C #if

The #if preprocessor directive evaluates the expression or condition. If condition is true, it executes the code otherwise #elseif or #else or #endif code is executed.

Syntax:

#if expression
//code
#endif

Syntax with #else:

#if expression
//if code
#else
//else code
#endif

Syntax with #elif and #else:

#if expression
//if code
#elif expression
//elif code
#else
//else code
#endif

C #if example

Let's see a simple example to use #if preprocessor directive.

#include <stdio.h>
#include <conio.h>
#define NUMBER 0
void main() {
#if (NUMBER==0)
printf("Value of Number is: %d",NUMBER);
#endif       
getch();
}

Output:

Value of Number is: 0

Let's see another example to understand the #if directive clearly.

#include <stdio.h>  
#include <conio.h>  
#define NUMBER 1
void main() {
clrscr();
#if (NUMBER==0)
printf("1 Value of Number is: %d",NUMBER);
#endif

#if (NUMBER==1)
printf("2 Value of Number is: %d",NUMBER);
#endif
getch();
}

Output: