6.1.2 Constant Variables

• Constant variables you declare yourself like variables. 
• The advantage of using Constant variables is that if you have to change constant later, then you do not need to change it in many places in the program. 
• You just change the value of constant variable and in that program your You change.
• You can declare constant variables in two ways. 
• These are being described below.

1. User #Define Directive 
2. Using const Keyword

❖ User #Define Directive :-

• #define is a pre processor directive and you declare constant variables by using it.
• This directive is declared before the main function at the beginning of the constant variables program. 
• You can use any of the program's constant variables defined by this directive.

The use of constant variables is being explained by the example below.

# include <stdio.h>

/* Defining constant */

#define result 10

int main()

{

int a=5, b=6;

/* Wrong, (error) Value of constant result variable can not be changed. */

result = a + b;

printf(“%d”, result);

return 0;

}

The above program produces the below given output.

error : lvalue required as left operand of assignment     result = a + b;

❖ Using const Keyword :-

• You can declare constant variables even by const keyword in C language. 
• If you want to use constant variable only in a function, you can declare constant variable through this keyword.

The use of const keyword in C language is being explained by the example below.

#include<stdio.h>

int main()

{

const int a=5;

const int b=6;

int c; /* Adding two constants */

c = a+b;

printf(“Result is : %d”,a);

return 0;

}

The above program produces the below given output.

Result is : 11

❖ Types of C Constants :-

1. Integer Constants
2. Floating-Point Constants
3. Character Constants
4. String Constants
5. Enumeration Constants

Apart from these, the escape sequence characters in C language are also considered

constants.