Output of the following code- macros in c

Question

For the following code I am getting output as- Geeks.

#include <stdio.h>
#define ISEQUAL(X, Y) X == Y
int main()
{
    #if ISEQUAL(X, 0)
        printf("Geeks");
    #else
        printf("Quiz");
    #endif
    return 0;
}

Explain the reason for such output.

Answer 1

The conditional macro #if ISEQUAL(X, 0) is expanded to #if X == 0 . After the pre-processing is over, all the undefined macros are initialized with default value 0 . Since macro X has not been defined, it is initialized with 0 . So, "Geeks" is printed.