GATE EXAM PREPARATION IN "C" - macro-preprocessor

#include <stdio.h>
#if X == 3
    #define Y 3
#else
    #define Y 5
#endif

int main()
{
    printf("%d", Y);
    return 0;
}

What is the output of the above program?
(A) 3
(B) 5
(C) 3 or 5 depending on value of X
(D) Compile time error


Answer: (B) 

Explanation: In the first look, the output seems to be compile-time error because macro X has not been defined. In C, if a macro is not defined, the pre-processor assigns 0 to it by default. Hence, the control goes to the conditional else part and 5 is printed. See the next question for better understanding.