C #ifdef

The #ifdef preprocessor directive checks if macro is defined by #define. If yes, it executes the code otherwise #else code is executed, if present.

Syntax:

Syntax with #else:

C #ifdef example

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

Output:

Value of a: 2

But, if you don't define NOINPUT, it will ask user to enter a number.

Output:

Enter a:5
Value of a: 5