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:

Syntax with #else:

C #ifndef example

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

Output:

Enter a:5
Value of a: 5

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

Output:

Value of a: 2