Constants in C

A constant is a value or variable that can't be changed in the program, for example: 10, 20, 'a', 3.4, "c programming" etc.

There are different types of constants in C programming.

List of Constants in C

Constant	Example
Decimal Constant	10, 20, 450 etc.
Real or Floating-point Constant	10.3, 20.2, 450.6 etc.
Octal Constant	021, 033, 046 etc.
Hexadecimal Constant	0x2a, 0x7b, 0xaa etc.
Character Constant	'a', 'b', 'x' etc.
String Constant	"c", "c program", "c in javatpoint" etc.

2 ways to define constant in C

There are two ways to define constant in C programming.

1. const keyword
2. #define preprocessor

1) C const keyword

The const keyword is used to define constant in C programming.

1. const float PI=3.14;  

Now, the value of PI variable can't be changed.

1. #include<stdio.h>    
2. int main(){    
3.     const float PI=3.14;    
4.     printf("The value of PI is: %f",PI);    
5.     return 0;  
6. }     

Output:

The value of PI is: 3.140000

If you try to change the the value of PI, it will render compile time error.

1. #include<stdio.h>    
2. int main(){    
3. const float PI=3.14;     
4. PI=4.5;    
5. printf("The value of PI is: %f",PI);    
6.     return 0;  
7. }     

Output: Compile Time Error: Cannot modify a const object

C #define

The #define preprocessor directive is used to define constant or micro substitution. It can use any basic data type.

Syntax:

1. #define token value  

Let's see an example of #define to define a constant.

1. #include <stdio.h>  
2. #define PI 3.14  
3. main() {  
4.    printf("%f",PI);  
5. }  

Output:

3.140000

Let's see an example of #define to create a macro.

1. #include <stdio.h>  
2. #define MIN(a,b) ((a)<(b)?(a):(b))  
3. void main() {  
4.    printf("Minimum between 10 and 20 is: %d\n", MIN(10,20));    
5. }  

Output:

Minimum between 10 and 20 is: 10