C #ifndef

The #ifndef preprocessor directive checks if macro is not defined by #define. If yes, it executes the code otherwise #else code is executed, if present.

Syntax:

#ifndef MACRO
//code
#endif

Syntax with #else:

#ifndef MACRO
//successful code
#else
//else code
#endif

C #ifndef example

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

#include <stdio.h>
#include <conio.h>
#define INPUT
void main() {
int a=0;
#ifndef INPUT
a=2;
#else
printf("Enter a:");
scanf("%d", &a);
#endif       
printf("Value of a: %d\n", a);
getch();
}

Output:

Enter a:5
Value of a: 5

But, if you don't define INPUT, it will execute the code of #ifndef.

#include <stdio.h>
#include <conio.h>
void main() {
int a=0;
#ifndef INPUT
a=2;
#else
printf("Enter a:");
scanf("%d", &a);
#endif       
printf("Value of a: %d\n", a);
getch();
}

Output: