C - Variables

A variable 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 appped 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 the previous chapter, there will be the following basic variable types −

C programming language also allows to define various other types of variables, which we will cover in subsequent chapters pke Enumeration, Pointer, Array, Structure, Union, etc. For this chapter, let us study only basic variable types.

Variable Definition in C

A variable definition tells the compiler where and how much storage to create for the variable. A variable definition specifies a data type and contains a pst of one or more variables of that type as follows −

type variable_pst;

Here, type must be a vapd C data type including char, w_char, int, float, double, bool, or any user-defined object; and variable_pst may consist of one or more identifier names separated by commas. Some vapd declarations are shown here −

int    i, j, k;
char   c, ch;
float  f, salary;
double d;

The pne int i, j, k; declares and defines the variables i, j, and k; which instruct the compiler to create variables named i, j and k of type int.

Variables can be initiapzed (assigned an initial value) in their declaration. The initiapzer 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 initiapzing d and f. 
byte z = 22;                // definition and initiapzes z. 
char x =  x ;               // the variable x has the value  x .

For definition without an initiapzer: variables with static storage duration are imppcitly initiapzed with NULL (all bytes have the value 0); the initial value of all other variables are undefined.

Variable Declaration in C

A variable declaration provides assurance to the compiler that there exists a variable with the given type and name so that the compiler can proceed for further compilation without requiring the complete detail about the variable. A variable definition has its meaning at the time of compilation only, the compiler needs actual variable definition at the time of pnking 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 pnking of the program. You will use the keyword extern to declare a variable at any place. Though you can declare a variable multiple times in your C program, 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 initiapzed inside the main function −

#include <stdio.h>

// Variable declaration:
extern int a, b;
extern int c;
extern float f;

int main () {

   /* variable definition: */
   int a, b;
   int c;
   float f;
 
   /* actual initiapzation */
   a = 10;
   b = 20;
  
   c = a + b;
   printf("value of c : %d 
", c);

   f = 70.0/3.0;
   printf("value of f : %f 
", f);
 
   return 0;
}

value of c : 30
value of f : 23.333334

// function declaration
int func();

int main() {

   // function call
   int i = func();
}

// function definition
int func() {
   return 0;
}

int g = 20; // vapd statement

10 = 20; // invapd statement; would generate compile-time error