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 with #else:
C #ifndef example
Let's see a simple example to use #ifndef preprocessor directive.
Enter a:5 Value of a: 5
But, if you don't define INPUT, it will execute the code of #ifndef.
Value of a: 2
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