How to learn C Programming

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Published Date:09-07-2017
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Let Us C Fifth Edition Yashavant P. Kanetkar 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 2 Let Us C 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 B 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 An opinion that is often heard today is – “C has been already superceded by languages like C++, C and Java, so why bother to Chapter 1: Getting Started 3 learn C today”.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. 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 Chapter 1: Getting Started 5 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 Steps in learning C: Alphabets Constants Digits Variables Special sy- Instructions Program Keywords mbols 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. 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. Chapter 1: Getting Started 7 x 3 x 5 x = 3 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. 8 Let Us C C Constants Primary Constants Secondary Constants Array Integer Constant Pointer Real Constant Structure Character Constant 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 (a) An integer constant must have at least one digit. (b) It must not have a decimal point. (c) It can be either positive or negative. (d) 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. Truly speaking the range of an Integer constant depends upon the compiler. For a 16-bit compiler like Turbo C or Turbo C++ the 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 16- bit 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) A real constant must have at least one digit. (b) It must have a decimal point. (c) It could be either positive or negative. (d) Default sign is positive. (e) 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. 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' '=' 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 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. Chapter 1: Getting Started 13 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. 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 ) ; 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. 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 ; 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 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 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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