the file doesn’t exist a new file is created. Returns NULL,, if unable to open file., Operations possible - writing new contents, reading them, back and modifying existing contents of the file., , "a+", , Searches file. If the file is opened successfully fopen( ), loads it into memory and sets up a pointer which points to, the first character in it. If the file doesn’t exist, a new file is, created. Returns NULL, if unable to open file., Operations possible - reading existing contents, appending, new contents to end of file. Cannot modify existing, contents., , String (line) I/O in Files, For many purposes, character I/O is just what is needed. However,, in some situations the usage of functions that read or write entire, strings might turn out to be more efficient., Reading or writing strings of characters from and to files is as easy, as reading and writing individual characters. Here is a program, that writes strings to a file using the function fputs( )., /* Receives strings from keyboard and writes them to file */, #include "stdio.h", main( ), {, FILE *fp ;, char s[80] ;
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428, , Let Us C, , fp = fopen ( "POEM.TXT", "w" ) ;, if ( fp == NULL ), {, puts ( "Cannot open file" ) ;, exit( ) ;, }, printf ( "\nEnter a few lines of text:\n" ) ;, while ( strlen ( gets ( s ) ) > 0 ), {, fputs ( s, fp ) ;, fputs ( "\n", fp ) ;, }, fclose ( fp ) ;, }, , And here is a sample run of the program..., Enter a few lines of text:, Shining and bright, they are forever,, so true about diamonds,, more so of memories,, especially yours !, , Note that each string is terminated by hitting enter. To terminate, the execution of the program, hit enter at the beginning of a line., This creates a string of zero length, which the program recognizes, as the signal to close the file and exit., We have set up a character array to receive the string; the fputs( ), function then writes the contents of the array to the disk. Since, fputs( ) does not automatically add a newline character to the end, of the string, we must do this explicitly to make it easier to read, the string back from the file., Here is a program that reads strings from a disk file.
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Chapter 12: File Input/Output, , 429, , /* Reads strings from the file and displays them on screen */, #include "stdio.h", main( ), {, FILE *fp ;, char s[80] ;, fp = fopen ( "POEM.TXT", "r" ) ;, if ( fp == NULL ), {, puts ( "Cannot open file" ) ;, exit( ) ;, }, while ( fgets ( s, 79, fp ) != NULL ), printf ( "%s" , s ) ;, fclose ( fp ) ;, }, , And here is the output..., Shining and bright, they are forever,, so true about diamonds,, more so of memories,, especially yours !, , The function fgets( ) takes three arguments. The first is the address, where the string is stored, and the second is the maximum length, of the string. This argument prevents fgets( ) from reading in too, long a string and overflowing the array. The third argument, as, usual, is the pointer to the structure FILE. When all the lines from, the file have been read, we attempt to read one more line, in which, case fgets( ) returns a NULL.
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430, , Let Us C, , The Awkward Newline, We had earlier written a program that counts the total number of, characters present in a file. If we use that program to count the, number of characters present in the above poem (stored in the file, “POEM.TXT”), it would give us the character count as 101. The, same file if seen in the directory, would be reported to contain 105, characters., This discrepancy occurs because when we attempt to write a “\n”, to the file using fputs( ), fputs( ) converts the \n to \r\n, combination. Here \r stands for carriage return and \n for linefeed., If we read the same line back using fgets( ) the reverse conversion, happens. Thus when we write the first line of the poem and a “\n”, using two calls to fputs( ), what gets written to the file is, Shining and bright, they are forever,\r\n, , When the same line is read back into the array s[ ] using fgets( ),, the array contains, Shining and bright, they are forever,\n\0, , Thus conversion of \n to \r\n during writing and \r\n conversion to, \n during reading is a feature of the standard library functions and, not that of the OS. Hence the OS counts \r and \n as separate, characters. In our poem there are four lines, therefore there is a, discrepancy of four characters (105 - 101)., , Record I/O in Files, So far we have dealt with reading and writing only characters and, strings. What if we want to read or write numbers from/to file?, Furthermore, what if we desire to read/write a combination of, characters, strings and numbers? For this first we would organize, this dissimilar data together in a structure and then use fprintf( )
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432, , Let Us C, , }, , And here is the output of the program..., Enter name, age and basic salary: Sunil 34 1250.50, Add another record (Y/N) Y, Enter name, age and basic salary: Sameer 21 1300.50, Add another record (Y/N) Y, Enter name, age and basic salary: Rahul 34 1400.55, Add another record (Y/N) N, , In this program we are just reading the data into a structure, variable using scanf( ), and then dumping it into a disk file using, fprintf( ). The user can input as many records as he desires. The, procedure ends when the user supplies ‘N’ for the question ‘Add, another record (Y/N)’., The key to this program is the function fprintf( ), which writes the, values in the structure variable to the file. This function is similar, to printf( ), except that a FILE pointer is included as the first, argument. As in printf( ), we can format the data in a variety of, ways, by using fprintf( ). In fact all the format conventions of, printf( ) function work with fprintf( ) as well., Perhaps you are wondering what for have we used the function, fflush( ). The reason is to get rid of a peculiarity of scanf( ). After, supplying data for one employee, we would hit the enter key. What, scanf( ) does is it assigns name, age and salary to appropriate, variables and keeps the enter key unread in the keyboard buffer., So when it’s time to supply Y or N for the question ‘Another, employee (Y/N)’, getch( ) will read the enter key from the buffer, thinking that user has entered the enter key. To avoid this problem, we use the function fflush( ). It is designed to remove or ‘flush, out’ any data remaining in the buffer. The argument to fflush( ), must be the buffer which we want to flush out. Here we have used, ‘stdin’, which means buffer related with standard input, device—keyboard.
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Chapter 12: File Input/Output, , 433, , Let us now write a program that reads the employee records, created by the above program. Here is how it can be done..., /* Read records from a file using structure */, #include "stdio.h", main( ), {, FILE *fp ;, struct emp, {, char name[40] ;, int age ;, float bs ;, };, struct emp e ;, fp = fopen ( "EMPLOYEE.DAT", "r" ) ;, if ( fp == NULL ), {, puts ( "Cannot open file" ) ;, exit( ) ;, }, while ( fscanf ( fp, "%s %d %f", e.name, &e.age, &e.bs ) != EOF ), printf ( "\n%s %d %f", e.name, e.age, e.bs ) ;, fclose ( fp ) ;, }, , And here is the output of the program..., Sunil 34 1250.500000, Sameer 21 1300.500000, Rahul 34 1400.500000
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434, , Let Us C, , Text Files and Binary Files, All the programs that we wrote in this chapter so far worked on, text files. Some of them would not work correctly on binary files., A text file contains only textual information like alphabets, digits, and special symbols. In actuality the ASCII codes of these, characters are stored in text files. A good example of a text file is, any C program, say PR1.C., As against this, a binary file is merely a collection of bytes. This, collection might be a compiled version of a C program (say, PR1.EXE), or music data stored in a wave file or a picture stored, in a graphic file. A very easy way to find out whether a file is a, text file or a binary file is to open that file in Turbo C/C++. If on, opening the file you can make out what is displayed then it is a, text file, otherwise it is a binary file., As mentioned while explaining the file-copy program, the program, cannot copy binary files successfully. We can improve the same, program to make it capable of copying text as well as binary files, as shown below., #include "stdio.h", main( ), {, FILE *fs, *ft ;, int ch ;, fs = fopen ( "pr1.exe", "rb" ) ;, if ( fs == NULL ), {, puts ( "Cannot open source file" ) ;, exit( ) ;, }, ft = fopen ( "newpr1.exe", "wb" ) ;
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Chapter 12: File Input/Output, , 435, , if ( ft == NULL ), {, puts ( "Cannot open target file" ) ;, fclose ( fs ) ;, exit( ) ;, }, while ( 1 ), {, ch = fgetc ( fs ) ;, if ( ch == EOF ), break ;, else, fputc ( ch, ft ) ;, }, fclose ( fs ) ;, fclose ( ft ) ;, }, , Using this program we can comfortably copy text as well as binary, files. Note that here we have opened the source and target files in, “rb” and “wb” modes respectively. While opening the file in text, mode we can use either “r” or “rt”, but since text mode is the, default mode we usually drop the ‘t’., From the programming angle there are three main areas where text, and binary mode files are different. These are:, (a) Handling of newlines, (b) Representation of end of file, (c) Storage of numbers, Let us explore these three differences.
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436, , Let Us C, , Text versus Binary Mode: Newlines, We have already seen that, in text mode, a newline character is, converted into the carriage return-linefeed combination before, being written to the disk. Likewise, the carriage return-linefeed, combination on the disk is converted back into a newline when the, file is read by a C program. However, if a file is opened in binary, mode, as opposed to text mode, these conversions will not take, place., , Text versus Binary Mode: End of File, The second difference between text and binary modes is in the way, the end-of-file is detected. In text mode, a special character, whose, ASCII value is 26, is inserted after the last character in the file to, mark the end of file. If this character is detected at any point in the, file, the read function would return the EOF signal to the program., As against this, there is no such special character present in the, binary mode files to mark the end of file. The binary mode files, keep track of the end of file from the number of characters present, in the directory entry of the file., There is a moral to be derived from the end of file marker of text, mode files. If a file stores numbers in binary mode, it is important, that binary mode only be used for reading the numbers back, since, one of the numbers we store might well be the number 26, (hexadecimal 1A). If this number is detected while we are reading, the file by opening it in text mode, reading would be terminated, prematurely at that point., Thus the two modes are not compatible. See to it that the file that, has been written in text mode is read back only in text mode., Similarly, the file that has been written in binary mode must be, read back only in binary mode.
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Chapter 12: File Input/Output, , 437, , Text versus Binary Mode: Storage of Numbers, The only function that is available for storing numbers in a disk, file is the fprintf( ) function. It is important to understand how, numerical data is stored on the disk by fprintf( ). Text and, characters are stored one character per byte, as we would expect., Are numbers stored as they are in memory, two bytes for an, integer, four bytes for a float, and so on? No., Numbers are stored as strings of characters. Thus, 1234, even, though it occupies two bytes in memory, when transferred to the, disk using fprintf( ), would occupy four bytes, one byte per, character. Similarly, the floating-point number 1234.56 would, occupy 7 bytes on disk. Thus, numbers with more digits would, require more disk space., Hence if large amount of numerical data is to be stored in a disk, file, using text mode may turn out to be inefficient. The solution is, to open the file in binary mode and use those functions (fread( ), and fwrite( ) which are discussed later) which store the numbers in, binary format. It means each number would occupy same number, of bytes on disk as it occupies in memory., , Record I/O Revisited, The record I/O program that we did in an earlier section has two, disadvantages:, (a) The numbers (basic salary) would occupy more number of, bytes, since the file has been opened in text mode. This is, because when the file is opened in text mode, each number is, stored as a character string., (b) If the number of fields in the structure increase (say, by, adding address, house rent allowance etc.), writing structures
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438, , Let Us C, , using fprintf( ), or reading them using fscanf( ), becomes, quite clumsy., Let us now see a more efficient way of reading/writing records, (structures). This makes use of two functions fread( ) and, fwrite( ). We will write two programs, first one would write, records to the file and the second would read these records from, the file and display them on the screen., /* Receives records from keyboard and writes them to a file in binary mode */, #include "stdio.h", main( ), {, FILE *fp ;, char another = 'Y' ;, struct emp, {, char name[40] ;, int age ;, float bs ;, };, struct emp e ;, fp = fopen ( "EMP.DAT", "wb" ) ;, if ( fp == NULL ), {, puts ( "Cannot open file" ) ;, exit( ) ;, }, while ( another == 'Y' ), {, printf ( "\nEnter name, age and basic salary: " ) ;, scanf ( "%s %d %f", e.name, &e.age, &e.bs ) ;, fwrite ( &e, sizeof ( e ), 1, fp ) ;, printf ( "Add another record (Y/N) " ) ;
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Chapter 12: File Input/Output, , 439, , fflush ( stdin ) ;, another = getche( ) ;, }, fclose ( fp ) ;, }, , And here is the output..., Enter name, age and basic salary: Suresh 24 1250.50, Add another record (Y/N) Y, Enter name, age and basic salary: Ranjan 21 1300.60, Add another record (Y/N) Y, Enter name, age and basic salary: Harish 28 1400.70, Add another record (Y/N) N, , Most of this program is similar to the one that we wrote earlier,, which used fprintf( ) instead of fwrite( ). Note, however, that the, file “EMP.DAT” has now been opened in binary mode., The information obtained from the keyboard about the employee is, placed in the structure variable e. Then, the following statement, writes the structure to the file:, fwrite ( &e, sizeof ( e ), 1, fp ) ;, , Here, the first argument is the address of the structure to be written, to the disk., The second argument is the size of the structure in bytes. Instead, of counting the bytes occupied by the structure ourselves, we let, the program do it for us by using the sizeof( ) operator. The, sizeof( ) operator gives the size of the variable in bytes. This keeps, the program unchanged in event of change in the elements of the, structure.
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440, , Let Us C, , The third argument is the number of such structures that we want, to write at one time. In this case, we want to write only one, structure at a time. Had we had an array of structures, for example,, we might have wanted to write the entire array at once., The last argument is the pointer to the file we want to write to., Now, let us write a program to read back the records written to the, disk by the previous program., /* Reads records from binary file and displays them on VDU */, #include "stdio.h", main( ), {, FILE *fp ;, struct emp, {, char name[40] ;, int age ;, float bs ;, };, struct emp e ;, fp = fopen ( "EMP.DAT", "rb" ) ;, if ( fp == NULL ), {, puts ( "Cannot open file" ) ;, exit( ) ;, }, while ( fread ( &e, sizeof ( e ), 1, fp ) == 1 ), printf ( "\n%s %d %f", e.name, e.age, e.bs ) ;, fclose ( fp ) ;, }
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Chapter 12: File Input/Output, , 441, , Here, the fread( ) function causes the data read from the disk to be, placed in the structure variable e. The format of fread( ) is same as, that of fwrite( ). The function fread( ) returns the number of, records read. Ordinarily, this should correspond to the third, argument, the number of records we asked for... 1 in this case. If, we have reached the end of file, since fread( ) cannot read, anything, it returns a 0. By testing for this situation, we know, when to stop reading., As you can now appreciate, any database management application, in C must make use of fread( ) and fwrite( ) functions, since they, store numbers more efficiently, and make writing/reading of, structures quite easy. Note that even if the number of elements, belonging to the structure increases, the format of fread( ) and, fwrite( ) remains same., , Database Management, So far we have learnt record I/O in bits and pieces. However, in, any serious database management application, we will have to, combine all that we have learnt in a proper manner to make sense., I have attempted to do this in the following menu driven program., There is a provision to Add, Modify, List and Delete records, the, operations that are imperative in any database management., Following comments would help you in understanding the, program easily:, −, , Addition of records must always take place at the end of, existing records in the file, much in the same way you would, add new records in a register manually., , −, , Listing records means displaying the existing records on the, screen. Naturally, records should be listed from first record to, last record., , −, , While modifying records, first we must ask the user which, record he intends to modify. Instead of asking the record
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442, , Let Us C, , number to be modified, it would be more meaningful to ask for, the name of the employee whose record is to be modified. On, modifying the record, the existing record gets overwritten by, the new record., −, , In deleting records, except for the record to be deleted, rest of, the records must first be written to a temporary file, then the, original file must be deleted, and the temporary file must be, renamed back to original., , −, , Observe carefully the way the file has been opened, first for, reading & writing, and if this fails (the first time you run this, program it would certainly fail, because that time the file is not, existing), for writing and reading. It is imperative that the file, should be opened in binary mode., , −, , Note that the file is being opened only once and closed only, once, which is quite logical., , −, , clrscr( ) function clears the contents of the screen and, gotoxy( ) places the cursor at appropriate position on the, screen. The parameters passed to gotoxy( ) are column number, followed by row number., , Given below is the complete listing of the program., /* A menu-driven program for elementary database management */, #include "stdio.h", main( ), {, FILE *fp, *ft ;, char another, choice ;, struct emp, {, char name[40] ;, int age ;, float bs ;, };
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446, , Let Us C, remove ( "EMP.DAT" ) ;, rename ( "TEMP.DAT", "EMP.DAT" ) ;, fp = fopen ( "EMP.DAT", "rb+" ) ;, printf ( "Delete another Record (Y/N) " ) ;, fflush ( stdin ) ;, another = getche( ) ;, }, break ;, case '0' :, fclose ( fp ) ;, exit( ) ;, }, , }, }, , To understand how this program works, you need to be familiar, with the concept of pointers. A pointer is initiated whenever we, open a file. On opening a file a pointer is set up which points to the, first record in the file. To be precise this pointer is present in the, structure to which the file pointer returned by fopen( ) points to., On using the functions fread( ) or fwrite( ), the pointer moves to, the beginning of the next record. On closing a file the pointer is, deactivated. Note that the pointer movement is of utmost, importance since fread( ) always reads that record where the, pointer is currently placed. Similarly, fwrite( ) always writes the, record where the pointer is currently placed., The rewind( ) function places the pointer to the beginning of the, file, irrespective of where it is present right now., The fseek( ) function lets us move the pointer from one record to, another. In the program above, to move the pointer to the previous, record from its current position, we used the function,
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Chapter 12: File Input/Output, , 447, , fseek ( fp, -recsize, SEEK_CUR ) ;, , Here, -recsize moves the pointer back by recsize bytes from the, current position. SEEK_CUR is a macro defined in “stdio.h”., Similarly, the following fseek( ) would place the pointer beyond, the last record in the file., fseek ( fp, 0, SEEK_END ) ;, , In fact -recsize or 0 are just the offsets that tell the compiler by, how many bytes should the pointer be moved from a particular, position. The third argument could be SEEK_END, SEEK_CUR, or SEEK_SET. All these act as a reference from which the pointer, should be offset. SEEK_END means move the pointer from the, end of the file, SEEK_CUR means move the pointer with, reference to its current position and SEEK_SET means move the, pointer with reference to the beginning of the file., If we wish to know where the pointer is positioned right now, we, can use the function ftell( ). It returns this position as a long int, which is an offset from the beginning of the file. The value, returned by ftell( ) can be used in subsequent calls to fseek( ). A, sample call to ftell( ) is shown below:, position = ftell ( fp ) ;, , where position is a long int., , Low Level Disk I/O, In low level disk I/O, data cannot be written as individual, characters, or as strings or as formatted data. There is only one, way data can be written or read in low level disk I/O functions—as, a buffer full of bytes.
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448, , Let Us C, , Writing a buffer full of data resembles the fwrite( ) function., However, unlike fwrite( ), the programmer must set up the buffer, for the data, place the appropriate values in it before writing, and, take them out after writing. Thus, the buffer in the low level I/O, functions is very much a part of the program, rather than being, invisible as in high level disk I/O functions., Low level disk I/O functions offer following advantages:, (a) Since these functions parallel the methods that the OS uses to, write to the disk, they are more efficient than the high level, disk I/O functions., (b) Since there are fewer layers of routines to go through, low, level I/O functions operate faster than their high level, counterparts., Let us now write a program that uses low level disk input/output, functions., , A Low Level File-copy Program, Earlier we had written a program to copy the contents of one file to, another. In that program we had read the file character by, character using fgetc( ). Each character that was read was written, into the target file using fputc( ). Instead of performing the I/O on, a character by character basis we can read a chunk of bytes from, the source file and then write this chunk into the target file. While, doing so the chunk would be read into the buffer and would be, written to the file from the buffer. While doing so we would, manage the buffer ourselves, rather than relying on the library, functions to do so. This is what is low-level about this program., Here is a program which shows how this can be done., /* File-copy program which copies text, .com and .exe files */, #include "fcntl.h", #include "types.h" /* if present in sys directory use
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450, , Let Us C, break ;, , }, close ( inhandle ) ;, close ( outhandle ) ;, }, , Declaring the Buffer, The first difference that you will notice in this program is that we, declare a character buffer,, char buffer[512] ;, , This is the buffer in which the data read from the disk will be, placed. The size of this buffer is important for efficient operation., Depending on the operating system, buffers of certain sizes are, handled more efficiently than others., , Opening a File, We have opened two files in our program, one is the source file, from which we read the information, and the other is the target file, into which we write the information read from the source file., As in high level disk I/O, the file must be opened before we can, access it. This is done using the statement,, inhandle = open ( source, O_RDONLY | O_BINARY ) ;, , We open the file for the same reason as we did earlier—to, establish communication with operating system about the file. As, usual, we have to supply to open( ), the filename and the mode in, which we want to open the file. The possible file opening modes, are given below:, O_APPEND, , - Opens a file for appending
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Chapter 12: File Input/Output, O_CREAT, O_RDONLY, O_RDWR, O_WRONLY, O_BINARY, O_TEXT, , 451, , - Creates a new file for writing (has no effect, if file already exists), - Creates a new file for reading only, - Creates a file for both reading and writing, - Creates a file for writing only, - Creates a file in binary mode, - Creates a file in text mode, , These ‘O-flags’ are defined in the file “fcntl.h”. So this file must, be included in the program while usng low level disk I/O. Note, that the file “stdio.h” is not necessary for low level disk I/O. When, two or more O-flags are used together, they are combined using, the bitwise OR operator ( | ). Chapter 14 discusses bitwise, operators in detail., The other statement used in our program to open the file is,, outhandle = open ( target, O_CREAT | O_BINARY | O_WRONLY,, S_IWRITE ) ;, , Note that since the target file is not existing when it is being, opened we have used the O_CREAT flag, and since we want to, write to the file and not read from it, therefore we have used, O_WRONLY. And finally, since we want to open the file in, binary mode we have used O_BINARY., Whenever O_CREAT flag is used, another argument must be, added to open( ) function to indicate the read/write status of the, file to be created. This argument is called ‘permission argument’., Permission arguments could be any of the following:, S_IWRITE, S_IREAD, , - Writing to the file permitted, - Reading from the file permitted
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452, , Let Us C, , To use these permissions, both the files “types.h” and “stat.h” must, be #included in the program alongwith “fcntl.h”., , File Handles, Instead of returning a FILE pointer as fopen( ) did, in low level, disk I/O, open( ) returns an integer value called ‘file handle’. This, is a number assigned to a particular file, which is used thereafter to, refer to the file. If open( ) returns a value of -1, it means that the, file couldn’t be successfully opened., , Interaction between Buffer and File, The following statement reads the file or as much of it as will fit, into the buffer:, bytes = read ( inhandle, buffer, 512 ) ;, , The read( ) function takes three arguments. The first argument is, the file handle, the second is the address of the buffer and the third, is the maximum number of bytes we want to read., The read( ) function returns the number of bytes actually read., This is an important number, since it may very well be less than, the buffer size (512 bytes), and we will need to know just how full, the buffer is before we can do anything with its contents. In our, program we have assigned this number to the variable bytes., For copying the file, we must use both the read( ) and the write( ), functions in a while loop. The read( ) function returns the number, of bytes actually read. This is assigned to the variable bytes. This, value will be equal to the buffer size (512 bytes) until the end of, file, when the buffer will only be partially full. The variable bytes, therefore is used to tell write( ), as to how many bytes to write, from the buffer to the target file.
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Chapter 12: File Input/Output, , 453, , Note that when large buffers are used they must be made global, variables otherwise stack overflow occurs., , I/O Under Windows, As said earlier I/O in C is carried out using functions present in the, library that comes with the C compiler targeted for a specific OS., Windows permits several applications to use the same screen, simultaneously. Hence there is a possibility that what is written by, one application to the console may get overwritten by the output, sent by another application to the console. To avoid such situations, Windows has completely abandoned console I/O functions. It uses, a separate mechanism to send output to a window representing an, application. The details of this mechanism are discussed in, Chapter 17., Though under Windows console I/O functions are not used, still, functions like fprintf( ), fscanf( ), fread( ), fwrite( ), sprintf( ),, sscanf( ) work exactly same under Windows as well., , Summary, (a) File I/O can be performed on a character by character basis, a, line by line basis, a record by record basis or a chunk by, chunk basis., (b) Different operations that can be performed on a file are—, creation of a new file, opening an existing file, reading from a, file, writing to a file, moving to a specific location in a file, (seeking) and closing a file., (c) File I/O is done using a buffer to improve the efficiency., (d) A file can be a text file or a binary file depending upon its, contents., (e) Library functions convert \n to \r\n or vice versa while, writing/reading to/from a file.
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454, , Let Us C, , (f) Many library functions convert a number to a numeric string, before writing it to a file, thereby using more space on disk., This can be avoided using functions fread( ) and fwrite( )., (g) In low level file I/O we can do the buffer management, ourselves., , Exercise, [A] Point out the errors, if any, in the following programs:, (a) #include "stdio.h", main( ), {, FILE *fp ;, openfile ( "Myfile.txt", fp ) ;, if ( fp == NULL ), printf ( "Unable to open file…" ) ;, }, openfile ( char *fn, FILE **f ), {, *f = fopen ( fn, "r" ) ;, }, (b) #include "stdio.h", main( ), {, FILE *fp ;, char c ;, fp = fopen ( "TRY.C" ,"r") ;, if ( fp == null ), {, puts ( "Cannot open file" ) ;, exit( ) ;, }, while ( ( c = getc ( fp ) ) != EOF ), putch ( c ) ;, fclose ( fp ) ;
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Chapter 12: File Input/Output, , 457, , }, (j), , main( ), {, int fp ;, fp = fopen ( "students.c", READ | BINARY ) ;, if ( fp == -1 ), puts ( "cannot open file" ) ;, else, close ( fp ) ;, }, , [B] Answer the following:, , (a) The macro FILE is defined in which of the following files:, 1., 2., 3., 4., , stdlib.h, stdio.c, io.h, stdio.h, , (b) If a file contains the line “I am a boy\r\n” then on reading this, line into the array str[ ] using fgets( ) what would str[ ], contain?, 1., 2., 3., 4., , I am a boy\r\n\0, I am a boy\r\0, I am a boy\n\0, I am a boy, , (c) State True or False:, 1. The disadvantage of High Level Disk I/O functions is that, the programmer has to manage the buffers., 2. If a file is opened for reading it is necessary that the file, must exist., 3. If a file opened for writing already exists its contents, would be overwritten.
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458, , Let Us C, , 4. For opening a file in append mode it is necessary that the, file should exist., (d) On opening a file for reading which of the following activities, are performed:, 1. The disk is searched for existence of the file., 2. The file is brought into memory., 3. A pointer is set up which points to the first character in the, file., 4. All the above., (e) Is it necessary that a file created in text mode must always be, opened in text mode for subsequent operations?, (f) State True or False:, A file opened in binary mode and read using fgetc( ) would, report the same number of characters in the file as reported by, DOS’s DIR command., (g) While using the statement,, fp = fopen ( "myfile.c", "r" ) ;, , what happens if,, −, −, , ‘myfile.c’ does not exist on the disk, ‘myfile.c’ exists on the disk, , (h) What is the purpose of the library function fflush( )?, (i) While using the statement,, fp = fopen ( "myfile.c", "wb" ) ;, what happens if,, , −, −, , ‘myfile.c’ does not exist on the disk., ‘myfile.c’ exists on the disk, , (j) A floating-point array contains percentage marks obtained by, students in an examination. To store these marks in a file, ‘marks.c’, in which mode would you open the file and why?
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Chapter 12: File Input/Output, , 459, , [C] Attempt the following:, , (a) Write a program to read a file and display contents with its, line numbers., (b) Write a program to find the size of a text file without, traversing it character by character., (c) Write a program to add the contents of one file at the end of, another., (d) Suppose a file contains student’s records with each record, containing name and age of a student. Write a program to read, these records and display them in sorted order by name., (e) Write a program to copy one file to another. While doing so, replace all lowercase characters to their equivalent uppercase, characters., (f) Write a program that merges lines alternately from two files, and writes the results to new file. If one file has less number, of lines than the other, the remaining lines from the larger file, should be simply copied into the target file., (g) Write a program to display the contents of a text file on the, screen. Make following provisions:, Display the contents inside a box drawn with opposite corner, co-ordinates being ( 0, 1 ) and ( 79, 23 ). Display the name of, the file whose contents are being displayed, and the page, numbers in the zeroth row. The moment one screenful of file, has been displayed, flash a message ‘Press any key...’ in 24th, row. When a key is hit, the next page’s contents should be, displayed, and so on till the end of file., (h) Write a program to encrypt/decrypt a file using:
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460, , Let Us C, , (1) An offset cipher: In an offset cipher each character from, the source file is offset with a fixed value and then, written to the target file., For example, if character read from the source file is ‘A’,, then convert this into a new character by offsetting ‘A’, by a fixed value, say 128, and then writing the new, character to the target file., (2) A substitution cipher: In this each character read from the, source file is substituted by a corresponding, predetermined character and this character is written to, the target file., For example, if character ‘A’ is read from the source file,, and if we have decided that every ‘A’ is to be substituted, by ‘!’, then a ‘!’ would be written to the target file in, place of every ‘A’ Similarly, every ‘B’ would be, substituted by ‘5’ and so on., (i) In the file ‘CUSTOMER.DAT’ there are 100 records with the, following structure:, struct customer, {, int accno ;, char name[30] ;, float balance ;, };, , In another file ‘TRANSACTIONS.DAT’ there are several, records with the following structure:, struct trans, {, int accno ,, char trans_type ;
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Chapter 12: File Input/Output, , 461, , float amount ;, };, , The parameter trans_type contains D/W indicating deposit or, withdrawal of amount. Write a program to update, ‘CUSTOMER.DAT’ file, i.e. if the trans_type is ‘D’ then, update the balance of ‘CUSTOMER.DAT’ by adding, amount to balance for the corresponding accno. Similarly, if, trans_type is ‘W’ then subtract the amount from balance., However, while subtracting the amount make sure that the, amount should not get overdrawn, i.e. at least 100 Rs. Should, remain in the account., (j) There are 100 records present in a file with the following, structure:, struct date, {, int d, m, y ;, };, struct employee, {, int empcode[6] ;, char empname[20] ;, struct date join_date ;, float salary ;, };, , Write a program to read these records, arrange them in, ascending order of join_date and write them in to a target, file., (k) A hospital keeps a file of blood donors in which each record, has the format:, Name: 20 Columns, Address: 40 Columns
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462, , Let Us C, , Age: 2 Columns, Blood Type: 1 Column (Type 1, 2, 3 or 4), Write a program to read the file and print a list of all blood, donors whose age is below 25 and blood is type 2., (l) Given a list of names of students in a class, write a program to, store the names in a file on disk. Make a provision to display, the nth name in the list (n is data to be read) and to display all, names starting with S., (m) Assume that a Master file contains two fields, Roll no. and, name of the student. At the end of the year, a set of students, join the class and another set leaves. A Transaction file, contains the roll numbers and an appropriate code to add or, delete a student., Write a program to create another file that contains the, updated list of names and roll numbers. Assume that the, Master file and the Transaction file are arranged in ascending, order by roll numbers. The updated file should also be in, ascending order by roll numbers., (n) In a small firm employee numbers are given in serial, numerical order, that is 1, 2, 3, etc., −, −, −, −, , Create a file of employee data with following information:, employee number, name, sex, gross salary., If more employees join, append their data to the file., If an employee with serial number 25 (say) leaves, delete, the record by making gross salary 0., If some employee’s gross salary increases, retrieve the, record and update the salary., , Write a program to implement the above operations., (o) Given a text file, write a program to create another text file, deleting the words “a”, “the”, “an” and replacing each one of, them with a blank space.
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Chapter 12: File Input/Output, , 463, , (p) You are given a data file EMPLOYEE.DAT with the, following record structure:, struct employee {, int empno ;, char name[30] ;, int basic, grade ;, };, , Every employee has a unique empno and there are supposed, to be no gaps between employee numbers. Records are, entered into the data file in ascending order of employee, number, empno. It is intended to check whether there are, missing employee numbers. Write a program segment to read, the data file records sequentially and display the list of, missing employee numbers., (q) Write a program to carry out the following:, −, −, −, , To read a text file “TRIAL.TXT” consisting of a, maximum of 50 lines of text, each line with a maximum, of 80 characters., Count and display the number of words contained in the, file., Display the total number of four letter words in the text, file., , Assume that the end of a word may be a space, comma or a, full-stop followed by one or more spaces or a newline, character., (r) Write a program to read a list of words, sort the words in, alphabetical order and display them one word per line. Also, give the total number of words in the list. Output format, should be:, Total Number of words in the list is _______, Alphabetical listing of words is:, -----------
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464, , Let Us C, , -----Assume the end of the list is indicated by ZZZZZZ and there, are maximum in 25 words in the Text file., (s) Write a program to carry out the following:, (a) Read a text file ‘INPUT.TXT’, (b) Print each word in reverse order, Example,, Input: INDIA IS MY COUNTRY, Output: AIDNI SI YM YRTNUOC, , Assume that each word length is maximum of 10 characters, and each word is separated by newline/blank characters., (t) Write a C program to read a large text file ‘NOTES.TXT’ and, print it on the printer in cut-sheets, introducing page breaks at, the end of every 50 lines and a pause message on the screen at, the end of every page for the user to change the paper.
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13, , More Issues In, Input/Output, •, •, •, •, , Using argc and argv, Detecting Errors in Reading/Writing, Standard I/O Devices, I/O Redirection, Redirecting the Output, Redirecting the Input, Both Ways at Once, • Summary, • Exercise, , 465
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466, , Let Us C, , I, , n Chapters 11 and 12 we saw how Console I/O and File I/O are, done in C. There are still some more issues related with, input/output that remain to be understood. These issues help in, making the I/O operations more elegant., , Using argc and argv, To execute the file-copy programs that we saw in Chapter 12 we, are required to first type the program, compile it, and then execute, it. This program can be improved in two ways:, (a) There should be no need to compile the program every time to, use the file-copy utility. It means the program must be, executable at command prompt (A> or C> if you are using, MS-DOS, Start | Run dialog if you are using Windows and $, prompt if you are using Unix)., (b) Instead of the program prompting us to enter the source and, target filenames, we must be able to supply them at command, prompt, in the form:, filecopy PR1.C PR2.C, , where, PR1.C is the source filename and PR2.C is the target, filename., The first improvement is simple. In MS-DOS, the executable file, (the one which can be executed at command prompt and has an, extension .EXE) can be created in Turbo C/C++ by using the key, F9 to compile the program. In VC++ compiler under Windows, same can be done by using F7 to compile the program. Under Unix, this is not required since in Unix every time we compile a program, we always get an executable file., The second improvement is possible by passing the source, filename and target filename to the function main( ). This is, illustrated below:
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468, , Let Us C, , fclose ( fs ) ;, fclose ( ft ) ;, }, , The arguments that we pass on to main( ) at the command prompt, are called command line arguments. The function main( ) can, have two arguments, traditionally named as argc and argv. Out of, these, argv is an array of pointers to strings and argc is an int, whose value is equal to the number of strings to which argv, points. When the program is executed, the strings on the command, line are passed to main( ). More precisely, the strings at the, command line are stored in memory and address of the first string, is stored in argv[0], address of the second string is stored in, argv[1] and so on. The argument argc is set to the number of, strings given on the command line. For example, in our sample, program, if at the command prompt we give,, filecopy PR1.C PR2.C, , then,, argc would contain 3, argv[0] would contain base address of the string “filecopy”, argv[1] would contain base address of the string “PR1.C”, argv[2] would contain base address of the string “PR2.C”, , Whenever we pass arguments to main( ), it is a good habit to, check whether the correct number of arguments have been passed, on to main( ) or not. In our program this has been done through,, if ( argc != 3 ), {, printf ( "Improper number of arguments" ) ;, exit( ) ;, }
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Chapter 13: More Issues In Input/Output, , 469, , Rest of the program is same as the earlier file-copy program. This, program is better than the earlier file-copy program on two counts:, (a) There is no need to recompile the program every time we, want to use this utility. It can be executed at command, prompt., (b) We are able to pass source file name and target file name to, main( ), and utilize them in main( )., One final comment... the while loop that we have used in our, program can be written in a more compact form, as shown below:, while ( ( ch = fgetc ( fs ) ) != EOF ), fputc ( ch, ft ) ;, , This avoids the usage of an indefinite loop and a break statement, to come out of this loop. Here, first fgetc ( fs ) gets the character, from the file, assigns it to the variable ch, and then ch is compared, against EOF. Remember that it is necessary to put the expression, ch = fgetc ( fs ), , within a pair of parentheses, so that first the character read is, assigned to variable ch and then it is compared with EOF., There is one more way of writing the while loop. It is shown, below:, while ( !feof ( fs ) ), {, ch = fgetc ( fs ) ;, fputc ( ch, ft ) ;, }, , Here, feof( ) is a macro which returns a 0 if end of file is not, reached. Hence we use the ! operator to negate this 0 to the truth, value. When the end of file is reached feof( ) returns a non-zero
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470, , Let Us C, , value, ! makes it 0 and since now the condition evaluates to false, the while loop gets terminated., Note that in each one of them the following three methods for, opening a file are same, since in each one of them, essentially a, base address of the string (pointer to a string) is being passed to, fopen( )., fs = fopen ( "PR1.C" , "r" ) ;, fs = fopen ( filename, "r" ) ;, fs = fopen ( argv[1] , "r" ) ;, , Detecting Errors in Reading/Writing, Not at all times when we perform a read or write operation on a, file are we successful in doing so. Naturally there must be a, provision to test whether our attempt to read/write was successful, or not., The standard library function ferror( ) reports any error that might, have occurred during a read/write operation on a file. It returns a, zero if the read/write is successful and a non-zero value in case of, a failure. The following program illustrates the usage of ferror( )., #include "stdio.h", main( ), {, FILE *fp ;, char ch ;, fp = fopen ( "TRIAL", "w" ) ;, while ( !feof ( fp ) ), {, ch = fgetc ( fp ) ;, if ( ferror( ) ), {
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Chapter 13: More Issues In Input/Output, , 471, , printf ( "Error in reading file" ) ;, break ;, }, else, printf ( "%c", ch ) ;, }, fclose ( fp ) ;, }, , In this program the fgetc( ) function would obviously fail first time, around since the file has been opened for writing, whereas fgetc( ), is attempting to read from the file. The moment the error occurs, ferror( ) returns a non-zero value and the if block gets executed., Instead of printing the error message using printf( ) we can use the, standard library function perror( ) which prints the error message, specified by the compiler. Thus in the above program the perror( ), function can be used as shown below., if ( ferror( ) ), {, perror ( "TRIAL" ) ;, break ;, }, , Note that when the error occurs the error message that is displayed, is:, TRIAL: Permission denied, , This means we can precede the system error message with any, message of our choice. In our program we have just displayed the, filename in place of the error message.
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472, , Let Us C, , Standard I/O Devices, To perform reading or writing operations on a file we need to use, the function fopen( ), which sets up a file pointer to refer to this, file. Most OSs also predefine pointers for three standard files. To, access these pointers we need not use fopen( ). These standard file, pointers are shown in Figure 13.1, , Standard File pointer, , Description, , stdin, , standard input device (Keyboard), , stdout, , standard output device (VDU), , stderr, , standard error device (VDU), , Figure 13.1, Thus the statement ch = fgetc ( stdin ) would read a character, from the keyboard rather than from a file. We can use this, statement without any need to use fopen( ) or fclose( ) function, calls., Note that under MS-DOS two more standard file pointers are, available—stdprn and stdaux. They stand for standard printing, device and standard auxiliary device (serial port). The following, program shows how to use the standard file pointers. It reads a file, from the disk and prints it on the printer., /* Prints file contents on printer */, #include "stdio.h", main( ), {, FILE *fp ;, char ch ;
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Chapter 13: More Issues In Input/Output, , 473, , fp = fopen ( "poem.txt", "r" ) ;, if ( fp == NULL ), {, printf ( "Cannot open file" ) ;, exit( ) ;, }, while ( ( ch = fgetc ( fp ) ) != EOF ), fputc ( ch, stdprn ) ;, fclose ( fp ) ;, }, , The statement fputc ( ch, stdprn ) writes a character read from the, file to the printer. Note that although we opened the file on the, disk we didn’t open stdprn, the printer. Standard files and their, use in redirection have been dealt with in more details in the next, section., Note that these standard file pointers have been defined in the file, “stdio.h”. Therefore, it is necessary to include this file in the, program that uses these standard file pointers., , I/O Redirection, Most operating systems incorporate a powerful feature that allows, a program to read and write files, even when such a capability has, not been incorporated in the program. This is done through a, process called ‘redirection’., Normally a C program receives its input from the standard input, device, which is assumed to be the keyboard, and sends its output, to the standard output device, which is assumed to be the VDU. In, other words, the OS makes certain assumptions about where input
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474, , Let Us C, , should come from and where output should go. Redirection, permits us to change these assumptions., For example, using redirection the output of the program that, normally goes to the VDU can be sent to the disk or the printer, without really making a provision for it in the program. This is, often a more convenient and flexible approach than providing a, separate function in the program to write to the disk or printer., Similarly, redirection can be used to read information from disk, file directly into a program, instead of receiving the input from, keyboard., To use redirection facility is to execute the program from the, command prompt, inserting the redirection symbols at appropriate, places. Let us understand this process with the help of a program., , Redirecting the Output, Let’s see how we can redirect the output of a program, from the, screen to a file. We’ll start by considering the simple program, shown below:, /* File name: util.c */, #include "stdio.h"<+>, main( ), {, char ch ;, while ( ( ch = getc ( stdin ) ) != EOF ), putc ( ch, stdout ) ;, }, , On compiling this program we would get an executable file, UTIL.EXE. Normally, when we execute this file, the putc( ), function will cause whatever we type to be printed on screen, until, we don’t type Ctrl-Z, at which point the program will terminate, as
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Chapter 13: More Issues In Input/Output, , 475, , shown in the following sample run. The Ctrl-Z character is often, called end of file character., C>UTIL.EXE, perhaps I had a wicked childhood,, perhaps I had a miserable youth,, but somewhere in my wicked miserable past,, there must have been a moment of truth ^Z, C>, , Now let’s see what happens when we invoke this program from in, a different way, using redirection:, C>UTIL.EXE > POEM.TXT, C>, , Here we are causing the output to be redirected to the file, POEM.TXT. Can we prove that this the output has indeed gone to, the file POEM.TXT? Yes, by using the TYPE command as, follows:, C>TYPE POEM.TXT, perhaps I had a wicked childhood,, perhaps I had a miserable youth,, but somewhere in my wicked miserable past,, there must have been a moment of truth, C>, , There’s the result of our typing sitting in the file. The redirection, operator, ‘>’, causes any output intended for the screen to be, written to the file whose name follows the operator., Note that the data to be redirected to a file doesn’t need to be typed, by a user at the keyboard; the program itself can generate it. Any, output normally sent to the screen can be redirected to a disk file., As an example consider the following program for generating the, ASCII table on screen:
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476, , Let Us C, , /* File name: ascii.c*/, main( ), {, int ch ;, for ( ch = 0 ; ch <= 255 ; ch++ ), printf ( "\n%d %c", ch, ch ) ;, }, , When this program is compiled and then executed at command, prompt using the redirection operator,, C>ASCII.EXE > TABLE.TXT, , the output is written to the file. This can be a useful capability any, time you want to capture the output in a file, rather than displaying, it on the screen., DOS predefines a number of filenames for its own use. One of, these names in PRN, which stands for the printer. Output can be, redirected to the printer by using this filename. For example, if you, invoke the “ascii.exe” program this way:, C>ASCII.EXE > PRN, , the ASCII table will be printed on the printer., , Redirecting the Input, We can also redirect input to a program so that, instead of reading, a character from the keyboard, a program reads it from a file. Let, us now see how this can be done., To redirect the input, we need to have a file containing something, to be displayed. Suppose we use a file called NEWPOEM.TXT, containing the following lines:
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Chapter 13: More Issues In Input/Output, , 477, , Let's start at the very beginning,, A very good place to start!, , We’ll assume that using some text editor these lines have been, placed in the file NEWPOEM.TXT. Now, we use the input, redirection operator ‘<’ before the file, as shown below:, C>UTIL.EXE < NEWPOEM.TXT, Let's start at the very beginning,, A very good place to start!, C>, , The lines are printed on the screen with no further effort on our, part. Using redirection we’ve made our program UTIL.C perform, the work of the TYPE command., , Both Ways at Once, Redirection of input and output can be used together; the input for, a program can come from a file via redirection, at the same time its, output can be redirected to a file. Such a program is called a filter., The following command demonstrates this process., C>UTIL < NEWPOEM.TXT > POETRY.TXT, , In this case our program receives the redirected input from the file, NEWPOEM.TXT and instead of sending the output to the screen it, would redirect it to the file POETRY.TXT., Similarly to send the contents of the file NEWPOEM.TXT to the, printer we can use the following command:, C>UTIL < NEWPOEM.TXT > PRN, , While using such multiple redirections don’t try to send output to, the same file from which you are receiving input. This is because
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478, , Let Us C, , the output file is erased before it’s written to. So by the time we, manage to receive the input from a file it is already erased., Redirection can be a powerful tool for developing utility programs, to examine or alter data in files. Thus, redirection is used to, establish a relationship between a program and a file. Another OS, operator can be used to relate two programs directly, so that the, output of one is fed directly into another, with no files involved., This is called ‘piping’, and is done using the operator ‘|’, called, pipe. We won’t pursue this topic, but you can read about it in the, OS help/manual., , Summary, (a) We can pass parameters to a program at command line using, the concept of ‘command line arguments’., (b) The command line argument argv contains values passed to, the program, whereas, argc contains number of arguments., (c) We can use the standard file pointer stdin to take input from, standard input device such as keyboard., (d) We can use the standard file pointer stdout to send output to, the standard output device such as a monitor., (e) We can use the standard file pointers stdprn and stdaux to, interact with printer and auxiliary devices respectively., (f) Redirection allows a program to read from or write to files at, command prompt., (g) The operators < and > are called redirection operators., , Exercise, [A] Answer the following:, , (a) How will you use the following program to, −, −, −, , Copy the contents of one file into another., Print a file on the printer., Create a new file and add some text to it.
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Chapter 13: More Issues In Input/Output, −, , 479, , Display the contents of an existing file., , #include "stdio.h", main( ), {, char ch, str[10] ;, while ( ( ch = getc ( stdin ) ) != -1 ), putc ( ch, stdout ) ;, }, , (b) State True or False:, 1. We can send arguments at command line even if we, define main( ) function without parameters., 2. To use standard file pointers we don’t need to open the, file using fopen( )., 3. Using stdaux we can send output to the printer if printer is, attached to the serial port., 4. The zeroth element of the argv array is always the name, of the exe file., (c) Point out the errors, if any, in the following program, main ( int ac, char ( * ) av[ ] ), {, printf ( "\n%d", ac ) ;, printf ( "\n%s", av[0] ) ;, }, [B] Attempt the following:, , (a) Write a program to carry out the following:, (a) Read a text file provided at command prompt, (b) Print each word in reverse order, For example if the file contains, INDIA IS MY COUNTRY, , Output should be
Page 494 : 480, , Let Us C, , AIDNI SI YM YRTNUOC, , (b) Write a program using command line arguments to search for, a word in a file and replace it with the specified word. The, usage of the program is shown below., C> change <old word> <new word> <filename>, (c) Write a program that can be used at command prompt as a, calculating utility. The usage of the program is shown below., C> calc <switch> <n> <m>, Where, n and m are two integer operands. switch can be any, one of the arithmetic or comparison operators. If arithmetic, operator is supplied, the output should be the result of the, operation. If comparison operator is supplied then the output, should be True or False.
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14, , Operations On, Bits, • Bitwise Operators, One’s Complement Operator, Right Shift Operator, Left Shift Operator, Bitwise AND Operator, Bitwise OR Operator, Bitwise XOR Operator, • The showbits( ) Function, • Summary, • Exercise, , 481
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482, , Let Us C, , S, , o far we have dealt with characters, integers, floats and their, variations. The smallest element in memory on which we are, able to operate as yet is a byte; and we operated on it by, making use of the data type char. However, we haven’t attempted, to look within these data types to see how they are constructed out, of individual bits, and how these bits can be manipulated. Being, able to operate on a bit level, can be very important in, programming, especially when a program must interact directly, with the hardware. This is because, the programming languages, are byte oriented, whereas hardware tends to be bit oriented. Let us, now delve inside the byte and see how it is constructed and how it, can be manipulated effectively. So let us take apart the byte... bit, by bit., , Bitwise Operators, One of C’s powerful features is a set of bit manipulation operators., These permit the programmer to access and manipulate individual, bits within a piece of data. The various Bitwise Operators available, in C are shown in Figure 14.1., , Operator, , Meaning, , ~, >>, <<, &, |, ^, , One’s complement, Right shift, Left shift, Bitwise AND, Bitwise OR, Bitwise XOR(Exclusive OR), , Figure 14.1, These operators can operate upon ints and chars but not on floats, and doubles. Before moving on to the details of the operators, let
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483, , Chapter 14: Operations On Bits, , us first take a look at the bit numbering scheme in integers and, characters. Bits are numbered from zero onwards, increasing from, right to left as shown below:, , 7, , 6, , 5, , 4, , 3, , 2, , 1, , 0, , Character, , 15 14 13 12 11 10 9, , 8, , 7, , 6, , 5, , 4, , 3, , 2, , 1, , 0, , 16-bit Integer, , Figure 14.2, Throughout this discussion of bitwise operators we are going to, use a function called showbits( ), but we are not going to show, you the details of the function immediately. The task of, showbits( ) is to display the binary representation of any integer or, character value., We begin with a plain-jane example with showbits( ) in action., /* Print binary equivalent of integers using showbits( ) function */, main( ), {, int j ;, for ( j = 0 ; j <<= 5 ; j++ ), {, printf ( "\nDecimal %d is same as binary ", j ) ;, showbits ( j ) ;, }
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484, , Let Us C, , }, , And here is the output..., Decimal 0 is same as binary 0000000000000000, Decimal 1 is same as binary 0000000000000001, Decimal 2 is same as binary 0000000000000010, Decimal 3 is same as binary 0000000000000011, Decimal 4 is same as binary 0000000000000100, Decimal 5 is same as binary 0000000000000101, , Let us now explore the various bitwise operators one by one., , One’s Complement Operator, On taking one’s complement of a number, all 1’s present in the, number are changed to 0’s and all 0’s are changed to 1’s. For, example one’s complement of 1010 is 0101. Similarly, one’s, complement of 1111 is 0000. Note that here when we talk of a, number we are talking of binary equivalent of the number. Thus,, one’s complement of 65 means one’s complement of 0000 0000, 0100 0001, which is binary equivalent of 65. One’s complement of, 65 therefore would be, 1111 1111 1011 1110. One’s complement, operator is represented by the symbol ~. Following program shows, one’s complement operator in action., main( ), {, int j, k ;, for ( j = 0 ; j <= 3 ; j++ ), {, printf ( "\nDecimal %d is same as binary ", j ) ;, showbits ( j ) ;, k = ~j ;, printf ( "\nOne’s complement of %d is ", j ) ;
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Chapter 14: Operations On Bits, , 485, , showbits ( k ) ;, }, }, , And here is the output of the above program..., Decimal 0 is same as binary 0000000000000000, One’s complement of 0 is 1111111111111111, Decimal 1 is same as binary 0000000000000001, One’s complement of 1 is 1111111111111110, Decimal 2 is same as binary 0000000000000010, One’s complement of 2 is 1111111111111101, Decimal 3 is same as binary 0000000000000011, One’s complement of 3 is 1111111111111100, , In real-world situations where could the one’s complement, operator be useful? Since it changes the original number beyond, recognition, one potential place where it can be effectively used is, in development of a file encryption utility as shown below:, /* File encryption utility */, #include "stdio.h", main( ), {, encrypt( ) ;, }, encrypt( ), {, FILE *fs, *ft ;, char ch ;, fs = fopen ( "SOURCE.C", "r" ) ; /* normal file */, ft = fopen ( "TARGET.C”, "w" ) ; /* encrypted file */, if ( fs == NULL || ft == NULL ), {
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486, , Let Us C, printf ( "\nFile opening error!" ) ;, exit ( 1 ) ;, , }, while ( ( ch = getc ( fs ) ) != EOF ), putc ( ~ch, ft ) ;, fclose ( fs ) ;, fclose ( ft ) ;, }, , How would you write the corresponding decrypt function? Would, there be any problem in tackling the end of file marker? It may be, recalled here that the end of file in text files is indicated by a, character whose ASCII value is 26., , Right Shift Operator, The right shift operator is represented by >>. It needs two, operands. It shifts each bit in its left operand to the right. The, number of places the bits are shifted depends on the number, following the operator (i.e. its right operand)., Thus, ch >> 3 would shift all bits in ch three places to the right., Similarly, ch >> 5 would shift all bits 5 places to the right., For example, if the variable ch contains the bit pattern 11010111,, then, ch >> 1 would give 01101011 and ch >> 2 would give, 00110101., Note that as the bits are shifted to the right, blanks are created on, the left. These blanks must be filled somehow. They are always, filled with zeros. The following program demonstrates the effect, of right shift operator., main( ), {
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Chapter 14: Operations On Bits, , 487, , int i = 5225, j, k ;, printf ( "\nDecimal %d is same as binary ", i ) ;, showbits ( i ) ;, for ( j = 0 ; j <= 5 ; j++ ), {, k = i >>j ;, printf ( "\n%d right shift %d gives ", i, j ) ;, showbits ( k ) ;, }, }, , The output of the above program would be..., Decimal 5225 is same as binary 0001010001101001, 5225 right shift 0 gives 0001010001101001, 5225 right shift 1 gives 0000101000110100, 5225 right shift 2 gives 0000010100011010, 5225 right shift 3 gives 0000001010001101, 5225 right shift 4 gives 0000000101000110, 5225 right shift 5 gives 0000000010100011, , Note that if the operand is a multiple of 2 then shifting the operand, one bit to right is same as dividing it by 2 and ignoring the, remainder. Thus,, 64 >> 1 gives 32, 64 >> 2 gives 16, 128 >> 2 gives 32, , but,, 27 >> 1 is 13, 49 >> 1 is 24 .
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488, , Let Us C, , A Word of Caution, In the explanation a >> b if b is negative the result is, unpredictable. If a is negative than its left most bit (sign bit) would, be 1. On some computer right shifting a would result in extending, the sign bit. For example, if a contains -1, its binary representation, would be 1111111111111111. Without sign extension, the, operation a >> 4 would be 0000111111111111. However, on the, machine on which we executed this expression the result turns out, to be 1111111111111111. Thus the sign bit 1 continues to get, extended., , Left Shift Operator, This is similar to the right shift operator, the only difference being, that the bits are shifted to the left, and for each bit shifted, a 0 is, added to the right of the number. The following program should, clarify my point., main( ), {, int i = 5225, j, k ;, printf ( "\nDecimal %d is same as ", i ) ;, showbits ( i ) ;, for ( j = 0 ; j <= 4 ; j++ ), {, k = i <<j ;, printf ( "\n%d left shift %d gives ", i, j ) ;, showbits ( k ) ;, }, }, , The output of the above program would be..., Decimal 5225 is same as binary 0001010001101001
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489, , Chapter 14: Operations On Bits, 5225 left shift 0 gives 0001010001101001, 5225 left shift 1 gives 0010100011010010, 5225 left shift 2 gives 0101000110100100, 5225 left shift 3 gives 1010001101001000, 5225 left shift 4 gives 0100011010010000, , Having acquainted ourselves with the left shift and right shift, operators, let us now find out the practical utility of these, operators., In DOS/Windows the date on which a file is created (or modified), is stored as a 2-byte entry in the 32 byte directory entry of that file., Similarly, a 2-byte entry is made of the time of creation or, modification of the file. Remember that DOS/Windows doesn’t, store the date (day, month, and year) of file creation as a 8 byte, string, but as a codified 2 byte entry, thereby saving 6 bytes for, each file entry in the directory. The bitwise distribution of year,, month and date in the 2-byte entry is shown in Figure 14.3., , 15 14 13 12 11 10 9, , 8, , 7, , 4, , 3, , 2, , 1, , 0, , Y, , M, , M M M D, month, , D, , D, , D, , D, , Y, , Y, , Y Y Y, year, , Y, , 6, , 5, , day, , Figure 14.3, DOS/Windows converts the actual date into a 2-byte value using, the following formula:, date = 512 * ( year - 1980 ) + 32 * month + day, , Suppose 09/03/1990 is the date, then on conversion the date will, be,, date = 512 * ( 1990 - 1980 ) + 32 * 3 + 9 = 5225
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490, , Let Us C, , The binary equivalent of 5225 is 0001 0100 0110 1001. This, binary value is placed in the date field in the directory entry of the, file as shown below., , 15 14 13 12 11 10 9, 0, , 0, , 0, , 1, , 0, , year, , 1, , 0, , 8, 0, , 7, 0, , 6, 1, , 5, 1, , 4, 0, , 3, , 2, , 1, , 0, , 1, , 0, , 0, , 1, , month, , day, , Figure 14.4, Just to verify this bit distribution, let us take the bits representing, the month,, month = 0011, =1*2+1*1, =3, , Similarly, the year and the day can also be verified., When we issue the command DIR or use Windows Explorer to list, the files, the file’s date is again presented on the screen in the, usual date format of mm/dd/yy. How does this integer to date, conversion take place? Obviously, using left shift and right shift, operators., When we take a look at Figure 14.4 depicting the bit pattern of the, 2- byte date field, we see that the year, month and day exist as a, bunch of bits in contiguous locations. Separating each of them is a, matter of applying the bitwise operators., For example, to get year as a separate entity from the two bytes, entry we right shift the entry by 9 to get the year. Just see, how...
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491, , Chapter 14: Operations On Bits, , 15 14 13 12 11 10, 0, , 0, , 0, , 1, , 0, , 1, , 9, 0, , 8, 0, , year, , 7, 0, , 6, 1, , 5, 1, , 4, 0, , 3, , 2, , 1, , 0, , 1, , 0, , 0, , 1, , month, , day, , Right shifting by 9 gives, 15 14 13 12 11 10, 0, , 0, , 0, , 0, , 0, , 0, , 9, 0, , 8, 0, , 7, 0, , 6, 0, , 5, 0, , 4, 0, , 3, , 2, , 1, , 0, , 1, , 0, , 1, , 0, , year, , Figure 14.5, On similar lines, left shifting by 7, followed by right shifting by 12, yields month.
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492, , Let Us C, , 15 14 13 12 11 10, 0, , 9, , 8, , 0, , 0, year, , 0, , 1, , 1, , 6, , 5, , 4, , 3, , 2, , 1, , 0, , 1, , 1, , 0, , 1, , 0, , 0, , 1, , month, , Left shifting by 7 gives,, 15 14 13 12 11 10, 0, , 7, , 0, , 9 8, , 1, , month, , 6, , 5, , 4, , 3, , 2, , 1, , 0, , 1, , 0, , 0, , 0, , 1, , 0, , 1, , 0, , 9 8 7, 0 0 0, , 6, 0, , 5, 0, , 4, 0, , 3, 0, , 2, 0, , 1, 1, , 0, 1, , 0, , 0, , 7, , day, , day, , Right shifting by 12 gives,, 15 14 13 12 11 10, 0 0 0 0 0 0, , month, , Figure 14.6, Finally, for obtaining the day, left shift date by 11 and then right, shift the result by 11. Left shifting by 11 gives, 0100100000000000., Right, shifting, by, 11, gives, 0000000000001001., This entire logic can be put into a program as shown below:, /* Decoding date field in directory entry using bitwise operators */, main( ), {, unsigned int d = 9, m = 3, y = 1990, year, month, day, date ;, date = ( y - 1980 ) * 512 + m * 32 + d ;, printf ( "\nDate = %u", date ) ;
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Chapter 14: Operations On Bits, , 493, , year = 1980 + ( date >> 9 ) ;, month = ( (date << 7 ) >> 12 ) ;, day = ( (date << 11 ) >> 11 ) ;, printf ( "\nYear = %u ", year ) ;, printf ( "Month = %u ", month ) ;, printf ( "Day = %u", day ) ;, }, , And here is the output..., Date = 5225, Year = 1990 Month = 3 Day = 9, , Bitwise AND Operator, This operator is represented as &. Remember it is different than, &&, the logical AND operator. The & operator operates on two, operands. While operating upon these two operands they are, compared on a bit-by-bit basis. Hence both the operands must be, of the same type (either char or int). The second operand is often, called an AND mask. The & operator operates on a pair of bits to, yield a resultant bit. The rules that decide the value of the resultant, bit are shown below:, , First bit, , Second bit, , First bit & Second bit, , 0, , 0, , 0, , 0, , 1, , 0, , 1, , 0, , 0, , 1, , 1, , 1, , Figure 14.7
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494, , Let Us C, , This can be represented in a more understandable form as a ‘Truth, Table’ shown in Figure 14.8., , &, , 0, , 1, , 0, , 0, , 0, , 1, , 0, , 1, , Figure 14.8, The example given below shows more clearly what happens while, ANDing one operand with another. The rules given in the Figure, 14.8 are applied to each pair of bits one by one., , 7, 1, 7, 1, , 7, 1, , 6, 0, 6, 1, , 6, 0, , 5, 1, 5, 0, , 5, 0, , 4, 0, 4, 0, , 4, 0, , 3, 1, 3, 0, , 3, 0, , 2, 0, 2, 0, , 2, 0, , 1, 1, 1, 1, , 1, 1, , 0, 0, , This operand when, ANDed bitwise, , 0, 1, , With this operand, yields, , 0, 0, , this result, , Figure 14.9, Work through the Truth Table and confirm that the result obtained, is really correct., Thus, it must be clear that the operation is being performed on, individual bits, and the operation performed on one pair of bits is
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Chapter 14: Operations On Bits, , 495, , completely independent of the operation performed on the other, pairs., Probably, the best use of the AND operator is to check whether a, particular bit of an operand is ON or OFF. This is explained in the, following example., Suppose, from the bit pattern 10101101 of an operand, we want to, check whether bit number 3 is ON (1) or OFF (0). Since we want, to check the bit number 3, the second operand for the AND, operation should be 1 * 23, which is equal to 8. This operand can, be represented bitwise as 00001000., Then the ANDing operation would be,, 10101101, 00001000, -------------00001000, , Original bit pattern, AND mask, Resulting bit pattern, , The resulting value we get in this case is 8, i.e the value of the, second operand. The result turned out to be 8 since the third bit of, the first operand was ON. Had it been OFF, the bit number 3 in the, resulting bit pattern would have evaluated to 0 and the complete, bit pattern would have been 00000000., Thus, depending upon the bit number to be checked in the first, operand we decide the second operand, and on ANDing these two, operands the result decides whether the bit was ON or OFF. If the, bit is ON (1), the resulting value turns out to be a non-zero value, which is equal to the value of second operand, and if the bit is OFF, (0) the result is zero as seen above. The following program puts, this logic into action., /* To test whether a bit in a number is ON or OFF */, main( ), {
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496, , Let Us C, , int i = 65, j ;, printf ( "\nvalue of i = %d", i ) ;, j = i & 32 ;, if ( j == 0 ), printf ( "\nand its fifth bit is off" ) ;, else, printf ( "\nand its fifth bit is on" ) ;, j = i & 64 ;, if ( j == 0 ), printf ( "\nwhereas its sixth bit is off" ) ;, else, printf ( "\nwhereas its sixth bit is on" ) ;, }, , And here is the output..., Value of i = 65, and its fifth bit is off, whereas its sixth bit is on, , In every file entry present in the directory, there is an attribute, byte. The status of a file is governed by the value of individual bits, in this attribute byte. The AND operator can be used to check the, status of the bits of this attribute byte. The meaning of each bit in, the attribute byte is shown in Figure 14.10.
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Chapter 14: Operations On Bits, , Bit numbers, 7, , 6, , 5, , 4, , 3, , 2, , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , 497, , Meaning, 1, , 0, , ., , ., , 1, , Read only, , ., , 1, , ., , Hidden, , ., , 1, , ., , ., , System, , ., , 1, , ., , ., , ., , Volume label entry, , ., , 1, , ., , ., , ., , ., , Sub-directory entry, , ., , 1, , ., , ., , ., , ., , ., , Archive bit, , ., , 1, , ., , ., , ., , ., , ., , ., , Unused, , 1, , ., , ., , ., , ., , ., , ., , ., , Unused, , Figure 14.10, Now, suppose we want to check whether a file is a hidden file or, not. A hidden file is one, which is never shown in the directory,, even though it exists on the disk. From the above bit classification, of attribute byte, we only need to check whether bit number 1 is, ON or OFF., So, our first operand in this case becomes the attribute byte of the, file in question, whereas the second operand is the 1 * 21 = 2, as, discussed earlier. Similarly, it can be checked whether the file is a, system file or not, whether the file is read-only file or not, and so, on., The second, and equally important use of the AND operator is in, changing the status of the bit, or more precisely to switch OFF a, particular bit.
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498, , Let Us C, , If the first operand happens to be 00000111, then to switch OFF, bit number 1, our AND mask bit pattern should be 11111101. On, applying this mask, we get,, 00000111, 11111101, -------------00000101, , Original bit pattern, AND mask, Resulting bit pattern, , Here in the AND mask we keep the value of all other bits as 1, except the one which is to be switched OFF (which is purposefully, kept as 0). Therefore, irrespective of whether the first bit is ON or, OFF previously, it is switched OFF. At the same time the value 1, provided in all the other bits of the AND mask (second operand), keeps the bit values of the other bits in the first operand unaltered., Let’s summarize the uses of bitwise AND operator:, (a) It is used to check whether a particular bit in a number is ON, or OFF., (b) It is used to turn OFF a particular bit in a number., , Bitwise OR Operator, Another important bitwise operator is the OR operator which is, represented as |. The rules that govern the value of the resulting bit, obtained after ORing of two bits is shown in the truth table below., , Figure 14.11, , |, , 0, , 1, , 0, , 0, , 1, , 1, , 1, , 1
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499, , Chapter 14: Operations On Bits, , Using the Truth table confirm the result obtained on ORing the, two operands as shown below., 11010000, 00000111, ------------11010111, , Original bit pattern, OR mask, Resulting bit pattern, , Bitwise OR operator is usually used to put ON a particular bit in a, number., Let us consider the bit pattern 11000011. If we want to put ON bit, number 3, then the OR mask to be used would be 00001000. Note, that all the other bits in the mask are set to 0 and only the bit,, which we want to set ON in the resulting value is set to 1., , Bitwise XOR Operator, The XOR operator is represented as ^ and is also called an, Exclusive OR Operator. The OR operator returns 1, when any one, of the two bits or both the bits are 1, whereas XOR returns 1 only, if one of the two bits is 1. The truth table for the XOR operator is, given below., , ^, , 0, , 1, , 0, , 0, , 1, , 1, , 1, , 0, , Figure 14.12, XOR operator is used to toggle a bit ON or OFF. A number, XORed with another number twice gives the original number. This, is shown in the following program.
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500, , Let Us C, , main( ), {, int b = 50 ;, b = b ^ 12 ;, printf ( "\n%d", b ) ; /* this will print 62 */, b = b ^ 12 ;, printf ( "\n%d", b ) ; /* this will print 50 */, }, , The showbits( ) Function, We have used this function quite often in this chapter. Now we, have sufficient knowledge of bitwise operators and hence are in a, position to understand it. The function is given below followed by, a brief explanation., showbits ( int n ), {, int i, k, andmask ;, for ( i = 15 ; i >= 0 ; i-- ), {, andmask = 1 << i ;, k = n & andmask ;, k == 0 ? printf ( "0" ) : printf ( "1" ) ;, }, }, , All that is being done in this function is using an AND operator, and a variable andmask we are checking the status of individual, bits. If the bit is OFF we print a 0 otherwise we print a 1., First time through the loop, the variable andmask will contain the, value 1000000000000000, which is obtained by left-shifting 1,
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Chapter 14: Operations On Bits, , 501, , fifteen places. If the variable n’s most significant bit is 0, then k, would contain a value 0, otherwise it would contain a non-zero, value. If k contains 0 then printf( ) will print out 0 otherwise it, will print out 1., On the second go-around of the loop, the value of i is decremented, and hence the value of andmask changes, which will now be, 0100000000000000. This checks whether the next most significant, bit is 1 or 0, and prints it out accordingly. The same operation is, repeated for all bits in the number., , Summary, (a) To help manipulate hardware oriented data—individual bits, rather than bytes a set of bitwise operators are used., (b) The bitwise operators include operators like one’s, complement, right-shift, left-shift, bitwise AND, OR, and, XOR., (c) The one’s complement converts all zeros in its operand to 1s, and all 1s to 0s., (d) The right-shift and left-shift operators are useful in, eliminating bits from a number—either from the left or from, the right., (e) The bitwise AND operators is useful in testing whether a bit is, on/off and in putting off a particular bit., (f) The bitwise OR operator is used to turn on a particular bit., (g) The XOR operator works almost same as the OR operator, except one minor variation., , Exercise, [A] Answer the following:, , (a) The information about colors is to be stored in bits of a char, variable called color. The bit number 0 to 6, each represent 7, colors of a rainbow, i.e. bit 0 represents violet, 1 represents
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502, , Let Us C, , indigo, and so on. Write a program that asks the user to enter, a number and based on this number it reports which colors in, the rainbow does the number represents., (b) A company planning to launch a new newspaper in market, conducts a survey. The various parameters considered in the, survey were, the economic status (upper, middle, and lower, class) the languages readers prefer (English, Hindi, Regional, language) and category of paper (daily, supplement, tabloid)., Write a program, which reads data of 10 respondents through, keyboard, and stores the information in an array of integers., The bit-wise information to be stored in an integer is given, below:, Bit Number, , Information, , 0, 1, 2, 3, 4, 5, 6, 7, 8, , Upper class, Middle class, Lower class, English, Hindi, Regional Language, Daily, Supplement, Tabloid, , At the end give the statistical data for number of persons who, read English daily, number of upper class people who read, tabloid and number of regional language readers., (c) In an inter-college competition, various sports and games are, played between different colleges like cricket, basketball,, football, hockey, lawn tennis, table tennis, carom and chess., The information regarding the games won by a particular, college is stored in bit numbers 0, 1, 2, 3, 4, 5, 6, 7 and 8, respectively of an integer variable called game. The college
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Chapter 14: Operations On Bits, , 503, , that wins in 5 or more than 5 games is awarded the Champion, of Champions trophy. If a number is entered through the, keyboard, then write a program to find out whether the, college won the Champion of the Champions trophy or not,, along with the names of the games won by the college., (d) An animal could be either a canine (dog, wolf, fox, etc.), a, feline (cat, lynx, jaguar, etc.), a cetacean (whale, narwhal,, etc.) or a marsupial (koala, wombat, etc.). The information, whether a particular animal is canine, feline, cetacean, or, marsupial is stored in bit number 0, 1, 2 and 3 respectively of, a integer variable called type. Bit number 4 of the variable, type stores the information about whether the animal is, Carnivore or Herbivore., For the following animal, complete the program to determine, whether the animal is a herbivore or a carnivore. Also, determine whether the animal is a canine, feline, cetacean or a, marsupial., struct animal, {, char name[30] ;, int type ;, }, struct animal a = { "OCELOT", 18 } ;, , (e) The time field in the directory entry is 2 bytes long., Distribution of different bits which account for hours, minutes, and seconds is given below. Write a function which would, receive the two-byte time entry and return to the calling, function, the hours, minutes and seconds.
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504, , Let Us C, , 15 14 13 12 11 10 9, , 8, , 7, , 6, , 5, , 4, , 3, , 2, , H H H H H M M M M M M S, , S, , S, , 1, , 0, , S S, , Figure 14.13, (f) In order to save disk space information about student is stored, in an integer variable. If bit number 0 is on then it indicates Ist, year student, bit number 1 to 3 stores IInd year, IIIrd year and, IVth year student respectively. The bit number 4 to 7 stores, stream Mechanical, Chemical, Electronics and IT. Rest of the, bits store room number. Based on the given data, write a, program that asks for the room number and displays the, information about the student, if its data exists in the array., The contents of array are,, int data[ ] = { 273, 548, 786, 1096 } ;, , (g) What will be the output of the following program:, main( ), {, int i = 32, j = 65, k, l, m, n, o, p ;, k = i | 35 ; l = ~k ; m = i & j ;, n = j ^ 32 ; o = j << 2 ; p = i >> 5 ;, printf ( "\nk = %d l = %d m = %d", k, l, m ) ;, printf ( "\nn = %d o = %d p = %d", n, o, p ) ;, }
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16, , C Under Windows, •, •, •, •, •, •, •, •, •, •, •, , Which Windows…, Integers, The Use of typedef, Pointers in the 32-bit World, Memory Management, Device Access, DOS Programming Model, Windows Programming Model, Event Driven Model, Windows Programming, a Closer Look, The First Windows Program, Hungarian Notation, Summary, Exercise, , 535, , 534
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536, , Let Us C, , S, , o far we have learnt every single keyword, operator and, instruction available in C. Thus we are through with the, language elements that were there to learn. We did all this, learning by compiling our programs using a 16-bit compiler like, Turbo C/C++. Now it is time to move on to more serious stuff. To, make a beginning one has to take a very important decision—, should we attempt to build programs that are targeted towards 16bit environments like MS-DOS or 32-bit environments like, Windows/Linux. Obviously we should choose the 32-bit platform, because that is what is in commercial use today and would remain, so until 64-bit environment takes over in future. That raises a very, important question—is it futile to learn C programming using 16bit compiler like Turbo C/C++? Absolutely not! The typical 32-bit, environment offers so many features that the beginner is likely to, feel lost. Contrasted with this, 16-bit compilers offer a very simple, environment that a novice can master quickly., Now that the C fundamentals are out of the way and you are, confident about the language features it is time for us to delve into, the modern 32-bit operating environments. In today’s commercial, world 16-bit operating environments like DOS are more or less, dead. More and more software is being created for 32-bit, environments like Windows and Linux. In this chapter we would, explore how C programming is done under Windows. Chapters 20, & 21 are devoted to exploring C under Linux., , Which Windows…, To a common user the differences amongst various versions of, Windows like Windows 95,98, ME, NT, 2000, XP, Server 2003 is, limited to only visual appearances—things like color of the title, bar, shape of the buttons, desktop, task bar, programs menu etc., But the truth is much farther than that. Architecturally there are, huge differences amongst them. So many are the differences that, Microsoft categorizes the different versions under two major, heads—Consumer Windows and Windows NT Family. Windows
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Chapter 16: C Under Windows, , 537, , 95, 98, ME fall under the Consumer Windows, whereas Windows, NT, 2000, XP, Server 2003 fall under the Windows NT Family., Consumer Windows was targeted towards the home or small office, users, whereas NT family was targeted towards business users., Microsoft no longer provides support for Consumer Windows., Hence in this book we would concentrate only on NT Family, Windows. So in the rest of this book whenever I refer to Windows, I mean Windows NT family, unless explicitly specified., Before we start writing C programs under Windows let us first see, some of the changes that have happened under Windows, environment., , Integers, Under 16-bit environment the size of integer is of 2 bytes. As, against this, under 32-bit environment an integer is of 4 bytes., Hence its range is -2147483648 to +2147483647. Thus there is no, difference between an int and a long int. But what if we wish to, store the age of a person in an integer? It would be improper to, sacrifice a 4-byte integer when we know that the number to be, stored in it is hardly going to exceed hundred. In such as case it, would be more sensible to use a short int since it is only 2 bytes, long., , The Use of typedef, Take a look at the following declarations:, COLORREF color ;, HANDLE h ;, WPARAM w ;, LPARAM l ;, BOOL b ;
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538, , Let Us C, , Are COLORREF, HANDLE, etc. new datatypes that have been, added in C under Windows compiler? Not at all. They are merely, typedef’s of the normal integer datatype., A typical C under Windows program would contain several such, typedefs. There are two reasons why Windows-based C programs, heavily make use of typedefs. These are:, (a) A typical Windows program is required to perform several, complex tasks. For example a program may print documents,, send mails, perform file I/O, manage multiple threads of, execution, draw in a window, play sound files, perform, operations over the network apart from normal data, processing tasks. Naturally a program that carries out so many, tasks would be very big in size. In such a program if we start, using the normal integer data type to represent variables that, hold different entities we would soon lose track of what that, integer value actually represents. This can be overcome by, suitably typedefining the integer as shown above., (b) At several places in Windows programming we are required, to gather and work with dissimilar but inter-related data. This, can be done using a structure. But when we define any, structure variable we are required to precede it with the, keyword struct. This can be avoided by using typedef as, shown below:, struct rect, {, int top ;, int left ;, int right ;, int bottom ;, };, typedef struct rect RECT ;, typedef struct rect* PRECT ;
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Chapter 16: C Under Windows, , 539, , RECT r ;, PRECT pr ;, , What have we achieved out of this? It makes user-defined, data types like structures look, act and behave similar to, standard data types like integers, floats, etc. You would agree, that the following declarations, RECT r ;, int i ;, , are more logical than, struct RECT r ;, int i ;, , Imagine a situation where each programmer typedefs the integer, to represent a color in different ways. Some of these could be as, follows:, typedef int COL ;, typedef int COLOR ;, typedef int COLOUR ;, typedef int COLORREF ;, , To avoid this chaos Microsoft has done several typedefs for, commonly required entities in Windows programming. All these, have been stored in header files. These header files are provided as, part of 32-bit compiler like Visual C++., , Pointers in the 32-bit World, In a 16-bit world (like MS-DOS) we could run only one, application at a time. If we were to run another program we were, required to terminate the first one before launching the second. As, only one program (task) could run at a time this environment was
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540, , Let Us C, , called single-tasking environment. Since only one program could, run at any given time entire resources of the machine like memory, and hardware devices were accessible to this program. Under 32bit environment like Windows several programs reside and work, in memory at the same time. Hence it is known as a multi-tasking, environment. But the moment there are multiple programs running, in memory there is a possibility of conflict if two programs, simultaneously access the machine resources. To prevent this,, Windows does not permit any application direct access to any, machine resource. To channelize the access without resulting into, conflict between applications several new mechanisms were, created in the Microprocessor & OS. This had a direct bearing on, the way the application programs are created. This is not a, Windows OS book. So we would restrict our discussion about the, new mechanisms that have been introduced in Windows to topics, that are related, to C programming. These topics are ‘Memory, Management and Device Access’., , Memory Management, Since users have become more demanding, modern day, applications have to contend with these demands and provide, several features in them. To add to this, under Windows several, such applications run in memory simultaneously. The maximum, allowable memory—1 MB—that was used in 16-bit environment, was just too small for this. Hence Windows had to evolve a new, memory management model. Since Windows runs on 32-bit, microprocessors each CPU register is 32-bit long. Whenever we, store a value at a memory location the address of this memory, location has to be stored in the CPU register at some point in time., Thus a 32-bit address can be stored in these registers. This means, that we can store 232 unique addresses in the registers at different, times. As a result, we can access 4 GB of memory locations using, 32-bit registers. As pointers store addresses, every pointer under, 32-bit environment also became a 4-byte entity.
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Chapter 16: C Under Windows, , 541, , However, if we decide to install 4 GB memory it would cost a lot., Hence Windows uses a memory model which makes use of as, much of physical memory (say 128 MB) as has been installed and, simulates the balance amount of memory (4 GB – 128 MB) on the, hard disk. Be aware that this balance memory is simulated as and, when the need to do so arises. Thus memory management is, demand based., Note that programs cannot execute straight-away from hard disk., They have to be first brought into physical memory before they, can get executed. Suppose there are multiple programs already in, memory and a new program starts executing. If this new program, needs more memory than what is available right now, then some of, the existing programs (or their parts) would be transferred to the, disk in order to free the physical memory to accommodate the new, program. This operation is often called page-out operation. Here, page stands for a block of memory (usually of size 4096 bytes)., When that part of the program that was paged out is needed it is, brought back into memory (called page-in operation) and some, other programs (or their parts) are paged out. This keeps on, happening without a common user’s knowledge all the time while, working with Windows. A few more facts that you must note, about paging are as follows:, (a) Part of the program that is currently executing might also be, paged out to the disk., (b) When the program is paged in (from disk to memory) there is, no guarantee that it would be brought back to the same, physical location where it was before it was paged out., Now imagine how the paging operations would affect our, programming. Suppose we have a pointer pointing to some data, present in a page. If this page gets paged out and is later paged in, to a different physical location then the pointer would obviously, have a wrong address. Hence under Windows the pointer never, holds the physical address of any memory location. It always holds, a virtual address of that location. What is this virtual address? At
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542, , Let Us C, , its name suggests it is certainly not a real address. It is a number,, which contains three parts. These parts when used in conjunction, with a CPU register called CR3 and contents of two tables called, Page Directory Table and Page Table leads to the actual physical, address. This is shown in Figure 16.1., , Page Dir. Index Page Table Index Page Byte Offset, 31, , 21, , Page Directory, PT0, PT1, , 0, , 11, Page Table, P0, P1, , Target Page, PFN n, , …, , Pn, …, , Page Frames, , PT n, Page Directory, Register, , PFN 0, Physical Memory, , Figure 16.1, The CR3 register holds the physical location of Page Directory, Table. The left part of the 32-bit virtual address holds the index, into the Page Directory Table. The value present at this index is, the starting address of the Page Table. The middle part of the 32bit virtual address holds the index into the Page Table. The value, present at this index is the starting address of the physical page in, memory. The right part of the 32-bit virtual address holds the byte
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Chapter 16: C Under Windows, , 543, , offset (from the start of the page) of the physical memory location, to be accessed., Note that the CR3 register is not accessible from an application., Hence an application can never directly reach a physical address., Also, as the paging activity is going on the OS would suitably keep, updating the values in the two tables., , Device Access, All devices under Windows are shared amongst all the running, programs. Hence no program is permitted a direct access to any of, the devices. The access to a device is routed through a device, driver program, which finally accesses the device. There is a, standard way in which an application can communicate with the, device driver. It is device driver’s responsibility to ensure that, multiple requests coming from different applications are handled, without causing any conflict. This standard way of communication, is discussed in detail in Chapter 17., , DOS Programming Model, Typical 16-bit environments like DOS use a sequential, programming model. In this model programs are executed from, top to bottom in an orderly fashion. The path along which the, control flows from start to finish may vary during each execution, depending on the input that the program receives or the conditions, under which it is run. However, the path remains fairly predictable., C programs written in this model begin execution with main( ), (often called entry point) and then call other functions present in, the program. If you assume some input data you can easily walk, through the program from beginning to end. In this programming, model it is the program and not the operating system that, determines which function gets called and when. The operating, system simply loads and executes the program and then waits for it, to finish. If the program wishes it can take help of the OS to carry
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544, , Let Us C, , out jobs like console I/O, file I/O, printing, etc. For other, operations like generating graphics, carrying out serial, communication, etc. the program has to call another set of, functions called ROM-BIOS functions., Unfortunately the DOS functions and the BIOS functions do not, have any names. Hence to call them the program had to use a, mechanism called interrupts. This is a messy affair since the, programmer has to remember interrupt numbers for calling, different functions. Moreover, communication with these functions, has to be done using CPU registers. This lead to lot of difficulties, since different functions use different registers for communication., To an extent these difficulties are reduced by providing library, functions that in turn call the DOS/BIOS functions using, interrupts. But the library doesn’t have a parallel function for every, DOS/BIOS function. DOS functions either call BIOS functions or, directly access the hardware., At times the programs are needed to directly interact with the, hardware. This has to be done because either there are no, DOS/BIOS functions to do this, or if they are there their reach is, limited., Figure 16.2 captures the essence of the DOS programming model.
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545, , Chapter 16: C Under Windows, , main( ), {, fun( ) ;, }, fun( ), {, …, …, }, , On execution, transfer control, to program, Interrupt, & CPU, Registers, Interrupt, & CPU, Registers, , DOS OS, , DOS, Functions, , BIOS, Function, , Hardware, Sequentially Executing, DOS program, , Figure 16.2, From the above discussion you can gather that there are several, limitations in the DOS programming model. These have been, listed below:, , No True Reuse, The library functions that are called from each program become, part of the executable file (.EXE) for that program. Thus the same, functions get replicated in several EXE files, thereby wasting, precious disk space.
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546, , Let Us C, , Inconsistent Look and Feel, Every DOS program has a different user interface that the user has, to get used to before he can start getting work out of the program., For example, successful DOS-based software like Lotus 1-2-3,, Foxpro, Wordstar offered different types of menus. This happened, because DOS/BIOS doesn’t provide any functions for creating, user interface elements like menus. As the look and feel of all, DOS based programs is different, the user takes a lot of time in, learning how to interact with the program, , Messy Calling Mechanism, It is difficult to remember interrupt numbers and the registers that, are to be used for communication with DOS/BIOS functions. For, example, if we are to position the cursor on the screen using a, BIOS function we are required to remember the following details:, Interrupt number – 16, CPU Registers to be used:, AH – 2 (service number), DH – Row number where cursor is to be positioned, DL – Column number where cursor is to be positioned, , While using these interrupt numbers and registers there is always a, chance of error., , Hardware Dependency, DOS programs are always required to bother about the details of, the hardware on which they are running. This is because for every, new piece of hardware introduced there are new interrupt numbers, and new register details. Hence DOS programmers are under the, constant fear that if the hardware on which the programs are, running changes then the program may crash.
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Chapter 16: C Under Windows, , 547, , Moreover the DOS programmer has to write lot of code to detect, the hardware on which his program is running and suitably make, use of the relevant interrupts and registers. Not only does this, make the program lengthy, the programmer has to understand a lot, of technical details of the hardware. As a result the programmer, has to spend more time in understanding the hardware than in the, actual application programming., , Windows Programming Model, From the perspective of the user the shift from MS-DOS to, Windows OS involves switching over to a Graphical User, Interface from the typical Text Interface that MS-DOS offers., Another change that the user may feel and appreciate is the ability, of Windows OS to execute several programs simultaneously,, switching effortlessly from one to another by pointing at windows, and clicking them with the mouse. Mastering this new GUI, environment and getting comfortable with the multitasking feature, is at the most a matter of a week or so. However, from the, programmer’s point of view programming for Windows is a whole, new ball game!, Windows programming model is designed with a view to:, (a), (b), (c), (d), , Eliminate the messy calling mechanism of DOS, Permit true reuse of commonly used functions, Provide consistent look and feel for all applications, Eliminate hardware dependency, , Let us discuss how Windows programming model achieves this., , Better Calling Mechanism, Instead of calling functions using Interrupt numbers and registers, Windows provides functions within itself which can be called, using names. These functions are called API (Application, Programming Interface) functions. There are literally hundreds of
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548, , Let Us C, , API functions available. They help an application to perform, various tasks such as creating a window, drawing a line,, performing file input/output, etc., , True Reuse, A C under Windows program calls several API functions during, course of its execution. Imagine how much disk space would have, been wasted had each of these functions become part of the EXE, file of each program. To avoid this, the API functions are stored in, special files that have an extension .DLL., DLL stands for Dynamic Link Libraries. A DLL is a binary file, that provides a library of functions. The functions present in DLLs, can be linked during execution. These functions can also be shared, between several applications running in Windows. Since linking is, done dynamically the functions do not become part of the, executable file. As a result, the size of EXE files does not go out of, hand. It is also possible to create your own DLLs. You would like, to do this for two reasons:, (a) Sharing common code between different executable files., (b) Breaking an application into component parts to provide a, way to easily upgrade application’s components., The Windows API functions come in three DLL files. Figure 16.3, lists these filenames along with purpose of each.
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Chapter 16: C Under Windows, , DLL, , Description, , USER32.DLL, , Contains functions that are responsible, for window management, including, menus, cursors, communications,, timer etc., , GDI32.DLL, , Contains functions for graphics drawing, and painting, , KERNEL32.DLL, , Contains functions to handle memory, management, threading, etc., , 549, , Figure 16.3, , Consistent Look and Feel, Consistent look and feel means that each program offers a, consistent and similar user interface. As a result, user doesn’t have, to spend long periods of time mastering a new program. Every, program occupies a window—a rectangular area on the screen. A, window is identified by a title bar. Most program functions are, initiated through the program’s menu. The display of information, too large to fit on a single screen can be viewed using scroll bars., Some menu items invoke dialog boxes, into which the user enters, additional information. One dialog box is found in almost every, Windows program. It opens a file. This dialog box looks the same, (or very similar) in many different Windows programs, and it is, almost always invoked from the same menu option., Once you know how to use one Windows program, you’re in a, good position to easily learn another. The menus and dialog boxes, allow user to experiment with a new program and explore its, features. Most Windows programs have both a keyboard interface, and a mouse interface. Although most functions of Windows, programs can be controlled through the keyboard, using the mouse, is often easier for many chores.
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550, , Let Us C, , From the programmer’s perspective, the consistent user interface, results from using the Windows API functions for constructing, menus and dialog boxes. All menus have the same keyboard and, mouse interfaces because Windows—rather than the application, program—handles this job., , Hardware Independent Programming, As we saw earlier a Windows program can always call Windows, API functions. Thus an application can easily communicate with, OS. What is new in Windows is that the OS can also communicate, with application. Let us understand why it does so with the help of, an example., Suppose we have written a program that contains a menu item,, which on selection is supposed to display a string “Hello World”, in the window. The menu item can be selected either using the, keyboard or using the mouse. On executing this program it will, perform the initializations and then wait for the user input. Sooner, or later the user would press the key or click the mouse to select, the menu-item. This key-press or mouse-click is known as an, ‘event’. The occurrence of this event is sensed by the keyboard or, mouse device driver. The device driver would now inform, Windows about it. Windows would in turn notify the application, about the occurrence of this event. This notification is known as a, ‘message’. Thus the OS has communicated with the application., When the application receives the message it communicates back, with the OS by calling a Windows API function to display the, string “Hello World” in the window. This API function in turn, communicates with the device driver of the graphics card (that, drives the screen) to display the string. Thus there is a two-way, communication between the OS and the application. This is shown, in Figure 16.4.
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Chapter 16: C Under Windows, , 551, , Application, API Call, , Message, , Windows OS, , Device Driver, , Hardware, , Figure 16.4, Suppose the keyboard and the mouse are now replaced with a new, keyboard and mouse. Doing so would not affect the application at, all. This is because at no time does the application carry out any, direct communication with the devices. Any differences that may, be there in the new set of mouse and keyboard would be handled, the device driver and not by the application program. Similarly, if, the screen or the graphics card is replaced no change would be, required in the program. In short hardware independence at work!, At times a change of device may necessitate a change in the device, driver program, but never a change in the application., , Event Driven Model, When a user interacts with a Windows program a lot of events, occur. For each event a message is sent to the program and the, program reacts to it. Since the order in which the user would, interact with the user-interface elements of the program cannot be, predicted the order of occurrence of events, and hence the order of, messages, also becomes unpredictable. As a result, the order of
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552, , Let Us C, , calling the functions in the program (that react to different, messages) is dictated by the order of occurrence of events. Hence, this programming model is called ‘Event Driven Programming, Model’., That’s really all that is there to event-driven programming. Your, job is to anticipate what users are likely to do with your, application’s user interface objects and have a function waiting,, ready to execute at the appropriate time. Just when that time is, no, one except the user can really say., , Windows Programming, a Closer Look, There can be hundreds of ways in which the user may interact with, an application. In addition to this some events may occur without, any user interaction. For example, events occur when we create a, window, when the window’s contents are to be drawn, etc. Not, only this, occurrence of one event may trigger a few more events., Thus literally hundreds of messages may be sent to an application, thereby creating a chaos. Naturally, a question comes—in which, order would these messages get processed by the application., Order is brought to this chaos by putting all the messages that, reach the application into a ‘Queue’. The messages in the queue, are processed in First In First Out (FIFO) order., In fact the OS maintains several such queues. There is one queue,, which is common for all applications. This queue is known as, ‘System Message Queue’. In addition there is one queue per, application. Such queues are called ‘Application Message, Queues’. Let us understand the need for maintaining so many, queues., When we click a mouse and an event occurs the device driver, posts a message into the System Message Queue. The OS retrieves, this message finds out with regard to which application the, message has been sent. Next it posts a message into the
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553, , Chapter 16: C Under Windows, , Application Message Queue of the application in which the mouse, was clicked. Refer Figure 16.5., , Event, , Event, , Device Driver, , Device Driver, , Msg., , Msg., System Msg., Queue, , Other, Mess, , Application1, , Application1, Msg. Queue, , OS, , Other, Messa, , Application2, Msg. Queue, , Application2, , Figure 16.5, I think now we have covered enough ground to be able to actually, start C under Windows programming. Here we go…
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554, , Let Us C, , The First Windows Program, To keep things simple we would begin with a program that merely, displays a “Hello” message in a message box. Here is the, program…, #include <windows.h>, int _stdcall WinMain ( HINSTANCE hInstance, HINSTANCE hPrevInstance,, LPSTR lpszCmdline, int nCmdShow ), {, MessageBox ( 0, “Hello!”, “Title”, 0 ) ;, return ( 0 ) ;, }, , Naturally a question would come to your mind—how do I create, and run this program and what output does it produce. Firstly take, a look at the output that it produces. Here it is…, , Figure 16.6, Let us now look at the steps that one needs to carry to create and, execute this program:, (a) Start VC++ from ‘Start | Programs | Microsoft Visual C++, 6.0’. The VC++ IDE window will get displayed., (b) From the File | New menu, select ‘Win32 Application’, and, give a project name, say, ‘sample1’. Click on OK., (c) From the File | New menu, select ‘C++ Source File’, and give, a suitable file name, say, ‘sample1’. Click on OK., (d) The ‘Win32 Application-Step 1 of 1’ window will appear., Select ‘An empty project’ option and click ‘Finish’ button.
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Chapter 16: C Under Windows, , 555, , (e) A ‘New Project Information’ dialog will appear. Close it by, clicking on OK., (f) Again select ‘File | New | C++ Source File’. Give the file, name as ‘sample1.c’. Click on OK., (g) Type the program in the ‘sample1.c’ file that gets opened in, the VC++ IDE., (h) Save this file using ‘Save’ option from the File menu., To execute the program follow the steps mentioned below:, (a) From the Build menu, select ‘Build sample1.exe’., (b) Assuming that no errors were reported in the program, select, ‘Execute sample1.exe’ from the Build menu., Let us now try to understand the program. The way every C under, DOS program begins its execution with main( ), every C under, Windows program begins its execution with WinMain( ). Thus, WinMain( ) becomes the entry point for a Windows program. A, typical WinMain( ) looks like this:, int __stdcall WinMain ( HINSTANCE hInstance, HINSTANCE hPrevInstance,, LPSTR lpszCmdLine, int nCmdShow ), , Note the __stdcall before WinMain( ). It indicates the calling, convention used by the WinMain( ) function. Calling Conventions, indicate two things:, (a) The order (left to right or right to left) in which the arguments, are pushed onto the stack when a function call is made., (b) Whether the caller function or called function removes the, arguments from the stack at the end of the call., , Out of the different calling conventions available most commonly, used conventions are __cdecl and __stdcall . Both these calling, conventions pass arguments to functions from right to left. In, __cdecl the stack is cleaned up by the calling function, whereas in, case of __stdcall the stack is cleaned up by the called function. All
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556, , Let Us C, , API functions use __stdcall calling convention. If not mentioned,, __cdecl calling convention is assumed by the compiler., HINSTANCE and LPSTR are nothing but typedefs. The first is an, unsigned int and the second is a pointer to a char. These macros, are defined in ‘windows.h’. This header file must always be, included while writing a C program under Windows. hInstance,, hPrevInstance, lpszCmdLine and nCmdShow are simple, variable names. In place of these we can use i, j, k and l, respectively. Let us now understand the meaning of these, parameters as well as the rest of the program., −, , WinMain( ) receives four parameters which are as under:, hInstance: This is the ‘instance handle’ for the running, application. Windows creates this ID number when the, application starts. We will use this value in many Windows, functions to identify an application’s data., A handle is simply a 32-bit number that refers to an entity., The entity could be an application, a window, an icon, a, brush, a cursor, a bitmap, a file, a device or any such entity., The actual value of the handle is unimportant to your, programs, but the Windows module that gives your program, the handle knows how to use it to refer to an entity. What is, important is that there is a unique handle for each entity and, we can refer and reach the entity only using its handle., hPrevInstance: This parameter is a remnant of earlier, versions of Windows and is no longer significant. Now it, always contains a value 0. It is being persisted with only to, ensure backward compatibility., lpszCmdLine: This is a pointer to a character string, containing the command line arguments passed to the, program. This is similar to the argv, argc parameters passed, to main( ) in a DOS program.
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Chapter 16: C Under Windows, , 557, , nCmdShow: This is an integer value that is passed to the, function. This integer tells the program whether the window, that it creates should appear minimized, as an icon, normal, or, maximized when it is displayed for the first time., −, , The MessageBox( ) function pops up a message box whose, title is ‘Title’ and which contains a message ‘Hello!’., , −, , Returning 0 from WinMain( ) indicates success, whereas,, returning a nonzero value indicates failure., , −, , Instead of printing ‘Hello!’ in the message box we can print, the command line arguments that the user may supply while, executing the program. The command line arguments can be, supplied to the program by executing it from Start | Run as, shown in Figure 16.7., , Figure 16.7, Note from Figure 16.7 that ‘myapp.exe’ is the name of our, application, whereas, ‘abc ijk xyz’ represents command line, arguments. The parameter lpszCmdline points to the string, “abc ijk xyz”. This string can be printed using the following, statement:, MessageBox ( 0, lpszCmdline, "Title", 0 ) ;, , If the entire command line including the filename is to be, retrieved we can use the GetCommandLine( ) function.
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558, , Let Us C, , Hungarian Notation, Hungarian Notation is a variable-naming convention so called in, the honor of the legendary Microsoft programmer Charles, Simonyi. According to this convention the variable name begins, with a lower case letter or letters that denotes the data type of the, variable. For example, the sz prefix in szCmdLine stands for, ‘string terminated by zero’; the prefix h in hInstance stands for, ‘handle’; the prefix n in nCmdShow stands for int. Prefixes are, often combined to form other prefixes, as lpsz in lpszCmdLine, stands for ‘long pointer to a zero terminated string’. Though, basically this notation is a good idea nowadays its usage is, discouraged. This is because when a transition happens from say a, 16-bit code to 32-bit code then a whole lot of variable names have, to be changed. For example, suppose the 16-bit code used 2-byte, and 4-byte integer variables called wParam and lParam, where w, indicated a 16-bit integer (word) and a 32-bit integer (long), respectively. When this code is ported to a 32-bit environment, wParam had to be changed to lParam since in this environment, every integer is 4 bytes long. You would agree that if we follow, the Hungarian notation then we would have to make a whole lot of, changes in the variable names when we port the code to a 32-bit or, a 64-bit environment. Hence the usage of this convention is, nowadays discouraged., , Summary, (a) Under Windows an integer is four bytes long. To use a twobyte integer pre-qualify it with short., (b) Under Windows a pointer is four bytes long., (c) Windows programming involves a heavy usage of typedefs., (d) DOS uses a Sequential Programming Model, whereas,, Windows uses an Event Driven Programming Model., (e) Entry point of every Windows program is a function called, WinMain( ).
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Chapter 16: C Under Windows, , 559, , (f) Windows does not permit direct access to memory or, hardware devices., (g) Windows uses a Demand-based Virtual Memory Model to, manage memory., (h) Under Windows there is two-way communication between the, program and the OS., (i) Windows maintains a system message queue common for all, applications., (j) Windows maintains an application message queue per running, application., (k) Calling convention decides the order in which the parameters, are passed to a function and whether the calling function or, the called function clears the stack., (l) Commonly used calling conventions are __cdecl and, __stdcall., (m) Hungarian notation though good its usage is not, recommended any more., , Exercise, [A] State True or False:, , (a) MS-DOS uses a procedural programming model., (b) A Windows program can directly call a device driver program, for a device., (c) API functions under Windows do not have names., (d) DOS functions are called using an interrupt mechanism., (e) Windows uses a 4 GB virtual memory space., (f) Size of a pointer under Windows depends upon whether it is, near or far., (g) Under Windows the address stored in a pointer is a virtual, address and not a physical address., (h) One of the parameters of WinMain( ) called hPrevInstance, is no longer relevant.
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560, , Let Us C, , [B] Answer the following:, , (a) Why is Event-driven Programming Model better than the, Sequential Programming Model?, (b) What is the meaning of different parts of the address stored in, a pointer under Windows environment?, (c) Why Windows does not permit direct access to hardware?, (d) What is the difference between an event and a message?, (e) Why Windows maintains a different message queue for each, application?, (f) In which different situations messages get posted into an, application message queue?, [C] Attempt the following:, , (a) Write a program that prints the value of hInstance in a, message box., (b) Write a program that displays three buttons ‘Yes’, ‘No’, ‘Cancel’ in the message box., (c) Write a program that receives a number as a command line, argument and prints its factorial value in a message box., (d) Write a program that displays command line arguments, including file name in a message box.
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17, , Windows, Programming, •, •, •, •, , The Role of a Message Box, Here comes the window…, More Windows, A Real-World Window, Creation and Displaying of Window, Interaction with Window, Reacting to Messages, • Program Instances, • Summary, • Exercise, , 561
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562, , Let Us C, , E, , event driven programming requires a change in mind set. I, hope Chapter 16 has been able to bring about this change., However this change would be bolstered by writing event, driven programs. This is what this chapter intends to do. I am, hopeful that by the time you reach the end of this chapter you, would be so comfortable with it as if you have been using it all, your life., , The Role of a Message Box, Often we are required to display certain results on the screen, during the course of execution of a program. We do this to, ascertain whether we are getting the results as per our, expectations. In a sequential DOS based program we can easily, achieve this using printf( ) statements. Under Windows screen is a, shared resource. So you can imagine what chaos would it create if, all running applications are permitted to write to the screen. You, would not be able to make out which output is of what application., Hence no Windows program is permitted to write anything directly, to the screen. That’s where a message box enters the scene. Using, it we can display intermediate results during the course of, execution of a program. It can be dismissed either by clicking the, ‘close button’ in its title bar or by clicking the OK button present, in it. There are numerous variations that you can try with the, MessageBox( ). Some of these are given below, MessageBox ( 0, “Are you sure”, “Caption”, MB_YESNO ) ;, MessageBox ( 0, “Print to the Printer”, “Caption”, MB_YESNO CANCEL) ;, MessageBox ( 0, “icon is all about style”, “Caption”, MB_OK |, MB_ICONINFORMATION ) ;, , You can put the above statements within WinMain( ) and see the, results for yourself. Though the above message boxes give you, flexibility in displaying results, button, icons, there is a limit to, which you can stretch them. What if we want to draw a free hand, drawing or display an image, etc. in the message box. This would
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Chapter 17: Windows Programming, , 563, , not be possible. To achieve this we need to create a full-fledged, window. The next section discusses how this can be done., , Here Comes the window…, Before we proceed with the actual creation of a window it would, be a good idea to identify the various elements of it. These are, shown in Figure 17.1., , Caption, Bar, , Minimize, Box, Close, Butt, on, , Icon, , Menu, Vertical, Scrol, , Client, Area, , Horizontal Scroll, , Figure 17.1, Note that every window drawn on the screen need not necessarily, have every element shown in the above figure. For example, a, window may not contain the minimize box, the maximize box, the, scroll bars and the menu., Let us now create a simple program that creates a window on the, screen. Here is the program…, #include <windows.h>
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564, , Let Us C, , int _stdcall WinMain ( HINSTANCE hInstance, HINSTANCE hPrevInstance,, LPSTR lpszCmdLine, int nCmdShow ), {, HWND h ;, h = CreateWindow ( “BUTTON”, “Hit Me”, WS_OVERLAPPEDWINDOW,, 10, 10, 150, 100, 0, 0, i, 0 ) ;, ShowWindow ( h, nCmdShow ) ;, MessageBox ( 0, “Hi!”, “Waiting”, MB_OK ) ;, return 0 ;, }, , Here is the output of the program…, , Figure 17.2, Let us now understand the program. Every window enjoys certain, properties—background color, shape of cursor, shape of icon, etc., All these properties taken together are known as ‘window class’., The meaning of ‘class’ here is ‘type’. Windows insists that a, window class should be registered with it before we attempt to, create windows of that type. Once a window class is registered we, can create several windows of that type. Each of these windows, would enjoy the same properties that have been registered through, the window class. There are several predefined window classes., Some of these are BUTTON, EDIT, LISTBOX, etc. Our program, has created one such window using the predefined BUTTON class.
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Chapter 17: Windows Programming, , 565, , To actually create a window we need to call the API function, CreateWindow( ). This function requires several parameters, starting with the window class. The second parameter indicates the, text that is going to appear on the button surface. The third, parameter, specifies, the, window, style., WS_OVERLAPPEDWINDOW is a commonly used style. The, next four parameters specify the window’s initial position and, size—the x and y screen coordinates of the window’s top left, corner and the window’s width and height in pixels. The next three, parameters specify the handles to the parent window, the menu and, the application instance respectively. The last parameter is the, pointer to the window-creation data., We can easily devote a section of this book to CreateWindow( ), and its parameters. But don’t get scared of it. Nobody is supposed, to remember all the parameters, their meaning and their order. You, can always use MSDN (Microsoft Developer Network) help to, understand the minute details of each parameter. This help is, available as part of VC++ 6.0 product. It is also available on the, net at http://www.msdn.microsoft.com/library., Note that CreateWindow( ) merely creates the window in, memory. We still are to display it on the screen. This can be done, using the ShowWindow( ) API function. CreateWindow( ), returns handle of the created window. Our program uses this, handle to refer to the window while calling ShowWindow( ). The, second parameter passed to ShowWindow( ) signifies whether the, window would appear minimized, maximized or normal. If the, value of this parameter is SW_SHOWNORMAL we get a normal, sized window, if it is SW_SHOWMINIMIZED we get a, minimized window and if it is SW_SHOWMINIMIZED we get a, maximized window. We have passed nCmdShow as the second, parameter. This variable contains SW_SHOWNORMAL by, default. Hence our program displays a normal sized window.
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566, , Let Us C, , The WS_OVERLAPPEDWINDOW style is a collection of the, following styles:, WS_OVERLAPPED | WS_CAPTION | WS_SYSMENU | WS_THICKFRAME |, WS_MINIMIZEBOX | WS_MAXIMIZEBOX, , As you can make out from these macros they essentially control, the look and feel of the window being created. All these macros, are #defined in the ‘Windows.h’ header file., On executing this program a window and a message box appears, on the screen as shown in the Figure 17.2. The window and the, message box disappear as soon as we click on OK. This is because, on doing so execution of WinMain( ) comes to an end and, moreover we have made no provision to interact with the window., You can try to remove the call to MessageBox( ) and see the, result. You would observe that no sooner does the window appear, it disappears. Thus a call to MessageBox( ) serves the similar, purpose as getch( ) does in sequential programming., , More Windows, Now that we know how to create a window let us create several, windows on the screen. The program to do this is given below., #include <windows.h>, int _stdcall WinMain ( HINSTANCE hInstance, HINSTANCE hPrevInstance,, LPSTR lpszCmdLine, int nCmdShow ), {, HWND h[10] ;, int x ;, for ( x = 0 ; x <= 9 ; x++ ), {
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Chapter 17: Windows Programming, , 567, , h[x] = CreateWindow ( "BUTTON", "Press Me",, WS_OVERLAPPEDWINDOW, x * 20,, x * 20, 150, 100, 0, 0, i, 0 ) ;, ShowWindow ( h[x], l ) ;, }, MessageBox ( 0, "Hi!", "Waiting", 0 ) ;, return 0 ;, }, , Figure 17.3, Note that each window created in this program is assigned a, different handle. You may experiment a bit by changing the name, of the window class to EDIT and see the result., , A Real-World Window, Suppose we wish to create a window and draw a few shapes in it., For creating such a window there is no standard window class, available. Hence we would have to create our own window class,, register it with Windows OS and then create a window on the basis, of it. Instead of straightway jumping to a program that draws
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Chapter 17: Windows Programming, , 569, , void OnDestroy ( HWND hWnd ), {, PostQuitMessage ( 0 ) ;, }, , On execution of this program the window shown in Figure 17.4, appears on the screen. We can use minimize and the maximize, button it its title bar to minimize and maximize the window. We, can stretch its size by dragging its boundaries. Finally, we can, close the window by clicking on the close window button in the, title bar., , Figure 17.4, Let us now try to understand this program step by step., , Creation and Displaying of Window, Creating and displaying a window on the screen is a 4-step, process. These steps are:, (a), (b), (c), (d), , Creation of a window class., Registering the window class with the OS., Creation of a window based on the registered class., Displaying the window on the screen., , Creation of a window class involves setting up of elements of a, structure called WNDCLASSEX. This structure contains several
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570, , Let Us C, , elements. They govern the properties of the window. Registration, of a window class, creation of a window and displaying of a, window involves calling of API functions RegisterClassEx( ),, CreateWindow( ) and ShowWindow( ) respectively. Since all the, 4 steps mentioned above would be required in almost every, program in this chapter I have written this code in a user-defined, function called InitInstance( ) in the file ‘helper.h’., Though writing code in a header file goes against the convention I, have still done so to achieve simplicity. The complete listing of, ‘helper.h’ file is available in Appendix F. Alternatively you can, download it from the following link:, www.kicit.com/books/letusc/sourcecode/helper.h, As expected WinMain( ) starts off by calling the function, InitInstance( ) present in ‘helper.h’ file. This file has been, #included at the beginning of the program. Remember to copy this, file to your project directory—the directory in which you are going, to create this program., Once the window has been created and displayed let us see how, we can interact with it., , Interaction with Window, As and when the user interacts with the window—by stretching its, boundaries or clicking the buttons in the title bar, etc. a suitable, message is posted into the message queue of our application. Our, application should now pick them up from the message queue and, process them., A message contains a message id and some other additional, information about the message. For example, a mouse click, message would contain additional information like handle to the, window with which the user has interacted, the coordinates of
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Chapter 17: Windows Programming, , 571, , mouse cursor and the status of mouse buttons. Since it is difficult, to memorize the message ids they have been suitably #defined in, ‘windows.h’. The message id and the additional information are, stored in a structure called MSG., In WinMain( ) this MSG structure is retrieved from the message, queue by calling the API function GetMessage( ). The first, parameter passed to this function is the address of the MSG, structure variable. GetMessage( ) would pick the message info, from the message queue and place it in the structure variable, passed to it. Don’t bother about the other parameters right now., After picking up the message from the message queue we need to, process it. This is done by calling the DispatchMessage( ) API, function. This function does several activities. These are as, follows:, (a) From the MSG structure that we pass to it,, DisplayMessage( ) extracts the handle of the window for, which this message is meant for., (b) From the handle it figures out the window class based on, which the window has been created., (c) From the window class structure it obtains the address of a, function called WndProc( ) (short for window procedure)., Well I didn’t tell you earlier that in InitInstance( ) while, filling the WNDCLASSEX structure one of the elements has, been set up with the address of a user-defined function called, WndProc( )., (d) Using this address it calls the function WndProc( )., Since several messages get posted into the message queue picking, of the message and processing it should be done repeatedly. Hence, calls to GetMesage( ) and DispatchMessage( ) have been made in, a while loop in WinMain( ). When GetMessage( ) encounters a, message with id WM_QUIT it returns a 0. Now the control comes, out of the loop and WinMain( ) comes to an end.
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572, , Let Us C, , Reacting to Messages, As we saw in the previous section, for every message picked up, from the message queue the control is transferred to the, WndProc( ) function. This function is shown below:, LRESULT CALLBACK WndProc ( HWND hWnd, UINT message,, WPARAM wParam, LPARAM lParam ), , This function always receives four parameters. The first parameter, is the handle to the window for which the message has been, received. The second parameter is the message id, whereas, the, third and fourth parameters contain additional information about, the message., LRESULT is a typedef of a long int and represents the return, value of this function. CALLBACK is a typedef of __stdcall., This typedef has been done in ‘windows.h’. CALLBACK, indicates that the WndProc function has been registered with, Windows (through WNDCLASSEX structure in InitInstance( ) ), with an intention that Windows would call this back (through, DispatchMessage( ) function)., In the WndProc( ) function we have checked the message id using, a switch. If the id is WM_DESTROY then we have called the, function OnDestroy( ). This message is posted to the message, queue when the user clicks on the ‘Close Window’ button in the, title bar. In OnDestroy( ) function we have called the API, function PostQuitMessage( ). This function posts a WM_QUIT, message into the message queue. As we saw earlier, when this, message is picked up the message loop and WinMain( ) is, terminated., For all messages other than WM_DESTROY the control lands in, the default clause of switch. Here we have simply made a call to, DefWindowProc( ) API function. This function does the default
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Chapter 17: Windows Programming, , 573, , processing of the message that we have decided not to tackle. The, default processing for different message would be different. For, example on double clicking the title bar DefWindowProc( ), maximizes the window., Actually speaking when we close the window a WM_CLOSE, message is posted into the message queue. Since we have not, handled this message the DefWindowProc( ) function gets called, to tackle this message. The DefWindowProc( ) function destroys, the window and places a WM_DESTROY message in the, message queue. As discussed earlier, in WndProc( ) we have, made the provision to terminate the application on encountering, WM_DESTROY., That brings us to the end of a lonnngggg explanation! You can, now heave a sigh of relief. I would urge you to go through the, above explanation till the time you are absolutely sure that you, have understood every detail of it. A very clear understanding of it, would help you make a good Windows programmer. For your, convenience I have given a flowchart of the entire working in, Figure 17.5.
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574, , Let Us C, , START Execution, Call InitInstance( ), , Fill WNDCLASSEX structure to define window class, Call RegisterCallEx( ) to register the window class with OS, Call CreateWindow( ) to create window in memory, Call ShowWindow( ) to display window on screen, , Pick message from message queue – GetMessage( ), Is, the message, WM_QUIT, , Yes, , STOP, , No, Process the message – DispatchMessage( ), , Call Window Procedure, , Is the message, WM_DESTROY, , Yes Post WM_QUIT –, PostQuitMessage( ), , No, Do default processing of, message – DefWindowProc( ), , Figure 17.5
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Chapter 17: Windows Programming, , 575, , Program Instances, Windows allows you to run more than one copy of a program at a, time. This is handy for cutting and pasting between two copies of, Notepad or when running more than one terminal session with a, terminal emulator program. Each running copy of a program is, called a ‘program instance’., Windows performs an interesting memory optimization trick. It, shares a single copy of the program’s code between all running, instances. For example, if you get three instances of Notepad, running, there will only be one copy of Notepad’s code in, memory. All three instances share the same code, but will have, separate memory areas to hold the text data being edited. The, difference between handling of the code and the data is logical, as, each instance of Notepad might edit a different file, so the data, must be unique to each instance. The program logic to edit the files, is the same for every instance, so there is no reason why a single, copy of Notepad’s code cannot be shared., , Summary, (a) A message box can be displayed by calling the, MessageBox( ) API function., (b) Message boxes are often used to ascertain the flow of a, program., (c) Appearance of a message box can be customized., (d) The CreateWindow( ) API function creates the window in, memory., (e) The window that is created in memory is displayed using the, ShowWindow( ) API function., (f) A ‘window class’ specifies various properties of the window, that we are creating., (g) The header file ‘Windows.h’ contains declaration of several, macros used in Windows programming.
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576, , Let Us C, , (h) When the user clicks in a window, or moves mouse pointer on, the window, messages are generated and posted in the, application message queue., (i) A message contains the message id and additional information, about the message., (j) The GetMessage( )-DispatchMessage( ) loop breaks when, GetMessage( ) encounters the WM_QUIT message., (k) If we don’t handle a message received by our application then, the DefWindowProc( ) function is called to do the default, processing., , Exercise, [A] State True or False:, , (a) MessageBox( ) is an API function., (b) Calling the MessageBox( ) function displays the specified, string in console window., (c) The CreateWindow( ) function creates and displays the, window on the screen., (d) The ShowWindow( ) function can display only the, maximized window., (e) Every window has to be created using pre-registered window, class., (f) Window classes are similar to classes in C++., (g) We can use the pre-defined window classes but cannot create, our own., (h) The style WS_OVERLAPPED | WS_CAPTION |, WS_MINIMIZEBOX will create a window with caption bar, and minimize box only., (i) To be able to interact with a window it is necessary to, implement the message loop., [B] Answer the following:, , (a) Outline the steps that a typical Windows program follows, during execution.
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Chapter 17: Windows Programming, , 577, , (b) Run any Windows based program and see whether you can, identify all the elements of the application window., (c) How would you minimize a window programmatically?, (d) What would happen if we do not place WM_QUIT message, in the message queue when the user tries to close the window., (e) Explain the need of RegisterClassEx( ) function., (f) What is the difference between GetMessage( ) and, DispatchMessage( ) function?, (g) Write a program, which receives an integer as a command line, argument, creates a button window, and based on the value of, the integer displays button window as maximized / minimized, / normal., (h) Try to display a window with different combinations of, window styles and observer the results.
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578, , Let Us C
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18 Graphics Under, Windows, •, •, •, •, •, •, •, •, •, •, •, , •, •, •, , Graphics as of Now, Device Independent Drawing, Hello Windows, Drawing Shapes, Types of Pens, Types of Brushes, Code and Resources, Freehand Drawing, the Paintbrush Style, Capturing the mouse, Device Context, A Closer Look, Displaying a Bitmap, Animation at Work, WM_CREATE and OnCreate( ), WM_TIMER and OnTimer( ), A Few More Points…, Windows, the Endless World…, Summary, Exercise, , 579
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580, , Let Us C, , S, , ince times immemorial colors and shapes have fascinated, mankind like nothing else. Otherwise people would have, still been using the character oriented interfaces of MS-DOS, or Unix. In fact the graphical ability of Windows has played a very, important role in its success story. Once you get a hang of how to, draw inside a window it would open up immense possibilities that, you never thought were possible., , Graphics as of Now, World has progressed much beyond 16 colors and 640 x 480, resolution graphics that Turbo C/C++ compilers offered under, MS-DOS environment. Today we are living in a world of 1024 x, 768 resolution offering 16.7 million colors. Graphical menus,, icons, colored cursors, bitmaps, wave files and animations are the, order of the day. So much so that a 16-color graphics program, built using Turbo C working on a poor resolution almost hurts the, eye. Moreover, with the whole lot of Windows API functions to, support graphics activity there is so much that can be achieved in a, graphics program under Windows. I am sure that this chapter will, help you understand and appreciate these new capabilities., , Device Independent Drawing, Windows allow programmers to write programs to display text or, graphics on the screen without concern over the specifics of the, video hardware. A Windows program that works on a VGA, display will work without modification on an SVGA or on a XGA, display that Windows supports., The key to this ‘device independence’ is Windows’ use of a, ‘device context’. We will explore how the device context can be, used for both text and graphics output, and how using the device, context keeps our programs from interfering with each other on the, screen.
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Chapter 18: Graphics Under Windows, , 581, , During the original design of Windows, one of the goals was to, provide ‘device independence’. Device independence means that, the same program should be able to work using different screens,, keyboards and printers without modification to the program., Windows takes care of the hardware, allowing the programmer to, concentrate on the program itself. If you have ever had to update, the code of an MS-DOS program for the latest printer, plotter,, video display, or keyboard, you will recognize device, independence as a huge advantage for the developer., Windows programs do not send data directly to the screen or, printer. A Windows program knows where (screen/printer) its, output is being sent. However, it does not know how it would be, sent there, neither does it need to bother to know this. This is, because Windows uses a standard and consistent way to send the, output to screen/printer. This standard way uses an entity called, Device Context, or simply a DC. Different DC’s are associated, with different devices. For example, a screen DC is associated, with a screen, a printer DC is associated with a printer, etc. Any, drawing that we do using the screen DC is directed to the screen., Similarly, any drawing done using the printer DC is directed to the, printer. Thus, the only thing that changes from drawing to screen, and drawing to printer is the DC that is used., A windows program obtains a handle (ID value) for the screen or, printer’s DC. The output data is sent to the screen/printer using its, DC, and then Windows and the Device Driver for the device takes, care of sending it to the real hardware. The advantage of using the, DC is that the graphics and text commands that we send using the, DC are always the same, regardless of where the physical output is, showing up., The part of Windows that converts the Windows graphics function, calls to the actual commands sent to the hardware is the GDI, or, Graphics Device Interface. The GDI is a program file called, GDI32.DLL and is stored in the Windows System directory. The
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582, , Let Us C, , Windows environment loads GDI32.DLL into memory when it is, needed for graphical output. Windows also loads a ‘device driver’, program if the hardware conversions are not part of GDI32.DLL., Common examples are VGA.SYS for VGA video screen and, HPPLC.SYS for the HP LaserJet printer. Drivers are just programs, that assist the GDI in converting Windows graphics commands to, hardware commands., Thus GDI provides all the basic drawing functionality for, Windows; the device context represents the device providing a, layer of abstraction that insulates your applications from the, trouble of drawing directly to the hardware. The GDI provides this, insulation by calling the appropriate device driver in response to, windows graphics function calls., , Hello Windows, We would begin our tryst with graphics programming under, windows by displaying a message “Hello Windows” in different, fonts. Note that though we are displaying text under Windows, even text gets drawn graphically in the window. First take a look at, the program given below before we set out to understand it., # include <windows.h>, # include "helper.h", void OnPaint ( HWND ) ;, void OnDestroy ( HWND ) ;, int __stdcall WinMain ( HINSTANCE hInstance, HINSTANCE hPrevInstance,, LPSTR lpszCmdline, int nCmdShow ), {, MSG m ;, /* Perform application initialization */, InitInstance ( hInstance, nCmdShow, "Text" ) ;
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Chapter 18: Graphics Under Windows, , 585, , Drawing to a window involves handling the WM_PAINT, message. This message is generated whenever the client area of the, window needs to be redrawn. This redrawing would be required in, the following situations:, (a) When the Window is displayed for the first time., (b) When the window is minimized and then maximized., (c) When some portion of the window is overlapped by another, window and the overlapped window is dismissed., (d) When the size of the window changes on stretching its, boundaries., (e) When the window is dragged out of the screen and then, brought back into the screen., Would a WM_PAINT message be generated when the cursor is, dragged in the window? No. In this case the window saves the area, overlapped by the cursor and restores it when the cursor moves to, another position., When the switch-case structure inside WndProc( ) finds that the, message ID passed to WndProc( ) is WM_PAINT, it calls the, function OnPaint( ). Within OnPaint( ) we have called the API, function BeginPaint( ). This function obtains a handle to the, device context. Additionally it also fills the PAINTSTRUCT, structure with information about the area of the window which, needs to be repainted. Lastly it removes WM_PAINT from the, message queue. After obtaining the device context handle, the, control enters a loop., Inside the loop we have displayed “Hello Windows” in three, different fonts. Each time through the loop we have setup a, LOGFONT structure f. This structure is used to indicate the font, properties like font name, font height, italic or normal, etc. Note, that in addition to these there are other font properties that may be, setup. The properties that we have not setup in the loop are all, initialized to 0. Once the font properties have been setup we have, called the CreateFontIndirect( ) API function to create the font.
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586, , Let Us C, , This function loads the relevant font file. Then using the, information in the font file and the font properties setup in the, LOGFONT structure it creates a font in memory., CreateFontIndirect( ) returns the handle to the font created in, memory. This handle is then passed to the SelectObject( ) API, function to get the font into the DC. This function returns the, handle to the existing font in the DC, which is preserved in, holdfont variable. Next we have used the SetTextColor( ) API, function to set the color of the text to be displayed through, TextOut( ). The RGB( ) macro uses the red, green and blue, component values to generate a 32-bit color value. Note that each, color component can take a value from 0 to 255. To TextOut( ), we have to pass the handle to the DC, position where the text is to, be displayed, the text to be displayed and its length., With hfont only one font can be associated at a time. Hence before, associating another font with it we have deleted the existing font, using the DeleteObject( ) API function. Once outside the loop we, have called the EndPaint( ) API function to release the DC, handle. If not released we would be wasting precious memory,, because the device context structure would remain in memory but, we would not be able access it., In place of TextOut( ) we can also use the DrawText( ) API, function. This function permits finer control over the way the text, is displayed. You can explore this function on your own., , Drawing Shapes, If text is so near can graphics be far behind? Now that we know, how to draw text in a window let us now create a simple program, that displays different shapes in a window. Instead of showing the, entire program given below is the listing of OnPaint( ). The rest of, the program is same as in the previous section. Here onwards I, would be showing only the OnPaint( ) handler unless otherwise, required.
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588, , Let Us C, , Figure 18.2, For drawing any shape we need a pen to draw its boundary and a, brush to paint the area enclosed by it. The DC contains a default, pen and brush. The default pen is a solid pen of black color and the, default brush is white in color. In this program we have used the, default pen and a blue colored solid brush for drawing the shapes., As before, we begin by obtaining a handle to the DC using, BeginPaint( ) function. For creating a solid colored brush we need, to call the CreateSolidBrush( ) API function. The second, parameter of this function specifies the color of the brush. The, function returns the handle of the brush which we have preserved
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Chapter 18: Graphics Under Windows, , 589, , in the hbr variable. Next we have selected this brush in the DC., The handle of the default brush in DC is collected in the holdbr, variable., Once we have selected the brush into the DC we are ready to draw, the shapes. For drawing the line we have used MoveToEx( ) and, LineTo( ) API functions. Similarly for drawing a rectangle we, have used the Rectangle( ) function., The RoundRect( ) function draws a rectangle with rounded, corners. In RoundRect ( x1, y1, x2, y2, x3, y3 ), x1, y1 represents, the x and y-coordinates of the upper-left corner of the rectangle., Likewise, x2, y2 represent coordinates of the bottom right corner, of the rectangle. x3, y3 specify the width and height of the ellipse, used to draw the rounded corners., Note that rectangle and the rounded rectangle are drawn from x1,, y1 up to x2-1, y2-1., Parameters of Ellipse( ) specify coordinates of bounding rectangle, of the ellipse., The Pie( ) function draws a pie-shaped wedge by drawing an, elliptical arc whose center and two endpoints are joined by lines., The center of the arc is the center of the bounding rectangle, specified by x1, y1 and x2, y2. In Pie( x1, y1, x2, y2, x3, y3, x4,, y4 ), x1, y1 and x2, y2 specify the x and y-coordinates of the upper, left corner and bottom right corner respectively, of the bounding, rectangle. x3, y3 and x4, y4 specify the x and y-coordinates of the, arc’s starting point and ending point respectively., In Polygon ( lpPoints, nCount ), lpPoints points to an array of, points that specifies the vertices of the polygon. Each point in the, array is a POINT structure. nCount specifies the number of, vertices stored in the array. The system closes the polygon, automatically, if necessary, by drawing a line from the last vertex, to the first.
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592, , Let Us C, , Figure 18.3, A new pen can be created using the CreatePen( ) API function., This function needs three parameters—pen style, pen thickness, and pen color. Different macros like PS_SOLID, PS_DOT, etc., have been defined in ‘windows.h’ to represent different pen styles., Note that for pen styles other than PS_SOLID the pen thickness, has to be 1 pixel., , Types of Brushes, The way we can create different types of pens, we can also create, three different types of brushes. These are—solid brush, hatch, brush and pattern brush. Let us now write a program that shows, how to build these brushes and then use them to fill rectangles., Here is the OnPaint( ) handler which achieves this., void OnPaint ( HWND hWnd ), {, HDC hdc ;, PAINTSTRUCT ps ;, HBRUSH hbr ;
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594, , Let Us C, , On execution of this program the window shown in Figure 18.4, appears., , Figure 18.4, In the OnPaint( ) handler we have drawn three rectangles—first, using a solid brush, second using a hatched brush and third using a, pattern brush. Creating and using a solid brush and hatched brush, is simple. We simply have to make calls to CreateSolidBrush( ), and CreateHatchBrush( ) respectively. For the hatch brush we, have used the style HS_CROSS. There are several other styles, defined in ‘windows.h’ that you can experiment with., For creating a pattern brush we need to first create a bitmap, (pattern). Instead of creating this pattern, we have used a, readymade bitmap file. You can use any other bitmap file present, on your hard disk., Bitmaps, menus, icons, cursors that a Windows program may use, are its resources. When the compile such a program we usually, want these resources to become a part of our EXE file. If so done, we do not have to ship these resources separately. To be able to, use a resource (bitmap file in our case) it is not enough to just copy, it in the project directory. Instead we need to carry out the steps, mentioned below to add a bitmap file to the project., (a) From the ‘Insert’ menu option of VC++ 6.0 select the, ‘Resource’ option.
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Chapter 18: Graphics Under Windows, , 595, , (b) From the dialog that pops up select ‘bitmap’ followed by the, import button., (c) Select the suitable .bmp file., (d) From the ‘File’ menu select the save option to save the, generated resource script file (Script1.rc). When we select, ‘Save’ one more file called ‘resource.h’ also gets created., (e) Add the ‘Script1.rc’ file to the project using the Project | Add, to Project | Files option., While using the bitmap in the program it is always referred using, an id. The id is #defined in the file ‘resource.h’. Somewhere, information has to be stored linking the id with the actual .bmp file, on the disk. This is done in the ‘Script1.rc’ file. We need to, include the ‘resource.h’ file in the program., To create the pattern brush we first need to load the bitmap in, memory. We have done this using the LoadBitmap( ) API, function. The first parameter passed to this function is the handle, to the instance of the program. When InitInstance( ) function is, called from WinMain( ) it stores the instance handle in a global, variable hInst. We have passed this hInst to LoadBitmap( ). The, second parameter passed to it is a string representing the bitmap., This string is created from the resource id using the, MAKEINTRESOURCE macro. The LoadBitmap( ) function, returns the handle to the bitmap. This handle is then passed to the, CreatePatternBrush( ) function. This brush is then selected into, the DC and then a rectangle is drawn using it., Note that if the size of the bitmap is bigger than the rectangle, being drawn then the bitmap is suitably clipped. On the other hand, if the bitmap is smaller than the rectangle it is suitably replicated., While doing the clean up firstly the brush is deleted followed by, the bitmap.
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596, , Let Us C, , Code and Resources, A program consists of both instructions and static data. Static data, is that portion of the program which is not executed as machine, instructions and which does not change as the program executes., Static data are character strings, data to create fonts, bitmaps, etc., The designers of Windows wisely decided that static data should, be handled separately from the program code. The Windows term, for static data is ‘Resource data’, or simply ‘Resources’. By, separating static data from the program code the creators of, Windows were able to use a standard C/C++ compiler to create the, code portion of the finished Windows program, and they only had, to write a ‘Resource compiler’ to create the resources that, Windows programs use. Separating the code from the resource, data has other advantages like reducing memory demands and, making programs more portable. It also means that a programmer, can work on a program’s logic, while a designer works on how the, program looks., , Freehand Drawing, the Paintbrush Style, Even if you are knee high in computers I am sure you must have, used PaintBrush. It provides a facility to draw a freehand drawing, using mouse. Let us see if we too can achieve this. We can indicate, where the freehand drawing begins by clicking the left mouse, button. Then as we move the mouse on the table with the left, mouse button depressed the freehand drawing should get drawn in, the window. This drawing should continue till we do not release, the left mouse button., The mouse input comes in the form of messages. For free hand, drawing we need to tackle three mouse messages—, WM_LBUTTONDOWN, for, left, button, click,, WM_MOUSEMOVE, for, mouse, movement, and, WM_LBUTTONUP for releasing the left mouse button. Let us, now see how these messages are tackled for drawing freehand. The
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Chapter 18: Graphics Under Windows, , 599, , Figure 18.5, It appears that for drawing the freehand we should simply receive, the mouse coordinates as it is moved and then highlight the pixels, at these coordinates using the SetPixel( ) API function. However,, if we do so the freehand would be broken at several places. This is, because usually the mouse is dragged pretty fast whereas the, mouse move messages won’t arrive so fast. A solution to this, problem is to construct the freehand using small little line, segments. This is what has been done in our program. These lines, are so small is size that you would not even recognize that the, freehand has been drawn by connecting these small lines.
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600, , Let Us C, , Let us now discuss each mouse handler. When the, WM_LBUTTONDOWN message arrives the WndProc( ), function calls the handler OnLButtonDown( ). While doing so,, we have passed the mouse coordinates where the click occurred., These coordinates are obtained in lParam in WndProc( ). In, lParam the low order 16 bits contain the current x - coordinate of, the mouse whereas the high order 16 bits contain the y coordinate. The LOWORD and HIWORD macros have been, used to separate out these x and y - coordinates from lParam., In OnLButtonDown( ) we have preserved the starting point of, freehand in global variables x1 and y1., When OnMouseMove( ) gets called it checks whether the left, mouse button stands depressed. If it stands depressed then the, flags variable contains MK_LBUTTON. If it does, then the, current mouse coordinates are set up in the global variables x2, y2., A line is then drawn between x1, y1 and x2, y2 using the functions, MoveToEx( ) and LineTo( ). Next time around x2, y2 should, become the starting of the next line. Hence the current values of, x2, y2 are stored in x1, y1., Note that here we have obtained the DC handle using the API, function GetDC( ). This is because we are carrying out the, drawing activity in reaction to a message other than WM_PAINT., Also, the handle obtained using GetDC( ) should be released using, a call to ReleaseDC( ) function., You can try using BeginPaint( ) / EndPaint( ) in mouse handlers, and GetDC( ) / ReleaseDC( ) in OnPaint( ). Can you draw any, conclusions?, , Capturing the Mouse, If in the process of drawing the freehand the mouse cursor goes, outside the client area then the window below our window would
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Chapter 18: Graphics Under Windows, , 601, , start getting mouse messages. So our window would not receive, any messages. If this has to be avoided then we should ensure that, our window continues to receive mouse messages even when the, cursor goes out of the client area of our window. The process of, doing this is known as mouse capturing., We have captured the mouse in OnLButtonDown( ) handler by, calling the API function SetCapture( ). As a result, the program, continues to respond to mouse events during freehand drawing, even if the mouse is moved outside the client area. In the, OnLButtonUp( ) handler we have released the captured mouse by, calling the ReleaseCapture( ) API function., , Device Context, a Closer Look, Now that we have written a few programs and are comfortable, with idea of selecting objects like font, pen and brush into the DC,, it is time for us to understand how Windows achieves the device, independent drawing using the concept of DC. In fact a DC is, nothing but a structure that holds handles of various drawing, objects like font, pen, brush, etc. A screen DC and its working is, shown in Figure 18.6.
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602, , Let Us C, , App1, , App2, , Screen DC, HPEN 200, HBRUSH 400, HBITMAP 600, HFONT 700, ., ., ., Other Info, , Screen DC, HPEN 900, HBRUSH 1000, HBITMAP 600, HFONT 800, ., ., ., Other Info, Screen, , Drawing Object, , Drawing Object, , O/P Device, , Arial, HFONT = 700, , Red, Pen, , Blue, Brush, , 900, , 1000, , Black Pen, 200, , White Brush, 400, , Mono. Bitmap, 600, , 800, , Font, , Default Drawing Objects, , Figure 18.6, You can make following observations from Figure 18.6:, (a) The DC doesn’t hold the drawing objects like pen, brush, etc., It merely holds their handles., (b) With each DC a default monochrome bitmap of size 1 pixel x, 1 pixel is associated., (c) Default objects like black pen, white brush, etc. are shared by, different DCs in same or different applications.
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Chapter 18: Graphics Under Windows, , 603, , (d) The drawing objects that an application explicitly creates can, be shared within DCs of the same application, but is never, shared between different applications., (e) Two different applications would need two different DCs, even though both would be used to draw to the same screen., In other words with one screen multiple DCs can exist., (f) A common Device Driver would serve the drawing requests, coming from different applications. (Truly speaking the, request comes from GDI functions that our application calls)., Screen and printer DC is OK, but what purpose would a memory, DC serve? Well, that is what the next program would explain., , Displaying a Bitmap, We are familiar with drawing normal shapes on screen using a, device context. How about drawing images on the screen?, Windows does not permit displaying a bitmap image directly using, a screen DC. This is because there might be color variations in the, screen on which the bitmap was created and the screen on which it, is being displayed. To account for such possibilities while, displaying a bitmap Windows uses a different mechanism—a, ‘Memory DC’, The way anything drawn using a screen DC goes to screen,, anything drawn using a printer DC goes to a printer, similarly, anything drawn using a memory DC goes to memory (RAM). But, where in RAM—in the 1 x 1 pixel bitmap whose handle is present, in memory DC. (Note that this handle was of little use In case of, screen/printer DC). Thus if we attempt to draw a line using a, memory DC it would end up on the 1 x 1 pixel bitmap. You would, agree 1 x 1 is too small a place to draw even a small line. Hence, we need to expand the size and color capability of this bitmap., How can this be done? Simple, just replace the handle of the 1 x 1, bitmap with the handle of a bigger and colored bitmap object. This, is shown in Figure 18.7.
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604, , Let Us C, , Default Memory DC, HPEN, HBRUSH, HBITMAP, HFONT, , 200, 400, 405, 800, ., ., ., Other Info, , Black Pen, 200, , White Brush, 400, , Default O/P Device, 405, 1x1 Monochrome, bitmap, , Font, 800, , Default Drawing Objects, , Memory DC after selecting bitmap, HPEN 200, HBRUSH 400, HBITMAP 40000, HFONT 800, ., ., ., Other Info, , Black Pen, 200, New O/P Device, , White Brush, 400, Font, 800, , 40000, 190x220 24 –color, bitmap, , Figure 18.7, What purpose would just increasing the bitmap size/color would, serve? Whatever we draw here would get drawn on the bitmap but, would still not be visible. We can make it visible by simply, copying the bitmap image (including what has been drawn on it) to, the screen DC by using the API function BitBlt( )., Before transferring the image to the screen DC we need to make, the memory DC compatible with the screen DC. Here making, compatible means making certain adjustments in the contents of, the memory DC structure. Looking at these values the screen, device driver would suitably adjust the colors when the pixels in
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606, , Let Us C, , Figure 18.7, As usual we begin our drawing activity in OnPaint( ) by first, getting the screen DC using the BeginPaint( ) function. Next we, have loaded the vulture bitmap image in memory by calling the, LoadBitmap( ) function. Its usage is similar to what we saw while, creating a pattern brush in an earlier section of this chapter. Then, we have created a memory device context and made its properties, compatible with that of the screen DC. To do this we have called, the API function CreateCompatibleDC( ). Note that we have, passed the handle to the screen DC to this function. The function, in turn returns the handle to the memory DC. After this we have, selected the loaded bitmap into the memory DC. Lastly, we have, performed a bit block transfer (a bit by bit copy) from memory DC, to screen DC using the function BitBlt( ). As a result of this the, vulture now appears in the window., We have made the call to BitBlt( ) as shown below:, BitBlt ( hdc, 10, 20, 190, 220, hmemdc, 0, 0, SRCCOPY ) ;
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Chapter 18: Graphics Under Windows, , 607, , Let us now understand its parameters. These are as under:, hdc – Handle to target DC where the bitmap is to be blitted, 10, 20 – Position where the bitmap is to be blitted, 190, 220 – Width and height of bitmap being blitted, 0, 0 – Top left corner of the source image. If we give 10, 20 then, the image from 10, 20 to bottom right corner of the bitmap would, get blitted., SRCCOPY – Specifies one of the raster-operation codes. These, codes define how the color data for the source rectangle is to be, combined with the color data for the destination rectangle to, achieve the final color. SRCCOPY means that the pixel color of, source should be copied onto the destination pixel of the target., , Animation at Work, Speed is the essence of life. So having the ability to display a, bitmap in a window is fine, but if we can add movement and sound, to it then nothing like it. So let us now see how to achieve this, animation and sound effect., If we are to animate an object in the window we need to carry out, the following steps:, (a) Create an image that is to be animated as a resource., (b) Prepare the image for later display., (c) Repeatedly display this prepared image at suitable places in, the window taking care that when the next image is displayed, the previous image is erased., (d) Check for collisions while displaying the prepared image., Let us now write a program that on execution makes a red colored, ball move in the window. As the ball strikes the walls of the
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Chapter 18: Graphics Under Windows, , 611, , The WM_CREATE message arrives whenever a new window is, created. Since usually a window is created only once, the one-time, activity that is to be carried out in a program is usually done in, OnCreate( ) handler. In our program to make the ball move we, need to display it at different places at different times. To do this it, would be necessary to blit the ball image several times. However,, we need to load the image only once. As this is a one-time activity, it has been done in the handler function OnCreate( )., You are already familiar with the steps involved in preparing the, image for blitting—loading the bitmap, creating a memory DC,, making it compatible with screen DC and selecting the bitmap in, the memory DC., Apart from preparing the image for blitting we have also done, some intialialisations like setting up values in some variables to, indicate the initial position of the ball. We have also called the, SetTimer( ) function. This function tells Windows to post a, message WM_TIMER into the message queue of our application, every 50 milliseconds., , WM_TIMER and OnTimer( ), If we are to perform an activity at regular intervals we have two, choices:, (a) Use a loop and monitor within the loop when is it time to, perform that activity., (b) Use a Windows mechanism of timer. This mechanism when, used posts a WM_TIMER message at regular intervals to our, application., The first method would seriously hamper the responsiveness of the, program. If the control is within the loop and a new message, arrives the message would not get processed unless the control, goes out of the loop. The second choice is better because it makes, the program event driven. That is, whenever WM_TIMER arrives
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612, , Let Us C, , that time its handler does the job that we want to get executed, periodically. At other times the application is free to handle other, messages that come to its queue., All that we have done in the OnTimer( ) handler is erase the ball, from previous position and draw it at a new position. We have also, checked if the ball has hit the boundaries of the window. If so we, have played a sound file using the PlaySound( ) API function and, then changed the direction of the ball., , A Few More Points…, A few more points worth noting before we close our discussion on, animation…, (a) One application can set up multiple timers to do different jobs, at different intervals. Hence we need to pass the id of the, timer that we want to set up to the SetTimer( ) function. In, our case we have specified the id as 1., (b) For multiple timers Windows would post multiple, WM_TIMER messages. Each time it would pass the timer id, as additional information about the message., (c) For drawing as well as erasing the ball we have used the same, function—BitBlt( ). While erasing we have used the raster, operation code WHITENESS. When we use this code the, color values of the source pixels get ignored. Thus red colored, pixels of ball would get ignored leading to erasure of the ball, in the window., (d) The size of client area of the window can be obtained using, the GetClientRect( ) API function., (e) We want that every time we run the application the initial, position of the ball should be different. To ensure this we, have generated its initial x, y coordinates using the standard, library function rand( ). However, this function doesn’t
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Chapter 18: Graphics Under Windows, , 613, , generate true random numbers. To ensure that we do get true, random numbers, somehow we need to tie the random number, generation with time, as time of each execution of our, program would be different. This has been achieved by, making the call, srand ( time ( NULL ) ) ;, , Here time( ) is function that returns the time. We have further, passed this time to the srand( ) function., (f) To be able to use rand( ) and srand( ) functions include the, file ‘stdlib.h’. Similarly for time( ) function to work include, the file ‘time.h’., (g) In the call to the PlaySound( ) function the first parameter is, the name of the wave file that is to be played. If first, parameter is filename then the second has to be NULL. The, third parameter is a set of flags. SND_FILENAME indicates, that the first parameter is the filename, whereas, SND_ASYNC indicates that the sound should be played in, the background., (h) To be able to use the PlaySound( ) function we need to link, the library ‘winmm.lib’. This is done by using ‘Project |, Settings’ menu item. On selection of this item a dialog pops, up. In the ‘Link’ tab of this dialog mention the name, ‘winmm.lib’ in the ‘Object / Library modules’ edit box., (i) When the application terminates we have to instruct Windows, not to send WM_TIMER messages to our application any, more. For this we have called the KillTimer( ) API function, passing to it the ID of the timer., , Windows, the Endless World…, The biggest hurdle in Windows programming is a sound, understanding of its programming model. In this chapter and in the
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614, , Let Us C, , last two I have tried to catch the essence of Windows’ Event, Driven Programming model. Once you have understood it, thoroughly rest is just a matter of understanding and calling the, suitable API functions to get your job done. Windows API is truly, an endless world. It covers areas like Networking, Internet, programming, Telephony, Drawing and Printing, Device I/O,, Imaging, Messaging, Multimedia, Windowing, Database, programming, Shell programming, to name a few. The programs, that we have written have merely scratched the surface. No matter, how many programs that we write under Windows, several still, remain to be written. The intention of this chapter was to unveil, before you, to give you the first glimpse of what is possible under, Windows. The intention all along was not to catch fish for you but, to show you how to catch fish so that you can do fishing all your, life. Having made a sound beginning, rest is for you to explore., Good luck and happy fishing!, , Summary, (a) In DOS, programmers had to write separate graphics code for, every new video adapter. In Windows, the code once written, works on any video adapter., (b) A Windows program cannot draw directly on an output device, like screen or printer. Instead, it draws to the logical display, surface using device context., (c) When the window is displayed for the first time, or when it is, moved or resized OnPaint( ) handler gets called., (d) It is necessary to obtain the device context before drawing, text or graphics in the client area., (j) A device context is a structure containing information, required to draw on a display surface. The information, includes color of pen and brush, screen resolution, color, palettes, etc., (e) To draw using a new pen or brush it is necessary to select, them into the device context.
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Chapter 18: Graphics Under Windows, , 615, , (f) If we don’t select any brush or pen into the device context, then the drawing drawn in the client area would be drawn, with the default pen (black pen) and default brush (white, brush)., (g) RGB is a macro representing the Red, Green and Blue, elements of a color. RGB ( 0, 0, 0 ) gives black color,, whereas, RGB ( 255, 255, 255 ) gives white color., (h) Animation involves repeatedly drawing the same image at, successive positions., , Exercise, [A] State True or False:, , (a) Device independence means the same program is able to work, using different screens, keyboards and printers without, modifications to the program., (b) The WM_PAINT message is generated whenever the client, area of the window needs to be redrawn., (c) The API function EndPaint( ) is used to release the DC., (d) The default pen in the DC is a solid pen of white color., (e) The pen thickness for the pen style other than PS_SOLID has, to be 1 pixel., (f) BeginPaint( ) and GetDC( ) can be used interchangeably., (g) If we drag the mouse from (10, 10) to (110, 100), 100, WM_MOUSEMOVE messages would be posted into the, message queue., (h) WM_PAINT message is raised when the window contents are, scrolled., (i) With each DC a default monochrome bitmap of size 1 pixel x, 1 pixel is associated., (j) The WM_CREATE message arrives whenever a window is, displayed., [B] Answer the following:, (a) What is meant by Device Independent Drawing and how it is, achieved?
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616, , Let Us C, , (b) Explain the significance of WM_PAINT message., (c) How Windows manages the code and various resources of a, program?, (d) Explain the Windows mechanism of timer., (e) What do you mean by capturing a mouse?, (f) Write down the steps that need to be carried out to animate an, object., [C] Attempt the following:, , (a) Write a program, which displays "hello" at any place in the, window where you click the left mouse button. If you click, the right mouse button the color of subsequent hellos should, change., (b) Write a program that would draw a line by joining the new, point where you have clicked the left mouse button with the, last point where you clicked the left mouse button., (c) Write a program to gradient fill the entire client area with, shades of blue color., (d) Write a program to create chessboard like boxes (8 X 8) in the, client area. If the window is resized the boxes should also get, resized so that all the 64 boxes are visible at all times., (e) Write a program that displays only the upper half of a bitmap, of size 40 x 40., (f) Write a program that displays different text in different colors, and fonts at different places after every 10 seconds.
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19, , Interaction With, Hardware, •, •, •, •, •, •, •, •, •, •, •, •, •, , Hardware Interaction, Hardware Interaction, DOS Perspective, Hardware Interaction, Windows Perspective, Communication with Storage Devices, The ReadSector( ) Function, Accessing Other Storage Devices, Communication with Keyboard, Dynamic Linking, Windows Hooks, Caps Locked, Permanently, Did You Press It TTwwiiccee…., Mangling Keys, KeyLogger, Where is This Leading, Summary, Exercise, , 617
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618, , Let Us C, , T, , here are two types of Windows programmers those who are, happy in knowing the things the way they are under, Windows and those who wish to know why the things are, the way they are. This chapter is for the second breed of, programmers. They are the real power users of Windows. Because, it is they who first understand the default working of different, mechanisms that Windows uses and then are able to make those, mechanisms work to their advantage. The focus here would be, restricted to mechanisms that are involved in interaction with the, hardware under the Windows world. Read on and I am sure you, would be on your path to become a powerful Windows, programmer., , Hardware Interaction, Primarily interaction with hardware suggests interaction with, peripheral devices. However, its reach is not limited to interaction, with peripherals. The interaction may also involve communicating, with chips present on the motherboard. Thus more correctly,, interaction with hardware would mean interaction with any chip, other than the microprocessor. During this interaction one or more, of the following activities may be performed:, (a) Reacting to events that occur because of user’s interaction, with the hardware. For example, if the user presses a key or, clicks the mouse button then our program may do something., (b) Reacting to events that do not need explicit user’s interaction., For example, on ticking of a timer our program may want to, do something., (c) Explicit communication from a program without the, occurrence of an event. For example, a program may want to, send a character to the printer, or a program may want to, read/write the contents of a sector from the hard disk.
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Chapter 19: Interaction With Hardware, , 619, , Let us now see how this interaction is done under different, platforms., , Hardware Interaction, DOS Perspective, Under DOS whenever an external event (like pressing a key or, ticking of timer) occurs a signal called hardware interrupt gets, generated. For different events there are different interrupts. As a, reaction to the occurrence of an interrupt a table called Interrupt, Vector Table (IVT) is looked up. IVT is present in memory. It is, populated with addresses of different BIOS routines during, booting. Depending upon which interrupt has occurred the, Microprocessor picks the address of the appropriate BIOS routine, from IVT and transfers execution control to it. Once the control, reaches the BIOS routine, the code in the BIOS routine interacts, with the hardware. Naturally, for different interrupts different, BIOS routines are called. Since these routines serve the interrupts, they are often called ‘Interrupt Service Routines’ or simply ISRs., Refer Figure 19.1 to understand this mechanism.
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620, , Let Us C, , Key hit / Mouse click, generates an interrupt, Microprocessor, Microprocessor, looks up IVT, , Address, of ISR1, , Address, of ISR2, , Suitable, ISR is, called, , ISR1, , ISR2, , IVT, , BIOS Routines, , Figure 19.1, If we want that instead of the default ISR our routine should get, called then it is necessary to store the address of this routine in, IVT. Once this is done whenever a hardware interrupt occurs our, routine’s address from IVT is picked up and the control is, transferred to our routine. For example, we may register our ISR in, IVT to gain finer control over the way key-hits from the keyboard, are tackled. This finer control may involve changing codes of keys, or handling hitting of multiple keys simultaneously., Explicit communication with the hardware can be done in four, different ways. These are shown in Figure 19.2.
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Chapter 19: Interaction With Hardware, , 621, , C Program, , BIOS, Functions, , DOS, Functions, , Library, Functions, , Direct, Interaction, , Hardware, , Figure 19.2, Let us now discuss the pros and cons of using these different, methods to interact with the hardware., (a) Calling DOS Functions, To interact with the hardware a program can call DOS, functions. These functions can either directly interact with the, hardware or they may call BIOS functions which in turn, interact with the hardware. As a result, the programmer is not, required to know all the hardware details to be able to interact, with it. However, since DOS functions do not have names, they have to be called through the mechanism of interrupts., This is difficult since the programmer has to remember, interrupt service numbers for calling different DOS functions., Moreover, communication with these functions has to be done, using CPU registers. This leads to lot of difficulties since, different functions use different registers for communication., So one has to know details of different CPU registers, how to, use them, which one to use when, etc., (b) Calling BIOS Functions
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622, , Let Us C, , DOS functions can carry out jobs like console I/O, file I/O,, printing, etc. For other operations like generating graphics,, carrying out serial communication, etc. the program has to, call another set of functions called ROM-BIOS functions., Note that there are some functions in ROM-BIOS that do, same jobs as equivalent DOS functions. BIOS functions suffer, from the same difficulty as DOS functions—they do not have, names. Hence they have to be called using interrupts and, involve heavy usage of registers., (c) Calling Library Functions, We can call library functions which in turn can call, DOS/BIOS functions to carry out the interaction with, hardware. Good examples of these functions are printf( ) /, scanf( ) / getch( ) for interaction with console, absread( ) /, abswrite( ) for interaction with disk, bioscom( ) for, interaction with serial port, etc. But the library doesn’t have a, parallel function for every DOS/BIOS function. Hence at, some point of time one has to learn how to call DOS/BIOS, functions., (d) Directly interacting with the hardware, At times the programs are needed to directly interact with the, hardware. This has to be done because either there are no, library functions or DOS/BIOS functions to do this, or if they, are there their reach is limited. For example, while writing, good video games one is required to watch the status of, multiple keys simultaneously. The library functions as well as, the DOS/BIOS functions are unable to do this. At such times, we have to interact with the keyboard controller chip directly., However, direct interaction with the hardware is difficult, because one has to have good knowledge of technical details, of the chip to be able to do so. Moreover, not every technical, detail about how the hardware from a particular manufacturer, works is well documented.
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Chapter 19: Interaction With Hardware, , 623, , Hardware Interaction, Windows Perspective, Like DOS, under Windows too a hardware interrupt gets generated, whenever an external event occurs. As a reaction to this signal a, table called Interrupt Descriptor Table (IDT) is looked up and a, corresponding routine for the interrupt gets called. Unlike DOS the, IDT contains addresses of various kernel routines (instead of BIOS, routines). These routines are part of the Windows OS itself. When, the kernel routine is called, it in turn calls the ISR present in the, appropriate device driver. This ISR interacts with the hardware., Two questions may now occur to you:, (a) Why the kernel routine does not interact with the hardware, directly?, (b) Why the ISR of the device driver not registered directly in the, IDT?, Let us find answer to the first question. Every piece of hardware, works differently than the other. As new pieces of hardware come, into existence new code has to be written to be able to interact with, them. If this code is written in the kernel then the kernel would, have to be rewritten and recompiled every time a new hardware, comes into existence. This is practically impossible. Hence the, new code to interact with the device is written in a separate, program called device driver. With every new piece of hardware a, new device driver is provided. This device driver is an extension, of the OS itself., Let us now answer the second question. Out of the several, components of Windows OS a component called kernel is tightly, integrated with the processor architecture. If the processor, architecture changes then the kernel is bound to change. One of, goals of Windows NT family was to keep the other components of, OS and the device drivers portable across different microprocessor, architectures. All processor architectures may not use IDT for the, registration and lookup mechanism. So, had registration of the, device driver’s ISR in IDT been allowed, then the mechanism
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624, , Let Us C, , would fail on processors which do not use IDT, thereby, compromising portability of device drivers., Refer Figure 19.3 for understanding the interrupt handling, mechanism under Windows., , Key hit / Mouse click, generates an interrupt, Microprocessor, Microprocessor, looks up IDT, , Address, of ISR1, , Address, of ISR2, , IDT, , Suitable, Kernel, routine, is, Kernel, called, routine1, , Kernel, routine2, , Suitable, ISR is, called, ISR, Device Driver, Suitable, ISR is, called, ISR, Device Driver, , Figure 19.3, If we are to gain finer control while reacting to interrupts we, would be required to write a device driver containing a new ISR to, do so., Under Windows explicit communication with hardware is much, different than the way it was done under DOS. This is primarily, because under Windows every device is shared amongst multiple, applications running in memory. To avoid conflict between, different programs accessing the same device simultaneously
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Chapter 19: Interaction With Hardware, , 625, , Windows does not permit an application program to directly, access any of the devices. Instead it provides several API functions, to carry out the interaction. These functions have names so calling, them is much easier than calling DOS/BIOS functions. When we, call an API function to interact with a device, it in turn accesses, the device driver program for the device. It is the device driver, program that finally accesses the device. There is a standard way, in which an application can communicate with the device driver. It, is device driver’s responsibility to ensure that multiple requests, coming from different applications are handled without causing, any conflict. In the sections to follow we would see how to, communicate with the device driver to be able to interact with the, hardware., One last question—won’t the API change if a new device comes, into existence? No it won’t. That is the beauty of the Windows, architecture. All that would change is the device driver program, for the new device. The API functions that we would need to, interact with this new device driver would remain same. This is, shown in Figure 19.4, , C Program, , Windows API, , Device Driver, , Hardware, , Figure 19.4
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626, , Let Us C, , Communication with Storage Devices, Since DOS is commercially dead the rest of the chapter would, focus on communication with the devices under Windows, platform. We would illustrate this with the help of several, programs., Let us begin with the one that interacts with the simplest storage, device, namely the floppy disk. Rather than the physical structure, of the floppy disk it is the way the stored information is laid out, and managed that concerns programmers most. Let us understand, how the information is laid out on a floppy disk. Each floppy disk, consists of four logical parts—Boot Sector, File Allocation Table, (FAT), Directory and Data space. Of these, the Boot Sector, contains information about how the disk is organized. That is, how, many sides does it contain, how many tracks are there on each, side, how many sectors are there per track, how many bytes are, there per sector, etc. The files and the directories are stored in the, Data Space. The Directory contains information about the files like, its attributes, name, size, etc. The FAT contains information about, where the files and directories are stored in the data space. Figure, 19.5 shows the four logical parts of a 1.44 MB disk.
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627, , Chapter 19: Interaction With Hardware, , 13, 12, 11, 10, , F2, F2, F1, , 14, , F2 F2 F2, DS, , 13, , 15, 16, , 8, , BS 1, F1, F1, F1, 2, F1 F1 F1, 3, 7, 4, 6, 5, Side 0, Track 0, BS - Boot Sector, F2 - Second copy of FAT, DS - Data space, , 14, , 15, , 16, DS, 17, DS, DS, F2 18, D 1, 9 D, D, D, 8, D, D, 2, D D D, 3, 7, 4, 6, 5, , D, F2, 11, 17, F2, D, F2 18 10 D, , F1, F1, , 9, , 12, , D D DS, , Side 0, Track 1, F1 - First copy of FAT, D - Root directory structure, , Figure 19.5, With the logical structure of the floppy disk behind us let us now, write a program that reads the boot sector of a floppy disk and, displays its contents on the screen. But why on earth would we, ever like to do this? Well, that’s what all Windows-based Antiviral softwares do when they scan for boot sector viruses. A good, enough reason for us to add the capability to read a boot sector to, our knowledge! Here is the program…, # include <stdafx.h>, # include <windows.h>, # include <stdio.h>, # include <conio.h>, # pragma pack ( 1 ), struct boot, {, BYTE jump [ 3 ] ;
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Chapter 19: Interaction With Hardware, , 629, , }, , The boot sector contains two parts—‘Boot Parameters’ and ‘Disk, Bootstrap Program’. The Boot Parameters are useful while, performing read/write operations on the disk. Figure 19.6 shows, the break up of the boot parameters for a floppy disk., , Description, , Length, , Typical Values, , Jump instruction, , 3, , EB3C90, , OEM name, , 8, , MSWIN4.1, , Bytes per sector, , 2, , 512, , Sectors per cluster, , 1, , 1, , Reserved sectors, , 2, , 1, , Number of FAT copies, , 1, , 2, , Max. Root directory entries, , 2, , 224, , Total sectors, , 2, , 2880, , Media descriptor, , 1, , F0, , Sectors per FAT, , 2, , 9, , Sectors per track, , 2, , 18, , No. Of sides, , 2, , 2, , Hidden sectors, , 4, , 0, , Huge sectors, , 4, , 0, , BIOS drive number, , 1, , 0, , Reserved sectors, , 1, , 0, , Boot signature, , 1, , 41, , Volume ID, , 4, , 349778522, , Volume label, , 11, , ICIT, , File system type, , 8, , FAT12, , Figure 19.6
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630, , Let Us C, , Using the breakup of bytes shown in Figure 19.6 our program has, first created a structure called boot. Notice the usage of #pragma, pack to ensure that all elements of the structure are aligned on a 1byte boundary, rather than the default 4-byte boundary. Then, comes the surprise—there is no WinMain( ) in the program. This, is because we want to display the boot sector contents on the, screen rather than in a window. This has been done only for the, sake of simplicity. Remember that our aim is to interact with the, floppy, and not in drawing and painting in a window. If you wish, you can of course adapt this program to display the same contents, in a window. So the program is still a Windows application. Only, difference is that it is built as a ‘Win32 Console Application’ using, VC++. A console application always begins with main( ) rather, than WinMain( )., To actually read the contents of boot sector of the floppy disk the, program makes a call to a user-defined function called, ReadSector( ). The ReadSector( ) function is quite similar to the, absread( ) library function available in Turbo C/C++ under DOS., The first parameter passed to ReadSector( ) is a string that, indicates the storage device from where the reading has to take, place. The syntax for this string is \\machine-name\storagedevice name. In \\.\\A:, we have used ‘.’ for the machine name. A, ‘.’ means the same machine on which the program is executing., Needless to say, A: refers to the floppy drive. The second, parameter is the logical sector number. We have specified this as 0, which means the boot sector in case of a floppy disk. The third, parameter is the number of sectors that we wish to read. This, parameter is specified as 1 since the boot sector occupies only a, single sector. The last parameter is the address of a buffer/variable, that would collect the data that is read from the floppy. Here we, have passed the address of the boot structure variable b. As a, result, the structure variable would be setup with the contents of, the boot sector data at the end of the function call.
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Chapter 19: Interaction With Hardware, , 631, , Once the contents of the boot sector have been read into the, structure variable b we have displayed the first few of them on the, screen using printf( ). If you wish you can print the rest of the, contents as well., , The ReadSector( ) Function, With the preliminaries over let us now concentrate on the real stuff, in this program, i.e. the ReadSector( ) function. This function, begins by making a call to the CreateFile( ) API function as, shown below:, h = CreateFile ( src, GENERIC_READ,, FILE_SHARE_READ, 0, OPEN_EXISTING, 0, 0 ) ;, , The CreateFile( ) API function is very versatile. Anytime we are, to communicate with a device we have to firstly call this API, function. The CreateFile( ) function opens the specified device as, a file. Windows treats all devices just like files on disk. Reading, from this file means reading from the device., The CreateFile( ) API function takes several parameters. The first, parameter is the string specifying the device to be opened. The, second parameter is a set of flags that are used to specify the, desired access to the file (representing the device) about to be, opened. By specifying the GENERIC_READ flag we have, indicated that we just wish to read from the file (device). The third, parameter specifies the sharing access for the file (device). Since, floppy drive is a shared resource across all the running, applications we have specified the FILE_SHARE_READ flag. In, general while interacting with any hardware the sharing flag for, the file (device) must always be set to this value since every piece, of hardware is shared amongst all the running applications. The, fourth parameter indicates security access for the file (device)., Since we are not concerned with security here we have specified, the value as 0. The fifth parameter specifies what action to take if
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632, , Let Us C, , the file already exists. When using CreateFile( ) for device access, we must always specify this parameter as OPEN_EXISTING., Since the floppy disk file was already opened by the OS a long, time back during the booting. The remaining two parameters are, not used when using CreateFile( ) API function for device access., Hence we have passed a 0 value for them. If the call to, CreateFile( ) succeeds then we obtain a handle to the file (device)., The device file mechanism allows us to read from the file (device), by setting the file pointer using the SetFilePointer( ) API function, and then reading the file using the ReadFile( ) API function. Since, every sector is 512 bytes long, to read from the nth sector we need, to set the file pointer to the 512 * n bytes from the start of the file., The first parameter to SetFilePointer( ) is the handle of the device, file that we obtained by calling the CreateFile( ) function. The, second parameter is the byte offset from where the reading is to, begin. This second parameter is relative to the third parameter. We, have specified the third parameter as FILE_BEGIN which means, the byte offset is relative to the start of the file., To actually read from the device file we have made a call to the, ReadFile( ) API function. The ReadFile( ) function is very easy to, use. The first parameter is the handle of the file (device), the, second parameter is the address of a buffer where the read contents, should be dumped. The third parameter is the count of bytes that, have to be read. We have specified the value as 512 * num so as to, read num sectors. The fourth parameter to ReadFile( ) is the, address of an unsigned int variable which is set up with the count, of bytes that the function was successfully able to read. Lastly,, once our work with the device is over we should close the file, (device) using the CloseHandle( ) API function., Though ReadSector( ) doesn’t need it, there does exist a, counterpart of the ReadFile( ) function. Its name is WriteFile( )., This API function can be used to write to the file (device). The, parameters of WriteFile( ) are same as that of ReadFile( ). Note
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Chapter 19: Interaction With Hardware, , 633, , that when WriteFile( ) is to be used we need to specify the, GENERIC_WRITE flag in the call to CreateFile( ) API, function. Given below is the code of WriteSector( ) function that, works exactly opposite to the ReadSector( ) function., void WriteSector ( char *src, int ss, int num, void* buff ), {, HANDLE h ;, unsigned int br ;, h = CreateFile ( src, GENERIC_WRITE,, FILE_SHARE_WRITE, 0, OPEN_EXISTING, 0, 0 ) ;, SetFilePointer ( h, ( ss * 512 ), NULL, FILE_BEGIN ) ;, WriteFile ( h, buff, 512 * num, &br, NULL ) ), CloseHandle ( h ) ;, }, , Accessing Other Storage Devices, Note that the mechanism of reading from or writing to any device, remains standard under Windows. We simply need to change the, string that specifies the device. Here are some sample calls for, reading/writing from/to various devices:, ReadSector ( "\\\\.\\a:", 0, 1, &b ) ; /* reading from 2nd floppy drive */, ReadSector ( "\\\\.\\d:", 0, 1, buffer ) ; /* reading from a CD-ROM drive */, WriteSector ( "\\\\.\\c:", 0, 1, &b ) ; /* writing to a hard disk */, ReadSector ( "\\\\.\\physicaldrive0", 0, 1, &b ) ; /* reading partition table */, , Here are a few interesting points that you must note., (a) If we are to read from the second floppy drive we should, replace A: with B: while calling ReadSector( )., (b) To read from storage devices like hard disk drive or CD-ROM, or ZIP drive, etc. use the string with appropriate drive letter., The string can be in the range \\.\C: to \\.\Z:.
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634, , Let Us C, , (c) To read from the CD-ROM just specify the drive letter of the, drive. Note that CD-ROMs follow a different storage, organization known as CD File System (CDFS)., (d) The hard disk is often divided into multiple partitions. Details, like the place at which each partition begins and ends, the size, of each partition, whether it is a bootable partition or not, etc., are stored in a table on the disk. This table is often called, ‘Partition Table’. If we are to read the partition table contents, we can do so by using the string \\.\physicaldrive0., (e) Using \\.\physicaldrive0 we can also read contents of any, other parts of the disk. Here 0 represents the first hard disk in, the system. If we are to read from the second hard disk we, need to use 1 in place of 0., , Communication with Keyboard, Like mouse messages there also exist messages for keyboard., These are WM_KEYDOWN, WM_KEYUP and WM_CHAR., Of these, WM_KEYDOWN and WM_KEYUP are sent to an, application (which has the input focus) whenever the key is, pressed and released respectively. The additional information in, case of these messages is the code of the key being pressed or, released. When we tackle WM_KEYDOWN or WM_KEYUP, we need to ourselves check the status of toggle keys like NumLock, and CapsLock and shift keys like Ctrl, Alt and Shift. If we wish to, avoid all this checking we can tackle the WM_CHAR message, instead., What is mentioned above is the normal procedure followed by, most Windows applications. However, if we wish to go a step, further and deal with the keyboard we need to tackle it differently., For example, suppose we are to perform one of the following jobs:, (a) Once you hit any key CapsLock should become on. Once it, becomes on it should remain permanently on.
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Chapter 19: Interaction With Hardware, , 635, , (b) If we hit a key once it should appear twice on the screen., (c) If we hit a key A then B should appear on the screen, if we hit, a B then C should occur and so on., Note that all these effects should work on a system-wide basis for, all Win32 applications. To be able to achieve these effect we need, understand two important mechanisms—‘Dynamic Linking’ and, ‘Windows Hooks’. Let us understand these mechanisms one by, one., , Dynamic Linking, As we saw in Chapter 16, Windows permits linking of libraries, stored in a .DLL file during execution. A .DLL file is a binary file, that cannot execute on its own. It contains functions that can be, shared between several applications running in memory., , Windows Hooks, As the name suggests, the hook mechanism permits us to intercept, and alter the flow of messages in the OS before they reach the, application. Since hooks are used to alter the messaging, mechanism on a system-wide basis the code for hooking has to be, written in a DLL. The hooking mechanism involves writing a hook, procedure in a DLL file and registering this procedure with the, OS. Since the DLL cannot execute on its own we need a separate, program that would load and execute the DLL., For different messages there are different types of hooks. For, example, for keyboard messages there is a keyboard hook, for, mouse messages there is mouse hook, etc. You can refer MSDN, for nearly a dozen more types of hooks. Here we would restrict our, discussion only to the keyboard hook.
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636, , Let Us C, , Before we proceed to write our own hook procedure let us, understand the normal working of the keyboard messages. This is, illustrated in Figure 19.7., , Interrupt, Kernel Routine, , Obtain key code by, interacting with, KB controller, , Device Driver ISR, Place key code in System, Msg. queue by calling, keybd_event( ), , System Msg. queue, , OS, , Application1, , Application1, Msg. Queue, , Application2, Msg. Queue, , Application2, , Figure 19.7, With reference to Figure 19.7 here is a list of steps that are carried, out when we press a key from the keyboard:
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Chapter 19: Interaction With Hardware, , 637, , (a) On pressing a key an interrupt occurs and the corresponding, kernel routine gets called., (b) The kernel routine calls the ISR of the keyboard device driver., (c) The ISR communicates with the keyboard controller and, obtains the code of the key pressed., (d) The ISR calls a OS function keybd_event( ) to post the key, code to the System Message Queue., (e) The OS retrieves the message from the System Message, Queue and posts it into the message queue of the application, with regard to which the key has been pressed., Let us now see what needs to be done if we are to alter this, procedure. We simply need to register our hook procedure with the, OS. As a result, our hook procedure would receive the message, before it is dispatched to the appropriate Application Message, Queue. Since our hook procedure gets a first shot at the message it, can now alter the working in the following three ways:, (a) It can suppress the message altogether, (b) It can change the message, (c) It can post more messages into the System Message Queue, using the keybd_event( ) function., Let us now put all this theory into practice by writing a few, programs., , Caps Locked, Permanently, Let us now write a program that keeps the CapsLock permanently, on. This effect would come into being when the first key is hit, subsequent to the execution of our program. In fact there would be, two programs:, (a) A DLL containing a hook procedure that achieves the, CapsLock effect., (b) An application EXE which loads the DLL in memory., Given below is the source code of the DLL program.
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Chapter 19: Interaction With Hardware, , 639, , {, keybd_event ( VK_CAPITAL , 0,, KEYEVENTF_EXTENDEDKEY, 0 ) ;, keybd_event ( VK_CAPITAL , 0,, KEYEVENTF_EXTENDEDKEY | KEYEVENTF_KEYUP, 0 ) ;, }, }, return CallNextHookEx ( hkb, nCode, wParam, lParam ) ;, }, BOOL __declspec ( dllexport ) removehook( ), {, return UnhookWindowsHookEx ( hkb ) ;, }, , Follow the steps mentioned below to create this program:, (a) Select ‘File | New’ option to start a new project in VC++., (b) From the ‘Project’ tab select ‘Win32 Dynamic-Link Library’, and click on the ‘Next’ button., (c) In the ‘Win32 Dynamic-link Library Step 1 of 1’ select “An, empty DLL project” and click on the ‘Finish’ button., (d) Select ‘File | New’ option., (e) From the ‘File’ tab select ‘C++ source file’ and give the file, name as ‘hook.c’. Type the code listed above in this file., (f) Compile the program to generate the .DLL file., Note that this program doesn’t contain WinMain( ) since the, program on compilation should not execute on its own. It has been, replaced by a function called DllMain( ). This function acts as, entry point of the DLL program. It gets called when the DLL is, loaded or unloaded., When the application loads the DLL the DllMain( ) function, would be called. In this function we have merely stored the handle, to the DLL that has been loaded in memory into a global variable, h for later use.
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Chapter 19: Interaction With Hardware, , 641, , As we know, the OnCreate( ) and OnDestroy( ) handlers would, be called when the WM_CREATE and WM_DESTROY, messages arrive respectively. In OnCreate( ) we have loaded the, DLL containing the hook procedure. To do this we have called the, LoadLibrary( ) API function. Once the DLL is loaded we have, obtained the address of the exported function installhook( ) using, the GetProcAddress( ) API function. The returned address is, stored in p, where p is a pointer to the installhook( ) function., Using this pointer we have then called the installhook( ) function., In the installhook( ) function we have called the API function, SetWindowsHookEx( ) to register our hook procedure with the, OS as shown below:, hkb = SetWindowsHookEx ( WH_KEYBOARD,, ( HOOKPROC ) KeyboardProc, ( HINSTANCE ) h, 0 ) ;, , Here the first parameter is the type of hook that we wish to, register, whereas the second parameter is the address of our hook, procedure KeyboardProc( ). hkb stores the handle of the hook, installed., From now on whenever a keyboard message is retrieved by the OS, from the System Message Queue the message is firstly passed to, our hook procedure, i.e. to KeyboardProc( ) function. Inside this, function we have written code to ensure that the CapsLock always, remains on. To begin with we have checked whether nCode, parameter is less than 0. If it so then it necessary to call the next, hook procedure. The MSDN documentation suggests that “if code, is less than zero, the hook procedure must pass the message to the, CallNextHookEx( ) function without further processing and, should return the value returned by CallNextHookEx( )”., Note that there can be several hook procedures installed by, different programs, thus forming a chain of hook procedures., These hook procedures always get called in an order that is
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642, , Let Us C, , opposite to their order of installation. This means the last hook, procedure installed is the first one to get called., If the nCode parameter contains a value HC_ACTION it means, that the message that was just removed form the system message, queue was a keyboard message. If it is so, then we have checked, the previous state of the key before the message was sent. If the, state of the key was ‘depressed’ (30th bit of lParam is 1) then we, have obtained the state of the CapsLock key by calling the, GetKeyState( ) API function. If it is off (0th bit of state variable is, 0) then we have turned on the CapsLock by simulating a keypress., For this simulation we have called the function keybd_event( ), twice—first call is for pressing the CapsLock and second is for, releasing it. Note that keybd_event( ) creates a keyboard message, from the parameters that we pass to it and posts it into the system, message queue. The parameter VK_CAPITAL represents the code, for the CapsLock key., A word of caution! When we use keybd_event( ) to post keyboard, message for a simulated CapsLock keypress, once again our hook, procedure would be called when these messages are retrieved from, the system message queue. But this time the CapsLock would be, on so we would end up passing control to the next hook procedure, through a call to CallNextHookEx( )., When we close the application window as usual the OnDestroy( ), would be called. In this handler we have obtained the address of, the removehook( ) exported function and called it. In the, removehook( ) function we have unregistered our hook procedure, by calling the UnhookWindowsHookEx( ) API function. Note, that to this function we have passed the handle to our hook. As a, result our hook procedure is now removed from the hook chain., Hereafter the CapsLock would behave normally. Having unhooked, our hook procedure the control would return to OnDestroy( ), handler where we have promptly unload the DLL from memory by, calling the FreeLibrary( ) API function.
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644, , Let Us C, , }, , In this hook procedure once again we have checked if the nCode, parameter contains a value HC_ACTION. If it does then we have, checked the present state of the key in question. If the present state, of the key is ‘pressed’ (31th bit of lParam is 0) then we have, posted the message for the same key into the system message, queue by calling the keybd_event( ). However, this may lead to a, serious problem. Can you imagine which? The message that we, post, once retrieved, would again bring the control to our hook, procedure. Once again the conditions would become true and we, would post the same message again. This would go on and on., This can be prevented by using a using a simple flag variable as, shown in the code., Note that the rest of the functions in the DLL file are exactly same, as in the previous program. So also is the application program., , Mangling Keys, How about one more program to bolster your confidence? Let us, try one that would mangle every key that is pressed. That is,, convert an A to a B, B to C, C to D, etc. This would be fairly, straight-forward. We simply have to increment the key code before, posting it into the system message queue. Also, further processing, of key has to be prevented. This can be achieved by simply, returning a non-zero value from the hook procedure (thus, bypassing the call to CallNextHookEx( )). This is shown in the, following hook procedure., LRESULT __declspec ( dllexport ) __stdcall KeyboardProc ( int nCode,, WPARAM wParam, LPARAM lParam ), {, static BYTE key ;, static BOOL flag = FALSE ;
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Chapter 19: Interaction With Hardware, , 645, , if ( nCode < 0 ), return CallNextHookEx ( hkb, nCode, wParam, lParam ) ;, if ( ( nCode == HC_ACTION ) &&, ( ( DWORD ) lParam & 0x80000000 ) == 0 ), {, if ( flag == FALSE ), {, key = wParam ;, key ++ ;, keybd_event ( key , 0, KEYEVENTF_EXTENDEDKEY, 0 ) ;, flag = TRUE ;, return 1 ;, }, else, {, if ( key == ( BYTE ) wParam ), flag = FALSE ;, }, }, return CallNextHookEx ( hkb, nCode, wParam, lParam ) ;, }, , KeyLogger, There are several malicious programs that are floating on the net, that steal away your passwords. These programs keep a log of, every key that is pressed while entering passwords or credit card, numbers. These programs make use of windows hooks to trap, every key that is pressed. With the knowledge that you have, gained from the past three programs this may not be a big deal., However, such key logger programs deviate from the ones that we, developed in three fundamental ways:, (a) They do not pop any window on the screen; otherwise the, program’s presence would get detected.
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646, , Let Us C, , (b) These programs also hide themselves from the Task Manager, so that the user cannot terminate them., (c) The logged keys are secretly sent over the net to the malicious, users who write such programs. Once the logged keys are, known it would be possible to break into the system., , Where is This Leading, Even for a moment do not create an impression in you mind that, Windows Hooks are only for notorious activities. There are many, good things that they can be put to use for. These activities, include:, (a) Multimedia keyboards have special key like Cut, Copy, Paste,, etc. Such keyboards also come with special programs which, when installed know how to tackle these special keys. On, pressing these keys these programs use the hook mechanism, to place the simulated keys in the system message queue., (b) Many demo programs once executed automatically move the, mouse pointer to a menu or a toolbar or any such item to, demonstrate some feature of the software. To manage these, actions a windows hook called Journal hook is used., (c) For physically impaired persons a keyboard can be simulated, on the screen and the mouse clicks on this keyboard can be, communicated to Windows as actual key hits. This again can, be achieved using mouse and keyboard hook., There can be many more such examples. But the above three I, believe would be ample to prove to you the constructive side of the, powerful mechanism called Windows Hooks.
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Chapter 19: Interaction With Hardware, , 647, , Summary, (a) Hardware interaction can happen in two ways: (1) When the, user interacts with the hardware and the program reacts to it., (2) When the program interacts with the hardware without any, user intervention., (b) In DOS when the user interacts with the hardware an ISR gets, called which interacts with the hardware. In Windows the, same thing is done by the device driver’s ISR., (c) In DOS when the program has to interact with the hardware it, can do so by using library functions, DOS/BIOS routines or, by directly interacting with the hardware. In Windows the, same thing can be done by using API functions., (d) Under Windows to gain finer control over the hardware we, are required to write a device driver program., (e) Interaction with the any device can be done using API, functions like CreateFile( ), ReadFile( ), WriteFile( ) and, CloseHandle( )., (f) Different strings have to be passed to the CreateFile( ), functions for interacting with different devices., (g) Windows provides a powerful mechanism called hooks that, can alter the flow of messages before they reach the, application., (h) Windows hook procedures should be written in a DLL since, they work on a system wide basis., (i) Windows hooks can be put to many good uses., , Exercise, [A] State True or False:, , (a) In MS-DOS on occurrence of an interrupt values from IDT, are used to call the appropriate kernel routine., (b) Under Windows on occurrence of an interrupt the kernel, routine calls the appropriate device driver’s ISR., (c) Under Windows an application can interact with the hardware, by directly calling its device driver’s routines.
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648, , Let Us C, , (d) Under Windows we can write device drivers to extend the OS, itself., (e) ReadSector( ) and WriteSector( ) are API functions., (f) While reading a sector from the disk the CreateFile( ), function creates a file on the disk., (g) The Windows API function to stop communication with a, device is CloseFile( )., (h) The ReadFile( ) and WriteFile( ) API functions can only, perform reading or writing from/to a disk file., [B] Answer the following:, , (a) How is hardware interaction under Windows different that, that under DOS?, (b) What is the advantage of writing code in a DLL?, (c) Explain the Windows hooks mechanism., (d) What is the standard way of communicating with a device, under Windows?, (e) Write a program to read the contents of Boot Sector of a 32bit FAT file system and print them on the screen. Refer, Appendix G for details about the contents of the boot sector., (f) Write a program that ensures that the key ‘A’ is completely, disabled across all applications., (g) Write a program that closes any window just by placing the, cursor on the ‘Close’ button in the title bar of it.
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20, , C Under Linux, •, •, •, •, •, •, •, •, •, •, , What is Linux, C Programming Under Linux, The ‘Hello Linux’ Program, Processes, Parent and Child Processes, More Processes, Zombies and Orphans, One Interesting Fact, Summary, Exercise, , 649
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650, , Let Us C, , T, , oday the programming world is divided into two major, camps—the Windows world and the Linux world. Since its, humble beginning about a decade ago, Linux has steadily, drawn the attention of programmers across the globe and has, successfully created a community of its own. How big and, committed is this community is one of the hottest debates that is, raging in all parts of the world. You can look at the hot discussions, and the flame wars on this issue on numerous sites on the internet., Before you decide to join the Windows or the Linux camp you, should first get familiar with both of them. The last 4 chapters, concentrated on Windows programming. This and the next one, would deal with Linux programming. Without any further, discussions let us now set out on the Linux voyage. I hope you, find the journey interesting and exciting., , What is Linux, Linux is a clone of the Unix operating system. Its kernel was, written from scratch by Linus Torvalds with assistance from a, loosely-knit team of programmers across the world on Internet. It, has all the features you would expect in a modern OS. Moreover,, unlike Windows or Unix, Linux is available completely free of, cost. The kernel of Linux is available in source code form., Anybody is free to change it to suit his requirement, with a, precondition that the changed kernel can be distributed only in the, source code form. Several programs, frameworks, utilities have, been built around the Linux kernel. A common user may not want, the headaches of downloading the kernel, going through the, complicated compilation process, then downloading the, frameworks, programs and utilities. Hence many organizations, have come forward to make this job easy. They distribute the, precompiled kernel, programs, utilities and frameworks on a, common media. Moreover, they also provide installation scripts, for easy installations of the Linux OS and applications. Some of, the popular distributions are RedHat, SUSE, Caldera, Debian,, Mandrake, Slackware, etc. Each of them contain the same kernel
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Chapter 19: Interaction With Hardware, , 651, , but may contain different application programs, libraries,, frameworks, installation scripts, utilities, etc. Which one is better, than the other is only a matter of taste., Linux was first developed for x86-based PCs (386 or higher)., These days it also runs on Compaq Alpha AXP, Sun SPARC,, Motorola 68000 machines (like Atari ST and Amiga), MIPS,, PowerPC, ARM, Intel Itanium, SuperH, etc. Thus Linux works on, literally every conceivable microprocessor architecture., Under Linux one is faced with simply too many choices of Linux, distributions, graphical shells and managers, editors, compilers,, linkers, debuggers, etc. For simplicity (in my opinion) I have, chosen the following combination:, Linux Distribution - Red Hat Linux 9.0, Console Shell, - BASH, Graphical Shell, - KDE 3.1-10, Editor, - KWrite, Compiler, - GNU C and C++ compiler (gcc), We would be using and discussing these in the sections to follow., , C Programming Under Linux, How is C under Linux any different than C under DOS or C under, Windows? Well, it is same as well as different. It is same to the, extent of using language elements like data types, control, instructions and the overall syntax. The usage of standard library, functions is also same even though the implementation of each, might be different under different OS. For example, a printf( ), would work under all OSs, but the way it is defined is likely to be, different for different OSs. The programmer however doesn’t, suffer because of this since he can continue to call printf( ) the, same way no matter how it is implemented.
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652, , Let Us C, , But there the similarity ends. If we are to build programs that, utilize the features offered by the OS then things are bound to be, different across OSs. For example, if we are to write a C program, that would create a Window and display a message “hello” at the, point where the user clicks the left mouse button. The architecture, of this program would be very closely tied with the OS under, which it is being built. This is because the mechanisms for creating, a window, reporting a mouse click, handling a mouse click,, displaying the message, closing the window, etc. are very closely, tied with the OS for which the program is being built. In short the, programming architecture (better known as programming model), for each OS is different. Hence naturally the program that achieves, the same task under different OS would have to be different., , The ‘Hello Linux’ Program, As with any new platform we would begin our journey in the, Linux world by creating a ‘hello world’ program. Here is the, source code...., int main( ), {, printf ( "Hello Linux\n" ) ;, return 0 ;, }, , The program is exactly same as compared to a console program, under DOS/Windows. It begins with main( ) and uses printf( ), standard library function to produce its output. So what is the, difference? The difference is in the way programs are typed,, compiled and executed. The steps for typing, compiling and, executing the program are discussed below., The first hurdle to cross is the typing of this program. Though any, editor can be used to do so, we have preferred to use the editor, called ‘KWrite’. This is because it is a very simple yet elegant
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Chapter 19: Interaction With Hardware, , 653, , editor compared to other editors like ‘vi’ or ‘emacs’. Note that, KWrite is a text editor and is a part of K Desktop environment, (KDE). Installation of Linux and KDE is discussed in Appendix H., Once KDE is started select the following command from the, desktop panel to start KWrite:, K Menu | Accessories | More Accessories | KWrite, If you face any difficulty in starting the KWrite editor please refer, Appendix H. Assuming that you have been able to start KWrite, successfully, carry out the following steps:, (a) Type the program and save it under the name ‘hello.c’., (b) At the command prompt switch to the directory containing, ‘hello.c’ using the cd command., (c) Now compile the program using the gcc compiler as shown, below:, # gcc hello.c, , (d) On successful compilation gcc produces a file named ‘a.out’., This file contains the machine code of the program which can, now be executed., (e) Execute the program using the following command., # ./a.out, , (f) Now you should be able to see the output ‘Hello Linux’ on, the screen., Having created a Hello Linux program and gone through the editcompile-execute cycle once let us now turn our attention to Linux, specific programming. We will begin with processes., , Processes, Gone are the days when only one job (task) could be executed in, memory at any time. Today the modern OSs like Windows and
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654, , Let Us C, , Linux permit execution of several tasks simultaneously. Hence, these OSs are aptly called ‘Multitasking’ OSs., In Linux each running task is known as a ‘process’. Even though it, may appear that several processes are being executed by the, microprocessor simultaneously, in actuality it is not so. What, happens is that the microprocessor divides the execution time, equally among all the running processes. Thus each process gets, the microprocessor’s attention in a round robin manner. Once the, time-slice allocated for a process expires the operation that it is, currently executing is put on hold and the microprocessor now, directs its attention to the next process. Thus at any given moment, if we take the snapshot of memory only one process is being, executed by the microprocessor. The switching of processes, happens so fast that we get a false impression that the processor is, executing several processes simultaneously., The scheduling of processes is done by a program called, ‘Scheduler’ which is a vital component of the Linux OS. This, scheduler program is fairly complex. Before switching over to the, next thread it stores the information about the current process. This, includes current values of CPU registers, contents of System Stack, and Application Stack, etc. When this process again gets the time, slot these values are restored. This process of shifting over from, one thread to another is often called a Context Switch. Note that, Linux uses preemptive scheduling, meaning thereby that the, context switch is performed as soon as the time slot allocated to, the process is over, no matter whether the process has completed, its job or not., Kernel assigns each process running in memory a unique ID to, distinguish it from other running processes. This ID is often known, as processes ID or simply PID. It is very simple to print the PID of, a running process programmatically. Here is the program that, achieves this…
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Chapter 19: Interaction With Hardware, , 655, , int main( ), {, printf ( "Process ID = %d", getpid( ) ) ;, }, , Here getpid( ) is a library function which returns the process ID, of the calling process. When the execution of the program comes, to an end the process stands terminated. Every time we run the, program a new process is created. Hence the kernel assigns a new, ID to the process each time. This can be verified by executing the, program several times—each time it would produce a different, output., , Parent and Child Processes, As we know, our running program is a process. From this process, we can create another process. There is a parent-child relationship, between the two processes. The way to achieve this is by using a, library function called fork( ). This function splits the running, process into two processes, the existing one is known as parent and, the new process is known as child. Here is a program that, demonstrates this…, # include <sys/types.h>, int main( ), {, printf ( "Before Forking\n" ) ;, fork( ) ;, printf ( "After Forking\n" ) ;, }, , Here is the output of the program…, Before Forking, After Forking, After Forking
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656, , Let Us C, , Watch the output of the program. You can notice that all the, statements after the fork( ) are executed twice—once by the parent, process and second time by the child process. In other words, fork( ) has managed to split our process into two., But why on earth would we like to do this? At times we want our, program to perform two jobs simultaneously. Since these jobs may, be inter-related we may not want to create two different programs, to perform them. Let me give you an example. Suppose we want, perform two jobs—copy contents of source file to target file and, display an animated GIF file indicating that the file copy is in, progress. The GIF file should continue to play till file copy is, taking place. Once the copying is over the playing of the GIF file, should be stopped. Since both these jobs are inter-related they, cannot be performed in two different programs. Also, they cannot, be performed one after another. Both jobs should be performed, simultaneously., At such times we would want to use fork( ) to create a child, process and then write the program in such a manner that file copy, is done by the parent and displaying of animated GIF file is done, by the child process. The following program shows how this can, be achieved. Note that the issue here is to show how to perform, two different but inter-related jobs simultaneously. Hence I have, skipped the actual code for file copying and playing the animated, GIF file., # include <sys/types.h>, int main( ), {, int pid ;, pid = fork( ) ;, if ( pid == 0 ), {, printf ( "In child process\n" ) ;, /* code to play animated GIF file */
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Chapter 19: Interaction With Hardware, , 657, , }, else, {, printf ( "In parent process\n" ) ;, /* code to copy file */, }, }, , As we know, fork( ) creates a child process and duplicates the, code of the parent process in the child process. There onwards the, execution of the fork( ) function continues in both the processes., Thus the duplication code inside fork( ) is executed once, whereas, the remaining code inside it is executed in both the parent as well, as the child process. Hence control would come back from fork( ), twice, even though it is actually called only once. When control, returns from fork( ) of the parent process it returns the PID of the, child process, whereas when control returns from fork( ) of the, child process it always returns a 0. This can be exploited by our, program to segregate the code that we want to execute in the, parent process from the code that we want to execute in the child, process. We have done this in our program using an if statement., In the parent process the ‘else block’ would get executed, whereas, in the child process the ‘if block’ would get executed., Let us now write one more program. This program would use the, fork( ) call to create a child process. In the child process we would, print the PID of child and its parent, whereas in the parent process, we would print the PID of the parent and its child. Here is the, program…, # include <sys/types.h>, int main( ), {, int pid ;, pid = fork( ) ;, if ( pid == 0 )
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658, , Let Us C, , {, printf ( "Child : Hello I am the child process\n" ) ;, printf ( "Child : Child’s PID: %d\n", getpid( ) ) ;, printf ( "Child : Parent’s PID: %d\n”, getppid( ) ) ;, }, else, {, printf ( "Parent : Hello I am the parent process\n" ) ;, printf ( "Parent : Parent’s PID: %d\n”, getpid( ) ) ;, printf ( "Parent : Child’s PID: %d\n", pid ) ;, }, }, , Given below is the output of the program:, Child : Hello I am the child process, Child : Child's PID: 4706, Child : Parent's PID: 4705, Parent : Hello I am the Parent process, Parent : Parent's PID: 4705, Parent : Child's PID: 4706, , In addition to getpid( ) there is another related function that we, have used in this program—getppid( ). As the name suggests, this, function returns the PID of the parent of the calling process., You can tally the PIDs from the output and convince yourself that, you have understood the fork( ) function well. A lot of things that, follow use the fork( ) function. So make sure that you understand, it thoroughly., Note that even Linux internally uses fork( ) to create new child, processes. Thus there is a inverted tree like structure of all the, processes running in memory. The father of all these processes is a, process called init. If we want to get a list of all the running, processes in memory we can do so using the ps command as, shown below.
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Chapter 19: Interaction With Hardware, , 659, , # ps –A, , Here the switch –A indicates that we want to list all the running, processes., , More Processes, Suppose we want to execute a program on the disk as part of a, child process. For this first we should create a child process using, fork( ) and then from within the child process we should call an, exec function to execute the program on the disk as part of a child, process. Note that there is a family of exec library functions, each, basically does the same job but with a minor variation. For, example, execl( ) function permits us to pass a list of command, line arguments to the program to be executed. execv( ) also does, the same job as execl( ) except that the command line arguments, can be passed to it in the form of an array of pointers to strings., There also exist other variations like execle( ) and execvp( )., Let us now see a program that uses execl( ) to run a new program, in the child process., # include <unistd.h>, int main( ), {, int pid ;, pid = fork( ) ;, if ( pid == 0 ), {, execl ( "/bin/ls","-al", "/etc", NULL ) ;, printf ( "Child: After exec( )\n") ;, }, else, printf ( "Parent process\n" ) ;, }
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660, , Let Us C, , After forking a child process we have called the execl( ) function., This function accepts variable number of arguments. The first, parameter to execl( ) is the absolute path of the program to be, executed. The remaining parameters describe the command line, arguments for the program to be executed. The last parameter is an, end of argument marker which must always be NULL. Thus in our, case the we have called upon the execl( ) function to execute the ls, program as shown below, ls -al /etc, , As a result, all the contents of the /etc directory are listed on the, screen. Note that the printf( ) below the call to execl( ) function is, not executed. This is because the exec family functions overwrite, the image of the calling process with the code and data of the, program that is to be executed. In our case the child process’s, memory was overwritten by the code and data of the ls program., Hence the call to printf( ) did not materialize., It would make little sense in calling execl( ) before fork( ). This is, because a child would not get created and execl( ) would simply, overwrite the main process itself. As a result, no statement beyond, the call to execl( ) would ever get executed. Hence fork( ) and, execl( ) usually go hand in hand., , Zombies and Orphans, We know that the ps –A command lists all the running processes., But from where does the ps program get this information? Well,, Linux maintains a table containing information about all the, processes. This table is called ‘Process Table’. Apart from other, information the process table contains an entry of ‘exit code’ of the, process. This integer value indicates the reason why the process, was terminated. Even though the process comes to an end its entry, would remain in the process table until such time that the parent of, the terminated process queries the exit code. This act of querying
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Chapter 19: Interaction With Hardware, , 661, , deletes the entry of the terminated process from the process table, and returns the exit code to the parent that raised the query., When we fork a new child process and the parent and the child, continue to execute there are two possibilities—either the child, process ends first or the parent process ends first. Let us discuss, both these possibilities., (a) Child terminates earlier than the parent, In this case till the time parent does not query the exit code of the, terminated child the entry of the child process would continue to, exist. Such a process in Linux terminology is known as a ‘Zombie’, process. Zombie means ghost, or in plain simple Hindi a ‘Bhoot’., Moral is, a parent process should query the process table, immediately after the child process has terminated. This would, prevent a zombie., What if the parent terminates without querying. In such a case the, zombie child process is treated as an ‘Orphan’ process., Immediately, the father of all processes—init—adopts the, orphaned process. Next, as a responsible parent init queries the, process table as a result of which the child process entry is, eliminated from the process table., (b) Parent terminates earlier than the child, Since every parent process is launched from the Linux shell, the, parent of the parent is the shell process. When our parent process, terminates, the shell queries the process table. Thus a proper, cleanup happens for the parent process. However, the child process, which is still running is left orphaned. Immediately the init process, would adopt it and when its execution is over init would query the, process table to clean up the entry for the child process. Note that, in this case the child process does not become a zombie., Thus, when a zombie or an orphan gets created the OS takes over, and ensures that a proper cleanup of the relevant process table
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662, , Let Us C, , entry happens. However, as a good programming practice our, program should get the exit code of the terminated process and, thereby ensure a proper cleanup. Note that here cleanup is, important (it happens anyway). Why is it important to get the exit, code of the terminated process. It is because, it is the exit code that, would give indication about whether the job assigned to the, process was completed successfully or not. The following program, shows how this can be done., # include <unistd.h>, # include <sys/types.h>, int main( ), {, unsigned int i = 0 ;, int pid, status ;, pid = fork( ) ;, if ( pid == 0 ), {, while ( i < 4294967295U ), i++ ;, printf ( "The child is now terminating\n" ) ;, }, else, {, waitpid ( pid, &status, 0 ) ;, if ( WIFEXITED ( status ) ), printf ( "Parent: Child terminated normally\n" ) ;, else, printf ( "Parent: Child terminated abnormally\n" ) ;, }, return 0 ;, }, , In this program we have applied a big loop in the child process., This loop ensures that the child does not terminate immediately., From within the parent process we have made a call to the, waitpid( ) function. This function makes the parent process wait
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Chapter 19: Interaction With Hardware, , 663, , till the time the execution of the child process does not come to an, end. This ensures that the child process never becomes orphaned., Once the child process, terminates the waitpid( ) function queries, its exit code and returns back to the parent. As a result of querying,, the child process does not become a zombie., The first parameter of waitpid( ) function is the pid of the child, process for which the wait has to be performed. The second, parameter is the address of an integer variable which is set up with, the exit status code of the child process. The third parameter is, used to specify some options to control the behavior of the wait, operation. We have not used this parameter and hence we have, passed a 0. Next we have made use of the WIFEXITED( ) macro, to test if the child process exited normally or not. This macro takes, the status value as a parameter and returns a non-zero value if the, process terminated normally. Using this macro the parent suitably, prints a message to report the status (normal/abnormal), termination of its child process., , One Interesting Fact, When we use fork( ) to create a child process the child process, does not contain the entire data and code of the parent process., Then does it mean that the child process contains the data and code, below the fork( ) call. Even this is not so. In actuality the code, never gets duplicated. Linux internally manages to intelligently, share it. As against this, some data is shared, some is not. Till the, time both the processes do not change the value of the variables, they keep getting shared. However, if any of the processes (either, child or parent) attempt to change the value of a variable it is no, longer shared. Instead a new copy of the variable is made for the, process that is attempting to change it. This not only ensures data, integrity but also saves precious memory.
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664, , Let Us C, , Summary, (a) Linux is a free OS whose kernel was built by Linus Trovalds, and friends., (b) A Linux distribution consists of the kernel with source code, along with a large collection of applications, libraries, scripts,, etc., (c) C programs under Linux can be compiled using the popular, gcc compiler., (d) Basic scheduling unit in Linux is a ‘Process’. Processes are, scheduled by a special program called ‘Scheduler’., (e) fork( ) library function can be used to create child processes., (f) Init process is the father of all processes., (g) execl( ) library function is used to execute another program, from within a running program,., (h) execl( ) function overwrites the image (code and data) of the, calling process., (i) execl( ) and fork( ) usually go hand in hand., (j) ps command can be used to get a list of all processes., (k) kill command can be used to terminate a process., (l) A ‘Zombie’ is a child process that has terminated but its, parent is running and has not called a function to get the exit, code of the child process., (m) An ‘Orphan’ is a child process whose parent has terminated., (n) Orphaned processes are adopted by init process, automatically., (o) A parent process can avoid creation of a Zombie and Orphan, processes using waitpid( ) function., , Exercise, [A] State True or False:, , (a) We can modify the kernel of Linux OS., (b) All distributions of Linux contain the same collection of, applications, libraries and installation scripts., (c) Basic scheduling unit in Linux is a file.
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Chapter 19: Interaction With Hardware, , 665, , (d) execl( ) library function can be used to create a new child, process., (e) The scheduler process is the father of all processes., (f) A family of fork( ) and exec( ) functions are available, each, doing basically the same job but with minor variations., (g) fork( ) completely duplicates the code and data of the parent, process into the child process., (h) fork( ) overwrites the image (code and data) of the calling, process., (i) fork( ) is called twice but returns once., (j) Every zombie process is essentially an orphan process., (k) Every orphan process is essentially an orphan process., [B] Answer the following:, , (a) If a program contains four calls to fork( ) one after the other, how many total processes would get created?, (b) What is the difference between a zombie process and an, orphan process?, (c) Write a program that prints the command line arguments that, it receives. What would be the output of the program if the, command line argument is * ?, (d) What purpose do the functions getpid( ), getppid( ),, getpppid( ) serve?, (e) Rewrite the program in the section ‘Zombies and Orphans’, replacing the while loop with a call to the sleep( ) function., Do you observe any change in the output of the program?, (f), , How does waitpid( ) prevent creation of Zombie or Orphan, processes?
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666, , Let Us C
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21, , More Linux, Programming, •, •, •, •, •, •, •, •, , Communication using Signals, Handling Multiple Signals, Registering a Common Handler, Blocking Signals, Event driven programming, Where Do You Go From Here, Summary, Exercise, , 667
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668, , Let Us C, , C, , ommunication is the essence of all progress. This is true in, real life as well as in programming. In today’s world a, program that runs in isolation is of little use. A worthwhile, program has to communicate with the outside world in general and, with the OS in particular. In Chapters 16 and 17 we saw how a, Windows based program communicates with Windows. In this, chapter let us explore how this communication happens under, Linux., , Communication using Signals, In the last chapter we used fork( ) and exec( ) library function to, create a child process and to execute a new program respectively., These library functions got the job done by communication with, the Linux OS. Thus the direction of communication was from the, program to the OS. The reverse communication—from the OS to, the program—is achieved using a mechanism called ‘Signal’. Let, us now write a simple program that would help you experience the, signal mechanism., int main( ), {, while ( 1 ), printf ( "Pogram Running\n" ) ;, return 0 ;, }, , The program is fairly straightforward. All that we have done here, is we have used an infinite while loop to print the message, "Program Running" on the screen. When the program is running, we can terminate it by pressing the Ctrl + C. When we press Ctrl +, C the keyboard device driver informs the Linux kernel about, pressing of this special key combination. The kernel reacts to this, by sending a signal to our program. Since we have done nothing to, handle this signal the default signal handler gets called. In this
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Chapter 21: More Linux Programming, , 669, , default signal handler there is code to terminate the program., Hence on pressing Ctrl + C the program gets terminated., But how on earth would the default signal handler get called. Well,, it is simple. There are several signals that can be sent to a program., A unique number is associated with each signal. To avoid, remembering these numbers, they have been defined as macros, like SIGINT, SIGKILL, SIGCONT, etc. in the file ‘signal.h’., Every process contains several ‘signal ID - function pointer’ pairs, indicating for which signal which function should be called. If we, do not decide to handle a signal then against that signal ID the, address of the default signal handler function is present. It is, precisely this default signal handler for SIGINT that got called, when we pressed Ctrl + C when the above program was executed., INT in SIGINT stands for interrupt., Let us know see how can we prevent the termination of our, program even after hitting Ctrl + C. This is shown in the following, program:, # include <signal.h>, void sighandler ( int signum ), {, printf ( "SIGINT received. Inside sighandler\n" ) ;, }, int main( ), {, signal ( SIGINT, ( void* ) sighandler ) ;, while ( 1 ), printf ( "Program Running\n" ) ;, return 0 ;, }, , In this program we have registered a signal handler for the SIGINT, signal by using the signal( ) library function. The first parameter
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670, , Let Us C, , of this function specifies the ID of the signal that we wish to, register. The second parameter is the address of a function that, should get called whenever the signal is received by our program., This address has to be typecasted to a void * before passing it to, the signal( ) function., Now when we press Ctrl + C the registered handler, namely,, sighandler( ) would get called. This function would display the, message ‘SIGINT received. Inside sighandler’ and return the, control back to main( ). Note that unlike the default handler, our, handler does not terminate the execution of our program. So only, way to terminate it is to kill the running process from a different, terminal. For this we need to open a new instance of command, prompt (terminal). How to start a new instace of command prompt, is discussed in Appendix H. Next do a ps –a to obtain the list of, processes running at all the command prompts that we have, launched. Note down the process id of a.out. Finally kill ‘a.out’, process by saying, # kill 3276, , In my case the terminal on which I executed a.out was tty1 and its, process id turned out to be 3276. In your case the terminal name, and the process id might be a different number., If we wish we can abort the execution of the program in the signal, handler itself by using the exit ( 0 ) beyond the printf( )., Note that signals work asynchronously. That is, when a signal is, received no matter what our program is doing, the signal handler, would immediately get called. Once the execution of the signal, handler is over the execution of the program is resumed from the, point where it left off when the signal was received.
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672, , Let Us C, , In this program apart from SIGINT we have additionally, registered two new signals, namely, SIGTERM and SIGCONT., The signal( ) function is called thrice to register a different handler, for each of the three signals. After registering the signals we enter, a infinite while loop to print the ‘Program running’ message on the, screen., As in the previous program, here too, when we press Ctrl + C the, handler for the SIGINT i.e. inthandler( ) is called. However,, when we try to kill the program from the second terminal using the, kill command the program does not terminate. This is because, when the kill command is used it sends the running program a, SIGTERM signal. The default handler for the message terminates, the program. Since we have handled this signal ourselves, the, handler for SIGTERM i.e. termhandler( ) gets called. As a, result the printf( ) statement in the termhandler( ) function gets, executed and the message ‘SIGTERM Received’ gets displayed on, the screen. Once the execution of termhandler( ) function is over, the program resumes its execution and continues to print ‘Program, Running’. Then how are we supposed to terminate the program?, Simple. Use the following command from the another terminal:, kill –SIGKILL 3276, , As the command indicates, we are trying to send a SIGKILL, signal to our program. A SIGKILL signal terminates the program., Most signals may be caught by the process, but there are a few, signals that the process cannot catch, and they cause the process to, terminate. Such signals are often known as un-catchable signals., The SIGKILL signal is an un-catchable signal that forcibly, terminates the execution of a process., Note that even if a process attempts to handle the SIGKILL signal, by registering a handler for it still the control would always land in, the default SIGKILL handler which would terminate the program.
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Chapter 21: More Linux Programming, , 673, , The SIGKILL signal is to be used as a last resort to terminate a, program that gets out of control. One such process that makes uses, of this signal is a system shutdown process. It first sends a, SIGTERM signal to all processes, waits for a while, thus giving a, ‘grace period’ to all the running processes. However, after the, grace period is over it forcibly terminates all the remaining, processes using the SIGKILL signal., That leaves only one question—when does a process receive the, SIGCONT signal? Let me try to answer this question., A process under Linux can be suspended using the Ctrl + Z, command. The process is stopped but is not terminated, i.e. it is, suspended. This gives rise to the un-catchable SIGSTOP signal., To resume the execution of the suspended process we can make, use of the fg (foreground) command. As a result of which the, suspended program resumes its execution and receives the, SIGCONT signal (CONT means continue execution)., , Registering a Common Handler, Instead of registering a separate handler for each signal we may, decide to handle all signals using a common signal handler. This is, shown in the following program:, # include <unistd.h>, # include <sys/types.h>, # include <signal.h>, void sighandler ( int signum ), {, switch ( signum ), {, case SIGINT :
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674, , Let Us C, printf ( "SIGINT Received\n" ) ;, break ;, , case SIGTERM :, printf ( "SIGTERM Received\n" ) ;, break ;, case SIGCONT :, printf ( "SIGCONT Received\n" ) ;, break ;, }, }, int main( ), {, signal ( SIGINT, sighandler ) ;, signal ( SIGTERM, sighandler ) ;, signal ( SIGCONT, sighandler ) ;, while ( 1 ), printf ( "\rProgram running" ) ;, return 0 ;, }, , In this program during each call to the signal( ) function we have, specified the address of a common signal handler named, sighandler( ). Thus the same signal handler function would get, called when one of the three signals are received. This does not, lead to a problem since the sighandler( ) we can figure out inside, the signal ID using the first parameter of the function. In our, program we have made use of the switch-case construct to print a, different message for each of the three signals., Note that we can easily afford to mix the two methods of, registering signals in a program. That is, we can register separate, signal handlers for some of the signals and a common handler for
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Chapter 21: More Linux Programming, , 675, , some other signals. Registering a common handler makes sense if, we want to react to different signals in exactly the same way., , Blocking Signals, Sometimes we may want that flow of execution of a critical/timecritical portion of the program should not be hampered by the, occurrence of one or more signals. In such a case we may decide, to block the signal. Once we are through with the critical/timecritical code we can unblock the signals(s). Note that if a signal, arrives when it is blocked it is simply queued into a signal queue., When the signals are unblocked the process immediately receives, all the pending signals one after another. Thus blocking of signals, defers the delivery of signals to a process till the execution of, some critical/time-critical code is over. Instead of completely, ignoring the signals or letting the signals interrupt the execution, it, is preferable to block the signals for the moment and deliver them, some time later. Let us now write a program to understand signal, blocking. Here is the program…, # include <unistd.h>, # include <sys/types.h>, # include <signal.h>, # include <stdio.h>, void sighandler ( int signum ), {, switch ( signum ), {, case SIGTERM :, printf ( "SIGTERM Received\n" ) ;, break ;, case SIGINT :, printf ( "SIGINT Received\n" ) ;, break ;
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Chapter 21: More Linux Programming, , 677, , repeatedly accept strings in a buffer and display them on the screen, till the time the user does not enter an ‘n’ from the keyboard., Additionally, we want that this activity of receiving input should, not be interrupted by the SIGINT or the SIGTERM signals., However, a SIGCONT should be permitted. So before we proceed, with the loop we must block the SIGINT and SIGTERM signals., Once we are through with the loop we must unblock these signals., This blocking and unblocking of signals can be achieved using the, sigprocmask( ) library function., The first parameter of the sigprocmask( ) function specifies, whether we want to block/unblock a set of signals. The next, parameter is the address of a structure (typedefed as sigset_t) that, describes a set of signals that we want to block/unblock. The last, parameter can be either NULL or the address of sigset_t type, variable which would be set up with the existing set of signals, before blocking/unblocking signals., There are library functions that help us to populate the sigset_t, structure. The sigemptyset( ) empties a sigset_t variable so that it, does not refer to any signals. The only parameter that this function, accepts is the address of the sigset_t variable. We have used this, function to quickly initialize the sigset_t variable block to a known, empty state. To block the SIGINT and SIGTERM we have to add, the signals to the empty set of signals. This can be achieved using, the sigaddset( ) library function. The first parameter of, sigaddset( ) is the address of the sigset_t variable and the second, parameter is the ID of the signal that we wish to add to the existing, set of signals., After the loop we have also used an infinite while loop to print the, ‘Program running’ message. This is done so that we can easily, check that till the time the loop that receives input is not over the, program cannot be terminated using Ctrl + C or kill command, since the signals are blocked. Once the user enters ‘n’ from the, keyboard the execution comes out of the while loop and unblocks
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678, , Let Us C, , the signals. As a result, pending signals, if any, are immediately, delivered to the program. So if we press Ctrl + C or use the kill, command when the execution of the loop that receives input is not, over these signals would be kept pending. Once we are through, with the loop the signal handlers would be called., , Event Driven programming, Having understood the mechanism of signal processing let us now, see how signaling is used by Linux – based libraries to create, event driven GUI programs. As you know, in a GUI program, events occur typically when we click on the window, type a, character, close the window, repaint the window, etc. We have, chosen the GTK library version 2.0 to create the GUI applications., Here, GTK stands for Gimp’s Tool Kit. Refer Appendix H for, installation of this toolkit. Given below is the first program that, uses this toolkit to create a window on the screen., /* mywindow.c */, # include <gtk/gtk.h>, int main ( int argc, char *argv[ ] ), {, GtkWidget *p ;, gtk_init ( &argc, &argv ) ;, p = gtk_window_new ( GTK_WINDOW_TOPLEVEL ) ;, gtk_window_set_title ( p , "Sample Window" ) ;, g_signal_connect ( p, "destroy", gtk_main_quit, NULL ) ;, gtk_widget_set_size_request ( p, 300, 300 ) ;, gtk_widget_show ( p ) ;, gtk_main( ) ;, return 0 ;, }
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Chapter 21: More Linux Programming, , 679, , We need to compile this program as follows:, gcc mywindow.c `pkg-config gtk+-2.0 - -cflags - -libs`, , Here we are compiling the program ‘mywindow.c’ and then, linking it with the necessary libraries from GTK toolkit. Note the, quotes that we have used in the command., Here is the output of the program…, , Figure 21.1, The GTK library provides a large number of functions that makes, it very easy for us to create GUI programs. Every window under, GTK is known as a widget. To create a simple window we have to, carry out the following steps:
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680, , Let Us C, , (a) Initialize the GTK library with a call to gtk_init( ) function., This function requires the addresses of the command line, arguments received in main( )., (b) Next, call the gtk_window_new( ) function to create a top, level window. The only parameter this function takes is the, type of windows to be created. A top level window can be, created by specifying the GTK_WINDOW_TOPLEVEL, value. This call creates a window in memory and returns a, pointer to the widget object. The widget object is a structure, (GtkWidget) variable that stores lots of information including, the attributes of window it represents. We have collected this, pointer in a GtkWidget structure pointer called p., (c) Set the title for the window by making a call to, gtk_window_set_title( ) function. The first parameter of this, function is a pointer to the GtkWidget structure representing, the window for which the title has to be set. The second, parameter is a string describing the text to be displayed in the, title of the window., (d) Register a signal handler for the destroy signal. The destroy, signal is received whenever we try to close the window. The, handler for the destroy signal should perform clean up, activities and then shutdown the application. GTK provides a, ready-made function called gtk_main_quit( ) that does this, job. We only need to associate this function with the destroy, signal. This can be achieved using the g_signal_connect( ), function. The first parameter of this function is the pointer to, the widget for which destroy signal handler has to be, registered. The second parameter is a string that specifies the, name of the signal. The third parameter is the address of the, signal handler routine. We have not used the fourth parameter., (e) Resize the window to the desired size using the, gtk_widget_set_size_request( ) function. The second and the
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Chapter 21: More Linux Programming, , 681, , third parameters specify the height and the width of the, window respectively., (f) Display the window on the screen using the function, gtk_widget_show( )., (g) Wait in a loop to receive events for the window. This can be, accomplished using the gtk_main( ) function., , How about another program that draws a few shapes in the, window? Here is the program…, /* myshapes.c */, # include <gtk/gtk.h>, int expose_event ( GtkWidget *widget, GdkEventExpose *event ), {, GdkGC* p ;, GdkPoint arr [ 5] = { 250, 150, 250, 300, 300, 350, 400, 300, 320, 190 } ;, p = gdk_gc_new ( widget -> window ) ;, gdk_draw_line ( widget -> window, p, 10, 10, 200, 10 ) ;, gdk_draw_rectangle ( widget -> window, p, TRUE, 10, 20, 200, 100 ) ;, gdk_draw_arc ( widget -> window, p, TRUE, 200, 10, 200, 200,, 2880, -2880*2 ) ;, gdk_draw_polygon ( widget -> window, p, TRUE , arr, 5 ) ;, gdk_gc_unref ( p ) ;, return TRUE ;, }, int main( int argc, char *argv[ ] ), {, GtkWidget *p ;, gtk_init ( &argc, &argv ) ;
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Chapter 21: More Linux Programming, , 683, , Figure 21.2, This program is similar to the first one. The only difference is that, in addition to the destroy signal we have registered a signal, handler for the expose_event using the g_signal_connect( ), function. This signal is sent to our process whenever the window, needs to be redrawn. By writing the code for drawing shapes in the, handler for this signal we are assured that the drawing would never, vanish if the windows is dragged outside the screen and then, brought back in, or some other window uncovers a portion of our, window which was previously overlapped, and so on. This is
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684, , Let Us C, , because a expose_event signal would be sent to our application, which would immediately redraw the shapes in our window., The way in Windows we have a device context, under Linux we, have a graphics context. In order to draw in the window we need, to obtain a graphics context for the window using the, gdk_gc_new( ) function. This function returns a pointer to the, graphics context structure. This pointer must be passed to the, drawing functions like gdk_draw_line( ), gdk_draw_rectangle(),, gdk_draw_arc( ), gdk_draw_polygon( ), etc. Once we are, through with drawing we should release the graphics context using, the gdk_gc_unref( ) function., , Where Do You Go From Here, You have now understood signal processing, the heart of, programming under Linux. With that knowledge under your belt, you are now capable of exploring the vast world of Linux on your, own. Complete Linux programming deserves a book on its own., Idea here was to raise the hood and show you what lies underneath, it. I am sure that if you have taken a good look at it you can try the, rest yourselves. Good luck!, , Summary, (a) Programs can communicate with the Linux OS using library, functions., (b) The Linux OS communicates with a program by means of, signals., (c) The interrupt signal (SIGINT) is sent by the kernel to our, program when we press Ctrl + C., (d) A term signal (SIGTERM) is sent to the program when we, use the kill command., (e) A process cannot handle an un-catchable signal., (f) The kill –SIGKILL variation of the kill command generates, an un-catchable SIGKILL signal that terminates a process.
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Chapter 21: More Linux Programming, , 685, , (g) A process can block a signal or a set of signals using the, sigprocmask( ) function., (h) Blocked signals are delivered to the process when the signals, are unblocked., (i) A SIGSTOP signal is generated when we press Ctrl + Z., (j) A SIGSTOP signal is un-catchable signal., (k) A suspended process can be resumed using the fg command., (l) A process receives the SIGCONT signal when it resumes, execution., (m) In GTK, the g_signal_connect( ) function can be used to, connect a function with an event., , Exercise, [A] State True or False:, , (a) All signals registered signals must have a separate signal, handler., (b) Blocked signals are ignored by a process., (c) Only one signal can be blocked at a time., (d) Blocked signals are ignored once the signals are unblocked., (e) If our signal handler gets called the default signal handler, automatically gets called., (f) gtk_main( ) function makes uses of a loop to prevent the, termination of the program., (g) Multiple signals can be registered at a time using a single call, to signal( ) function., (h) The sigprocmask( ) function can block as well as unblock, signals., [B] Answer the following:, , (a) How does the Linux OS know if we have registered a signal, or not?, (b) What happens when we register a handler for a signal?
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686, , Let Us C, , (c) Write a program to verify that SIGSTOP and SIGKILL, signals are un-catchable signals., (d) Write a program to handle the SIGINT and SIGTERM, signals. From inside the handler for SIGINT signal write an, infinite loop to print the message ‘Processing Signal’. Run the, program and make use of Ctrl + C more than once. Run the, program once again and press Ctrl + C once then use the kill, command. What are your observations?, (e) Write a program that blocks the SIGTERM signal during, execution of the SIGINT signal.
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A, , Precedence, Table, , 687
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688, , Let Us C, , Description, , Operator, , Associativity, , Function expression, , (), , Left to Right, , Array Expression, , [], , Left to Right, , Structure operator, , ->, , Left to Right, , Structure operator, , ., , Left to Right, , Unary minus, , -, , Right to left, , Increment/Decrement, , ++, , One’s compliment, , ~, , Right to left, , Negation, , !, , Right to Left, , --, , Right to Left, , Address of, , &, , Right to left, , Value of address, , *, , Right to left, , Type cast, , ( type ), , Right to left, , Size in bytes, , sizeof, , Right to left, , Multiplication, , *, , Left to right, , Division, , /, , Left to right, , Modulus, , %, , Left to right, , Addition, , +, , Left to right, , Subtraction, , -, , Left to right, , Left shift, , <<, , Left to right, , Right shift, , >>, , Left to right, , Less than, , <, , Left to right, , Less than or equal to, , <=, , Left to right, , Greater than, , >, , Left to right, , Greater than or equal to, , >=, , Left to right, , Equal to, , ==, , Left to right, , Not equal to, , !=, , Left to right, , Continued…
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689, , Appendix A: Precedence Table, Continued…, Description, , Operator, , Associativity, , Bitwise AND, , &, , Left to right, , Bitwise exclusive OR, , ^, , Left to right, , Bitwise inclusive OR, , |, , Left to right, , Logical AND, , &&, , Left to right, , Logical OR, , ||, , Left to right, , Conditional, , ? :, , Right to left, , Assignment, , =, , Right to left, , Comma, , Figure A1.1, , *=, , /=, , %=, , Right to left, , +=, , -=, , &=, , Right to left, , ^=, , |=, , Right to left, , <<=, , >>=, , Right to left, , ,, , Right to left
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690, , Let Us C
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B, , Standard Library, Functions, •, •, •, •, •, •, •, •, •, •, •, •, •, •, •, •, •, , Standard Library Functions, Arithmetic Functions, Data Conversion Functions, Character Classification Functions, String Manipulation Functions, Searching and Sorting Functions, I/O Functions, File Handling Functions, Directory Control Functions, Buffer Manipulation Functions, Disk I/O Functions, Memory Allocation Functions, Process Control Functions, Graphics Functions, Time Related Functions, Miscellaneous Functions, DOS Interface Functions, , 691
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692, , Let Us C, , L, , et alone discussing each standard library function in detail,, even a complete list of these functions would occupy scores, of pages. However, this book would be incomplete if it has, nothing to say about standard library functions. I have tried to, reach a compromise and have given a list of standard library, functions that are more popularly used so that you know what to, search for in the manual. An excellent book dedicated totally to, standard library functions is Waite group’s, Turbo C Bible, written, by Nabjyoti Barkakti., Following is the list of selected standard library functions. The, functions have been classified into broad categories., , Arithmetic Functions, Function, , Use, , abs, cos, cosh, exp, fabs, floor, fmod, hypot, log, log10, modf, pow, sin, sinh, sqrt, tan, tanh, , Returns the absolute value of an integer, Calculates cosine, Calculates hyperbolic cosine, Raises the exponential e to the xth power, Finds absolute value, Finds largest integer less than or equal to argument, Finds floating-point remainder, Calculates hypotenuse of right triangle, Calculates natural logarithm, Calculates base 10 logarithm, Breaks down argument into integer and fractional parts, Calculates a value raised to a power, Calculates sine, Calculates hyperbolic sine, Finds square root, Calculates tangent, Calculates hyperbolic tangent
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Appendix B: Standard Library Functions, , Data Conversion Functions, Function, , Use, , atof, atoi, atol, ecvt, fcvt, gcvt, itoa, ltoa, strtod, strtol, strtoul, ultoa, , Converts string to float, Converts string to int, Converts string to long, Converts double to string, Converts double to string, Converts double to string, Converts int to string, Converts long to string, Converts string to double, Converts string to long integer, Converts string to an unsigned long integer, Converts unsigned long to string, , Character classification Functions, Function, , Use, , isalnum, isalpha, isdigit, islower, isspace, isupper, isxdigit, tolower, toupper, , Tests for alphanumeric character, Tests for alphabetic character, Tests for decimal digit, Tests for lowercase character, Tests for white space character, Tests for uppercase character, Tests for hexadecimal digit, Tests character and converts to lowercase if uppercase, Tests character and converts to uppercase if lowercase, , 693
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694, , Let Us C, , String Manipulation Functions, Function, , Use, , strcat, strchr, strcmp, strcmpi, strcpy, strdup, stricmp, , Appends one string to another, Finds first occurrence of a given character in a string, Compares two strings, Compares two strings without regard to case, Copies one string to another, Duplicates a string, Compares two strings without regard to case (identical to, strcmpi), Finds length of a string, Converts a string to lowercase, Appends a portion of one string to another, Compares a portion of one string with portion of another, string, Copies a given number of characters of one string to another, Compares a portion of one string with a portion of another, without regard to case, Finds last occurrence of a given character in a string, Reverses a string, Sets all characters in a string to a given character, Finds first occurrence of a given string in another string, Converts a string to uppercase, , strlen, strlwr, strncat, strncmp, strncpy, strnicmp, strrchr, strrev, strset, strstr, strupr, , Searching and Sorting Functions, Function, , Use, , bsearch, lfind, qsort, , Performs binary search, Performs linear search for a given value, Performs quick sort
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Appendix B: Standard Library Functions, , I/O Functions, Function, , Use, , Close, fclose, feof, fgetc, fgetchar, fgets, fopen, fprintf, fputc, fputchar, fputs, fscanf, fseek, ftell, getc, getch, getche, getchar, gets, inport, inportb, kbhit, lseek, open, outport, outportb, printf, putc, putch, putchar, puts, read, , Closes a file, Closes a file, Detects end-of-file, Reads a character from a file, Reads a character from keyboard (function version), Reads a string from a file, Opens a file, Writes formatted data to a file, Writes a character to a file, Writes a character to screen (function version), Writes a string to a file, Reads formatted data from a file, Repositions file pointer to given location, Gets current file pointer position, Reads a character from a file (macro version), Reads a character from the keyboard, Reads a character from keyboard and echoes it, Reads a character from keyboard (macro version), Reads a line from keyboard, Reads a two-byte word from the specified I/O port, Reads one byte from the specified I/O port, Checks for a keystroke at the keyboard, Repositions file pointer to a given location, Opens a file, Writes a two-byte word to the specified I/O port, Writes one byte to the specified I/O port, Writes formatted data to screen, Writes a character to a file (macro version), Writes a character to the screen, Writes a character to screen (macro version), Writes a line to file, Reads data from a file, , 695
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696, rewind, scanf, sscanf, sprintf, tell, write, , Let Us C, Repositions file pointer to beginning of a file, Reads formatted data from keyboard, Reads formatted input from a string, Writes formatted output to a string, Gets current file pointer position, Writes data to a file, , File Handling Functions, Function, , Use, , remove, rename, unlink, , Deletes file, Renames file, Deletes file, , Directory Control Functions, Function, , Use, , chdir, getcwd, fnsplit, findfirst, findnext, mkdir, rmdir, , Changes current working directory, Gets current working directory, Splits a full path name into its components, Searches a disk directory, Continues findfirst search, Makes a new directory, Removes a directory, , Buffer Manipulation Functions, Function, , Use, , memchr, , Returns a pointer to the first occurrence, within a specified, number of characters, of a given character in the buffer, Compares a specified number of characters from two, buffers, , memcmp
Page 699 :
Appendix B: Standard Library Functions, memcpy, memicmp, memmove, memset, , 697, , Copies a specified number of characters from one buffer to, another, Compares a specified number of characters from two, buffers without regard to the case of the characters, Copies a specified number of characters from one buffer to, another, Uses a given character to initialize a specified number of, bytes in the buffer, , Disk I/O Functions, Function, , Use, , absread, abswrite, biosdisk, getdisk, setdisk, , Reads absolute disk sectors, Writes absolute disk sectors, Performs BIOS disk services, Gets current drive number, Sets current disk drive, , Memory Allocation Functions, Function, , Use, , calloc, farmalloc, farfree, free, malloc, realloc, , Allocates a block of memory, Allocates memory from far heap, Frees a block from far heap, Frees a block allocated with malloc, Allocates a block of memory, Reallocates a block of memory, , Process Control Functions, Function, , Use, , abort, atexit, , Aborts a process, Executes function at program termination
Page 700 :
698, execl, exit, spawnl, spawnlp, system, , Let Us C, Executes child process with argument list, Terminates the process, Executes child process with argument list, Executes child process using PATH variable and argument, list, Executes an MS-DOS command, , Graphics Functions, Function, , Use, , arc, ellipse, floodfill, getimage, getlinestyle, getpixel, lineto, , Draws an arc, Draws an ellipse, Fills an area of the screen with the current color, Stores a screen image in memory, Obtains the current line style, Obtains the pixel’s value, Draws a line from the current graphic output position to the, specified point, Moves the current graphic output position to a specified, point, Draws a pie-slice-shaped figure, Retrieves an image from memory and displays it, Draws a rectangle, Sets the current color, Sets the current line style, Plots a pixel at a specified point, Limits graphic output and positions the logical origin, within the limited area, , moveto, pieslice, putimage, rectangle, setcolor, setlinestyle, putpixel, setviewport, , Time Related Functions, Function, , Use, , clock, difftime, , Returns the elapsed CPU time for a process, Computes the difference between two times
Page 701 :
Appendix B: Standard Library Functions, ftime, strdate, strtime, time, setdate, getdate, , 699, , Gets current system time as structure, Returns the current system date as a string, Returns the current system time as a string, Gets current system time as long integer, Sets DOS date, Gets system date, , Miscellaneous Functions, Function, , Use, , delay, getenv, getpsp, perror, putenv, random, randomize, , Suspends execution for an interval (milliseconds), Gets value of environment variable, Gets the Program Segment Prefix, Prints error message, Adds or modifies value of environment variable, Generates random numbers, Initializes random number generation with a random value, based on time, Turns PC speaker on at specified frequency, Turns PC speaker off, , sound, nosound, , DOS Interface Functions, Function, , Use, , FP_OFF, FP_SEG, getvect, keep, int86, int86x, intdos, intdosx, MK_FP, , Returns offset portion of a far pointer, Returns segment portion of a far pointer, Gets the current value of the specified interrupt vector, Installs terminate-and-stay-resident (TSR) programs, Issues interrupts, Issues interrupts with segment register values, Issues interrupt 21h using registers other than DX and AL, Issues interrupt 21h using segment register values, Makes a far pointer
Page 702 :
700, segread, setvect, , Let Us C, Returns current values of segment registers, Sets the current value of the specified interrupt vector
Page 703 :
C, , Chasing The, Bugs, , 701
Page 704 :
702, , Let Us C, , programmers are great innovators of our times. Unhappily,, among their most enduring accomplishments are several, new techniques for wasting time. There is no shortage of, horror stories about programs that took twenty times to ‘debug’ as, they did to ‘write’. And one hears again and again about programs, that had to be rewritten all over again because the bugs present in, it could not be located. A typical C programmer’s ‘morning after’, is red eyes, blue face and a pile of crumpled printouts and dozens, of reference books all over the floor. Bugs are C programmer's, birthright. But how do we chase them away. No sure-shot way for, that. I thought if I make a list of more common programming, mistakes it might be of help. They are not arranged in any, particular order. But as you would realize surely a great help!, , C, , [1] Omitting the ampersand before the variables used in scanf( )., For example,, int choice ;, scanf ( "%d", choice ) ;, , Here, the & before the variable choice is missing. Another, common mistake with scanf( ) is to give blanks either just, before the format string or immediately after the format string, as in,, int choice ;, scanf ( " %d ", choice ) ;, , Note that this is not a mistake, but till you don't understand, scanf( ) thoroughly, this is going to cause trouble. Safety is in, eliminating the blanks. Thus, the correct form would be,, int choice ;, scanf ( "%d", &choice ) ;
Page 705 :
Appendix C: Chasing The Bugs, , 703, , [2] Using the operator = instead of the operator = =., What do you think will be the output of the following, program:, main( ), {, int i = 10 ;, while ( i = 10 ), {, printf ( "got to get out" ) ;, i++ ;, }, }, , At first glance it appears the message will be printed once and, the control will come out of the loop since i becomes 11. But,, actually we have fallen in an indefinite loop. This is because, the = used in the condition always assigns the value 10 to i,, and since i is non-zero the condition is satisfied and the body, of the loop is executed over and over again., [3] Ending a loop with a semicolon., Observe the following program., main( ), {, int j = 1 ;, while ( j <= 100 ) ;, {, printf ( "\nCompguard" ) ;, j++ ;, }, }
Page 706 :
704, , Let Us C, , Inadvertently, we have fallen in an indefinite loop. Cause is, the semicolon after while. This in effect makes the compiler, feel that you wanted the loop to work in the following, manner:, while ( j <= 100 ) ;, , This is an indefinite loop since j never gets incremented and, hence eternally remains less that 100., [4] Omitting the break statement at the end of a case in a switch, statement., Remember that if a break is not included at the end of a case,, then execution will continue into the next case., main( ), {, int ch = 1 ;, switch ( ch ), {, case 1 :, printf ( "\nGoodbye" ) ;, case 2 :, printf ( "\nLieutenant" ) ;, }, }, , Here, since the break has not been given after the printf( ) in, case 1, the control runs into case 2 and executes the second, printf( ) as well., However, this sometimes turns out to be a blessing in, disguise. Especially, in cases when we are checking whether, the value of a variable equals a capital letter or a small case
Page 707 :
Appendix C: Chasing The Bugs, , 705, , letter. This example has been succinctly explained in Chapter, 4., [5] Using continue in a switch., It is a common error to believe that the way the keyword, break is used with loops and a switch; similarly the keyword, continue can also be used with them. Remember that, continue works only with loops, never with a switch., [6] A mismatch in the number, type and order of actual and formal, arguments., yr = romanise ( year, 1000, 'm' ) ;, , Here, three arguments in the order int, int and char are being, passed to romanise( ). When romanise( ) receives these, arguments into formal arguments they must be received in the, same order. A careless mismatch might give strange results., [7] Omitting provisions for returning a non-integer value from a, function., If we make the following function call,, area = area_circle ( 1.5 ) ;, , then while defining area_circle( ) function later in the, program, care should be taken to make it capable of returning, a floating point value. Note that unless otherwise mentioned, the compiler would assume that this function returns a value, of the type int., [8] Inserting a semicolon at the end of a macro definition.
Page 708 :
706, , Let Us C, , How do you recognize a C programmer? Ask him to write a, paragraph in English and watch whether he ends each, sentence with a semicolon. This usually happens because a C, programmer becomes habitual to ending all statements with a, semicolon. However, a semicolon at the end of a macro, definition might create a problem. For example,, #define UPPER 25 ;, , would lead to a syntax error if used in an expression such as, if ( counter == UPPER ), , This is because on preprocessing, the if statement would take, the form, if ( counter == 25 ), , [9] Omitting parentheses around a macro expansion., #define SQR(x) x * x, main( ), {, int a ;, a = 25 / SQR ( 5 ) ;, printf ( "\n%d", a ) ;, }, , In this example we expect the value of a to be 1, whereas it, turns out to be 25. This so happens because on preprocessing, the arithmetic statement takes the following form:, a = 25 / 5 * 5 ;
Page 709 :
Appendix C: Chasing The Bugs, , 707, , [10] Leaving a blank space between the macro template and the, macro expansion., #define ABS (a) ( a = 0 ? a : -a ), , Here, the space between ABS and (a) makes the preprocessor, believe that you want to expand ABS into (a), which is, certainly not what you want., [11] Using an expression that has side effects in a macro call., #define SUM ( a ) ( a + a ), main( ), {, int w, b = 5 ;, w = SUM( b++ ) ;, printf ( "\n%d", w ) ;, }, , On preprocessing, the macro would be expanded to,, w = ( b++ ) + ( b++ ) ;, , If you are wanting to first get sum of 5 and 5 and then, increment b to 6, that would not happen using the above, macro definition., [12] Confusing a character constant and a character string., In the statement, ch = 'z' ;, , a single character is assigned to ch. In the statement
Page 710 :
708, , Let Us C, , ch = "z" ;, , a pointer to the character string “a” is assigned to ch., Note that in the first case, the declaration of ch would be,, char ch ;, , whereas in the second case it would be,, char *ch ;, , [13] Forgetting the bounds of an array., main( ), {, int num[50], i ;, for ( i = 1 ; i <= 50 ; i++ ), num[i] = i * i ;, }, , Here, in the array num there is no such element as num[50],, since array counting begins with 0 and not 1. Compiler would, not give a warning if our program exceeds the bounds. If not, taken care of, in extreme cases the above code might even, hang the computer., [14] Forgetting to reserve an extra location in a character array for the, null terminator., Remember each character array ends with a ‘\0’, therefore its, dimension should be declared big enough to hold the normal, characters as well as the ‘\0’.
Page 711 :
Appendix C: Chasing The Bugs, , 709, , For example, the dimension of the array word[ ] should be 9, if a string “Jamboree” is to be stored in it., [15] Confusing the precedences of the various operators., main( ), {, char ch ;, FILE *fp ;, fp = fopen ( "text.c", "r" ) ;, while ( ch = getc ( fp ) != EOF ), putch ( ch ) ;, fclose ( fp ) ;, }, , Here, the value returned by getc( ) will be first compared with, EOF, since != has a higher priority than =. As a result, the, value that is assigned to ch will be the true/false result of the, test—1 if the value returned by getc( ) is not equal to EOF,, and 0 otherwise. The correct form of the above while would, be,, while ( ( ch = getc ( fp ) ) != EOF ), putch ( ch ) ;, , [16] Confusing the operator -> with the operator . while referring to a, structure element., Remember, on the left of the operator . only a structure, variable can occur, whereas on the left of the operator -> only, a pointer to a structure can occur. Following example, demonstrates this., main( )
Page 712 :
710, , Let Us C, , {, struct emp, {, char name[35] ;, int age ;, };, struct emp e = { "Dubhashi", 40 } ;, struct emp *ee ;, printf ( "\n%d", e.age ) ;, ee = &e ;, printf ( "\n%d", ee->>age ) ;, }, , [17] Forgetting to use the far keyword for referring memory locations, beyond the data segment., main( ), {, unsigned int *s ;, s = 0x413 ;, printf ( "\n%d", *s ) ;, }, , Here, it is necessary to use the keyword far in the declaration, of variable s, since the address that we are storing in s (0x413), is a address of location present in BIOS Data Area, which is, far away from the data segment. Thus, the correct declaration, would look like,, unsigned int far *s ;, , The far pointers are 4-byte pointers and are specific to DOS., Under Windows every pointer is 4-byte pointer., [18] Exceeding the range of integers and chars.
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Appendix C: Chasing The Bugs, , 711, , main( ), {, char ch ;, for ( ch = 0 ; ch <= 255 ; ch++ ), printf ( "\n%c %d", ch, ch ) ;, }, , Can you believe that this is an indefinite loop? Probably, a, closer look would confirm it. Reason is, ch has been declared, as a char and the valid range of char constant is -128 to, +127. Hence, the moment ch tries to become 128 (through, ch++), the value of character range is exceeded, therefore the, first number from the negative side of the range, -128, gets, assigned to ch. Naturally the condition is satisfied and the, control remains within the loop externally.
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712, , Let Us C
Page 715 :
C, , Creating, Libraries, , 701
Page 716 :
702, , Let Us C, , I, , n Chapter 5 we saw how to add/delete functions to/from, existing libraries. At times we may want to create our own, library of functions. Here we would assume that we wish to, create a library containing the functions factorial( ), prime( ) and, fibonacci( ). As their names suggest, factorial( ) calculates and, returns the factorial value of the integer passed to it, prime( ), reports whether the number passed to it is a prime number or not, and fibonacci( ) prints the first n terms of the Fibonacci series,, where n is the number passed to it. Here are the steps that need to, be carried out to create this library. Note that these steps are, specific to Turbo C/C++ compiler and would vary for other, compilers., , (a) Define the functions factorial( ), prime( ) and fibonacci( ) in, a file, say ‘myfuncs.c’. Do not define main( ) in this file., (b) Create a file ‘myfuncs.h’ and declare the prototypes of, factorial( ), prime( ) and fibonacci( ) in it as shown below:, int factorial ( int ) ;, int prime ( int ) ;, void fibonacci ( int ) ;, , (c) From the Options menu select the menu-item ‘Application’., From the dialog that pops us select the option ‘Library’., Select OK., (d) Compile the program using Alt F9. This would create the, library file called ‘myfuncs.lib’., That’s it. The library now stands created. Now we have to use the, functions defined in this library. Here is how it can be done., (a) Create a file, say ‘sample.c’ and type the following code in it., #include "myfuncs.h", main( )
Page 717 :
Appendix C: Creating Libraries, , 703, , {, int f, result ;, f = factorial ( 5 ) ;, result = prime ( 13 ) ;, fibonacci ( 6 ) ;, printf ( "\n%d %d", f, result ) ;, }, , Note that the file ‘myfuncs.h’ should be in the same directory, as the file ‘sample.c’. If not, then while including ‘myfuncs.h’, mention the appropriate path., (b) Go to the ‘Project’ menu and select ‘Open Project…’ option., On doing so a dialog would pop up. Give the name of the, project, say ‘sample.prj’ and select OK., (c) From the ‘Project’ menu select ‘Add Item’. On doing so a file, dialog would appear. Select the file ‘sample.c’ and then select, ‘Add’. Also add the file ‘myfuncs.lib’ in the same manner., Finally select ‘Done’., (d) Compile and execute the project using Ctrl F9.
Page 718 :
704, , Let Us C
Page 719 :
D, , Hexadecimal, Numbering, • Numbering Systems, • Relation Between Binary and Hex, , 713
Page 720 :
714, , Let Us C, , W, , hile working with computers we are often required to use, hexadecimal numbers. The reason for this is—, everything a computer does is based on binary numbers,, and hexadecimal notation is a convenient way of expressing binary, numbers. Before justifying this statement let us first discuss what, numbering systems are, why computers use binary numbering, system, how binary and hexadecimal numbering systems are, related and how to use hexadecimal numbering system in everyday, life., , Numbering Systems, When we talk about different numbering systems we are really, talking about the base of the numbering system. For example,, binary numbering system has base 2 and hexadecimal numbering, system has base 16, just the way decimal numbering system has, base 10. What in fact is the ‘base’ of the numbering system? Base, represents number of digits you can use before you run out of, digits. For example, in decimal numbering system, when we have, used digits from 0 to 9, we run out of digits. That’s the time we put, a 1 in the column to the left - the ten’s column - and start again in, the one’s column with 0, as shown below:, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 last available digit, 10 start using a new column, 11, 12, 13
Page 721 :
Appendix D: Hexadecimal Numbering, , 715, , 14, ..., ..., , Since decimal numbering system is a base 10 numbering system, any number in it is constructed using some combination of digits 0, to 9. This seems perfectly natural. However, the choice of 10 as a, base is quite arbitrary, having its origin possibly in the fact that, man has 10 fingers. It is very easy to use other bases as well. For, example, if we wanted to use base 8 or octal numbering system,, which uses only eight digits (0 to 7), here’s how the counting, would look like:, 0, 1, 2, 3, 4, 5, 6, 7 last available digit, 10 start using a new column, 11, 12, ..., ..., , Similarly, a hexadecimal numbering system has a base 16. In hex, notation, the ten digits 0 through 9 are used to represent the values, zero through nine, and the remaining six values, ten through, fifteen, are represented by symbols A to F. The hex digits A to F, are usually written in capitals, but lowercase letters are also, perfectly acceptable. Here is how the counting in hex would look, like:, 0, 1
Page 722 :
716, , Let Us C, , 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F last available digit, 10 start using a new column, 11, ..., ..., , Many other numbering systems can also be imagined. For, example, we use a base 60 numbering system, for measuring, minutes and seconds. From the base 12 system we retain our 12, hour system for time, the number of inches in a foot and so on., The moral is that any base can be used in a numbering system,, although some bases are convenient than others., The hex numbers are built out of hex digits in much the same way, the decimal numbers are built out of decimal digits. For example,, when we write the decimal number 342, we mean,, 3 times 100 (square of 10), + 4 times 10, + 2 times 1, , Similarly, if we use number 342 as a hex number, we mean,, 3 times 256 (square of 16)
Page 723 :
Appendix D: Hexadecimal Numbering, , 717, , + 4 times 16, + 2 times 1, , Relation Between Binary and Hex, As it turns out, computers are more comfortable with binary, numbering system. In a binary system, there are only two digits 0, and 1. This means you can’t count very far before you need to start, using the next column:, 0, 1 last available digit, 10 start using a new column, 11, ..., ..., , Binary numbering system is a natural system for computers, because each of the thousands of electronic circuits in the, computer can be in one of the two states—on or off. Thus, binary, numbering system corresponds nicely with the circuits in the, computer—0 means off, and 1 means on. 0 and 1 are called bits, a, short-form of binary digits., Hex numbers are used primarily as shorthand for binary numbers, that the computers work with. Every hex digit represents four bits, of binary information (Refer Figure D.1). In binary numbering, system 4 bits taken at a time can give rise to sixteen different, numbers, so the only way to represent each of these sixteen 4-bit, binary numbers in a simple and short way is to use a base sixteen, numbering system., Suppose we want to represent a binary number 11000101 in a, short way. One way is to find it decimal equivalent by multiplying, each binary digit with an appropriate power of 2 as shown below:
Page 724 :
718, , Let Us C, , 1 * 2 7 + 1 * 2 6 + 0 * 2 5 + 0 * 2 4 + 0 * 2 3 + 1 * 2 2 + 0 * 21 + 1 * 2 0, which is equal to 197., , Hex, , Binary, , Hex, , Binary, , 0, , 0000, , 8, , 1000, , 1, , 0001, , 9, , 1001, , 2, , 0010, , A, , 1010, , 3, , 0011, , B, , 1011, , 4, , 0100, , C, , 1100, , 5, , 0101, , D, , 1101, , 6, , 0110, , E, , 1110, , 7, , 0111, , F, , 1111, , Figure D.1, Another method is much simpler. Just look at Figure D.1. From it, find out the hex digits for the two four-bit sets (1100 and 0101)., These happen to be C and 5. Therefore, the binary number’s hex, equivalent is C5. You would agree this is a easier way to represent, the binary number than to find its decimal equivalent. In this, method neither multiplication nor addition is needed. In fact, since, there are only 16 hex digits, it’s fairly easy to memorize the binary, equivalent of each one. Quick now, what’s binary 1100 in hex?, That’s right C. You are already getting the feel of it. With a little, practice it is easy to translate even long numbers into hex. Thus,, 1100 0101 0011 1010 binary is C53A hex., As it happens with many unfamiliar subjects, learning hexadecimal, requires a little practice. Try your hand at converting some binary, numbers and vice versa. Soon you will be talking hexadecimal as, if you had known it all your life.
Page 725 :
E, , ASCII Chart, , 719
Page 726 :
720, , Let Us C, , T, , here are 256 distinct characters used by IBM compatible, family of microcomputers. Their values range from 0 to, 255. These can be grouped as under:, , Character Type, , No. of Characters, , Capital letters, , 26, , Small-case Letters, , 26, , Digits, , 10, , Special Symbols, , 32, , Control Character, , 34, , Graphics Character, , 128, , Total, , 256, , Figure E.1, Out of the 256 character set, the first 128 are often called ASCII, characters and the next 128 as Extended ASCII characters. Each, ASCII character has a unique appearance. The following simple, program can generate the ASCII chart:, main( ), {, int ch ;, for ( ch = 0 ; ch <<= 255 ; ch++ ), printf ( "%d %c\n", ch, ch ) ;, }, , This chart is shown on the following page. Out of the 128 graphic, characters (Extended ASCII characters), there are characters that, are used for drawing single line and double line boxes in text, mode. For convenience these characters are shown in Figure E.2.
Page 727 :
721, , Appendix E: ASCII Chart, , 218 129, , 194, , ┌ ─ ┬, , 191, , ┐, , └, 213╒, , ┼, , ┤180, , 193, , ┴, ╤, , ┘, ╕184, , 217, , ╪, , ╠204, 200, , ╚, ╓, , 214, , 209, 198╞, , ╡181, , ╟ 199, , ╘, , Figure E.2, , ╗, , ╧, , 190, , ╬, , ╣185, , 206, 202, , ╩, ╥, , ╝188, ╖183, , 210, , ╫, , ╢182, , 215, , 216, 212, , 187, , 186, , 197, 192, , 203, , ═, , ─, , 179, , 195├, , 201 205, , ╔ ═ ╦, , ╛196, , ╙, , 211, , ╨, , 208, , ╜, , 189
Page 728 :
722, , Let Us C, , Value, , Char, , Value, , Char, , Value, , 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, , ☺, ☻, ♥, ♦, ♣, ♠, ●, ◘, ○, ◙, ♂, ♀, ♪, ♫, ☼, ►, ◄, ↕, ‼, ¶, §, , 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, , ▬, ↕, ↑, ↓, →, ←, ⌐, ↔, ▲, ▼, , 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, , !, ", #, $, %, &, ’, (, ), *, +, , Char, ,, ., /, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, :, ;, <, =, >, ?, @, A, , Value, , Char, , Value, , Char, , Value, , Char, , 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, , B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, , 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, , X, Y, Z, [, \, ], ^, _, `, a, b, c, d, e, f, g, h, i, j, k, l, m, , 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, , n, o, p, q, r, s, t, u, v, w, x, y, z, {, |, }, ~, м, н, Ç, ü, é, â
Page 730 :
724, , Let Us C
Page 731 :
F, , Helper.h File, , 725
Page 734 :
728, , Let Us C
Page 735 :
G, , Boot Parameters, , 729
Page 736 :
730, , Let Us C, , T, , he disk drives in DOS and Windows are organized as zerobased drives. That is, drive A is drive number 0, drive B is, drive number 1, drive C is drive number 2, etc. The hard, disk drive can be further partitioned into logical partitions. Each, drive consists of four logical parts—Boot Sector, File Allocation, Table (FAT), Directory and Data space. When a file/directory is, created on the disk, instead of allocating a sector for it, a group of, sectors is allocated. This group of sectors is often known as a, cluster. How many sectors together form one cluster depends, upon the capacity of the disk. As the capacity goes on increasing,, so also does the maximum cluster number. Accordingly, we have, 12-bit, 16-bit or 32-bit FAT. In a 12-bit FAT each entry is of 12, bits. Since each entry in FAT represents a cluster number, the, maximum cluster number possible in a 12-bit FAT is 212 (4096)., Similarly, in case of a 16-bit FAT the maximum cluster number is, 216 (65536). Also, for a 32-bit FAT the maximum cluster number, is 228 (268435456. Only 28 of the 32 bits are used in this FAT)., All FAT systems are not supported by all versions of DOS and, Windows. For example, the 32-bit FAT system is supported only, in Win 95 OSR2 version or later. There are differences in the, organization of contents of Boot Sector, FAT and Directory in, FAT12/FAT16 system on one hand and FAT32 on the other., In Chapter 19 Figure 19.6 we saw the breakup of the contents of, the boot sector of a 12-bit FAT. Given below are the contents of a, boot sector of 16-bit FAT and a 32-bit FAT.
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731, , Appendix G: Boot Parameters, , Description, , Length, , Typical Values, , Jump instruction, , 3, , EB3C90, , OEM name, , 8, , MSWIN4.1, , Bytes per sector, , 2, , 512, , Sectors per cluster, , 1, , 64, , Reserved sectors, , 2, , 1, , Number of FAT copies, , 1, , 2, , Max. Root directory entries, , 2, , 512, , Total sectors, , 2, , 0, , Media descriptor, , 1, , F8, , Sectors per FAT, , 2, , 256, , Sectors per track, , 2, , 63, , No. of sides, , 2, , 255, , Hidden sectors, , 4, , 63, , Huge sectors, , 4, , 4192902, , BIOS drive number, , 1, , 128, , Reserved sectors, , 1, , 1, , Boot signature, , 1, , 41, , Volume ID, , 4, , 4084677574, , Volume label, , 11, , ICIT, , File system type, , 8, , FAT16, , Figure G.1, Let us now take a look at the 32-bit FAT system’s boot sector, contents. These are shown in Figure G.2.
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732, , Let Us C, , Description, , Length, , Typical Values, , Jump instruction, , 3, , EB5890, , OEM name, , 8, , MSWIN4.1, , Bytes per sector, , 2, , 512, , Sectors per cluster, , 1, , 8, , Reserved sectors, , 2, , 51, , Number of FAT copies, , 1, , 2, , Root directory entries, , 2, , 0, , Total sectors, , 2, , 0, , Media descriptor, , 1, , F8, , Sectors per FAT, , 2, , 0, , Sectors per track, , 2, , 63, , No. of sides, , 2, , 255, , Hidden sectors, , 2, , 63, , High word of hidden sectors, , 4, , 63, , Huge sectors, , 4, , 4192902, , High word of huge sectors, , 2, , 4192902, , Sectors per FAT, , 2, , 4095, , High word of sectors per FAT, , 2, , 4095, , Drive description flag, , 2, , 0, , File system version, , 2, , 0, , Root directory starting cluster, , 2, , 2, , High word of root directory, starting cluster, , 2, , 2, , File system information sector, , 2, , 1, , Back up boot sector, , 2, , 6, , Reserved, , 6, , 0, continued…
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733, , Appendix G: Boot Parameters, …continued, BIOS drive number, , 1, , 128, , Reserved, , 1, , 0, , Boot signature, , 1, , 41, , Volume ID, , 4, , 649825316, , Volume label, , 11, , ICIT, , File system type, , 8, , FAT32, , Figure G.2, There are significant changes in the contents of the boo t sector of, a 32-bit FAT system. The entries ‘Number of hidden sectors’ and, ‘Huge sectors’ have now been made 4-byte entries. The first two, bytes contain the low word of the value, whereas, the next two, bytes contain the high word value., The number of sectors per FAT in a 32-bit file system is likely to, exceed what can be accommodated in two bytes. Hence the entry, ‘Sectors per FAT’ for a disk with a 32-bit file system would, typically have a value 0. The value of ‘Sectors per FAT’ is now, stored as a 4-byte entity, with the similar arrangement of low word, and high word as discussed earlier., The boot sector of a 32-bit FAT system also has new entries like, ‘Drive description flag’, ‘File system version’ ‘Starting cluster, number of the root directory’, ‘Sector number of the file system, information sector’, and the sector number of the ‘Backup boot, sector’., The ‘Drive description flag’ is a two-byte entity. Bit 8 of this flag, indicates whether or not the information written to the active FAT, will be written to all copies of the FAT. The low four bits of this, entry contains the 0-based FAT number of the active FAT. These, bits are meaningful only if bit 8 is set.
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734, , Let Us C, , In the entry ‘File system version number’ the high byte contains, the major version number, whereas, the low byte contains the, minor version number., The entry ‘File system information sector’ contains a value, indicating the sector number where the file system information is, present. This file system information consists of the fields shown, in Figure G.3., , Description, , Length, , File system signature, , 4, , Total number of free clusters, , 4, , Sector number of the next free cluster, , 4, , Reserved, , 6, , Figure G.3, The entry ‘File information sector’ contains a value OFFFFh if, there is no such sector. The entry ‘Backup boot sector’ contains a, value 0FFFFh is there is no backup boot sector. Otherwise this, value is any non-zero value less than the reserved sector count.
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H, , Linux Installation, , 735
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736, , Let Us C, , T, , his appendix gives the steps that are to be carried out for, installing Red Hat Linux 9.0. In addition I have also, indicated a few commands that are necessary to compile, and execute the programs given in Chapters 20 and 21. Follow the, steps mentioned below to carry out the installation., (a), (b), (c), (d), (e), (f), (g), (h), (i), (j), (k), , (l), (m), (n), (o), (p), (q), (r), , Configure the system to boot from CDROM drive., Insert the first CD in the drive and boot the system from it., Hit ‘Enter’ key when the ‘boot’ prompt appears., Select the ‘Skip’ option in the "CD Found" dialog box., Click on the ‘Next’ button in the ‘Welcome’ screen., Click on the ‘Next’ button in the ‘Language selection’ screen., Click on the ‘Next’ button in the ‘Keyboard’ screen., Click on the ‘Next’ button in the ‘Mouse Configuration’, screen., Select the ‘Custom’ option in the ‘Installation Type’ screen, and then click on the ‘Next’ button., Click on the ‘Next’ button in the ‘Disk Partitioning Setup’, screen., Select the ‘Keep all partitions and use existing free space’, option in the ‘Automatic Partitioning’ screen and then click, on the ‘Next’ button. Ignore any warnings generated by, clicking on the ‘OK’ button., Click on the ‘Next’ button in the ‘Boot loader configuration’, screen., Click on the ‘Next’ button in the ‘Network configuration’, screen., Click on the ‘Next’ button in the ‘Firewall configuration’, screen., Click on the ‘Next’ button in the ‘additional language, support’ screen., Select a suitable option in the ‘Time zone offset’ screen and, click on the ‘Next’ button., Type a password for the root account in the ‘Set root, password’ screen and then click on the ‘Next’ button., Click on the ‘Next’ button in the ‘Authentication, configuration’ screen.
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Appendix H: Linux Installation, , 737, , (s) In the ‘Package group selection’ screen make sure that you, select the following options—X window system, K desktop, environment, Development tools, GNOME software, development and then click on the ‘Next’ button., (t) Select ‘No’ option in the ‘Boot diskette creation’ screen, (u) Click on the ‘Next’ button in the ‘Graphical Interface (x), configuration’ screen., (v) Click on the ‘Next’ button in the ‘Monitor configuration’, screen., (w) In the ‘Customize graphical configuration’ screen select the, ‘Graphical’ option and then click on ‘Next’ button., (x) Once the system restarts configure the system to boot from, Hard Disk., , Using Red Hat Linux, For logging into the system enter the username and password and, select the session as KDE (K Desktop Environment). Once you, have logged in, to start typing the program use the following menu, options:, KMenu | Run Command, , A dialog would now pop up. In this dialog in the command edit, box type KWrite and then click on the Ok button. Now you can, type the program and save it., To compile the program you need to go the command prompt., This can be done using the following menu option., KMenu | System Tools | Terminal, , Once at the command prompt you can use the gcc compiler to, compile and execute your programs. You can launch another, instance of the command prompt by repeating the step mentioned, above.
