c preprocessor undefine

C #undef:

A macro value in C replaces a part of a program with a value already defined by #define directive. The #undef is a preprocessor directive which is used to undefine the value defined by #define.

Syntax:

#undef macro_Name

Example:

#include<stdio.h>  
#define e 2.718
#undef e
void main()
{    
printf("e :%f\n",e );    
}

Output

error: 'e' undeclared