c preprocessor ifdef

by

The #ifdef is a preprocessor directive in C which is used to execute a piece of code, only if the specified macro is already defined by #define.

Syntax:

#ifdef MACRO

// code to be executed if macro is defined

#endif

OR

#ifdef MACRO

// code to be executed if macro is defined

#else

// code to be executed if macro is not defined

#endif

Example:

#include<stdio.h>  
#define E 2.718
void main()
{    
#ifdef E
printf("E: %f", E);
 
#else  
printf("E is not defined.");  
 
#endif    	 
}

Output

E: 2.718000