C programming chapter 6

  

C programming by UAH TECHTUBE

C programming chapter 6

C Variables

Avariable is nothing but a name given to a storage area that our programs can

manipulate. Each variable in C has a specific type, which determines the size and layout of 

the variable's memory; the range of values that can be stored within that memory; and the 

set of operations that can be applied to the variable.

The name of a variable can be composed of letters, digits, and the underscore character. It 

must begin with either a letter or an underscore. Upper and lowercase letters are distinct 

because C is case-sensitive. Based on the basic types explained in previous chapter, there 

will be the following basic variable types:

Type Description

Char Typically a single octet(one byte). This is an integer type.

Int The most natural size of integer for the machine.

Float A single-precision floating point value.

Double A double-precision floating point value.

Void Represents the absence of type.

C programming language also allows to define various other types of variables, which we 

will cover in subsequent chapters like Enumeration, Pointer, Array, Structure, Union, etc. 

For this chapter, let us study only basic variable types.

Variable Definition in C:

A variable definition means to tell the compiler where and how much to create the storage for the 

variable. A variable definition specifies a data type and contains a list of one or more variables of 

that type as follows:

type variable_list;

Here, type must be a valid C data type including char, w_char, int, float, double, bool or any user￾defined object, etc., and variable_list may consist of one or more identifier names separated by 

commas. Some valid declarations are shown here:

int i, j, k;

char c, ch;

float f, salary;

double d;

The line int i, j, k; both declares and defines the variables i, j and k; which instructs the compiler 

to create variables named i, j and k of type int.

Variables can be initialized (assigned an initial value) in their declaration. The initializer consists of 

an equal sign followed by a constant expression as follows:

type variable_name = value;

Some examples are:

extern int d = 3, f = 5; // declaration of d and f.

int d = 3, f = 5; // definition and initializing d and f.

byte z = 22; // definition and initializes z.

char x = 'x'; // the variable x has the value 'x'.

For definition without an initializer: variables with static storage duration are implicitly initialized 

with NULL (all bytes have the value 0); the initial value of all other variables is undefined.

Variable Declaration in C:

A variable declaration provides assurance to the compiler that there is one variable existing with 

the given type and name so that compiler proceed for further compilation without needing 

complete detail about the variable. A variable declaration has its meaning at the time of 

compilation only, compiler needs actual variable declaration at the time of linking of the program.

A variable declaration is useful when you are using multiple files and you define your variable in 

one of the files, which will be available at the time of linking of the program. You will 

use extern keyword to declare a variable at any place. Though you can declare a variable multiple 

times in your C program but it can be defined only once in a file, a function or a block of code.

Example

Try the following example, where variables have been declared at the top, but they have been 

defined and initialized inside the main function:

#include <stdio.h>

// Variable definition:

extern int a, b;

extern int c;

extern float f;

int main ()

{

// Variable definition:

int a, b;

int c;

float f;

// actual initialization

 a =10;

 b =20;

 c = a + b;

 printf("value of c : %d \n", c);

 f = 70.0/3.0;

 printf("value of f : %f \n", f);

 return 0;

}

When the above code is compiled and executed, it produces the following result:

value of c : 30

value of f : 23.333334

Same concept applies on function declaration where you provide a function name at the time of its 

declaration and its actual definition can be given anywhere else. For example:

// function declaration

int func();

int main()

{ 

 // function call 

 int i = func();

} 

// function definition

int func()

{ 

 return 0;

}

Lvalues and Rvalues in C

There are two kinds of expressions in C:

1. lvalue: An expression that is an lvalue may appear as either the left-hand or right-hand 

side of an assignment.

2. rvalue: An expression that is an rvalue may appear on the right- but not left-hand side 

of an assignment.

Variables are lvalues and so may appear on the left-hand side of an assignment. Numeric 

literals are rvalues and so may not be assigned and cannot appear on the left-hand side. 

Following is a valid statement:

int g = 20;

But following is not a valid statement and would generate compile-time error:

10  -  20