How does the below program output `C89` when compiled in C89 mode and `C99` when compiled in C99 mode?

Question

I've found this C program from the web:

#include <stdio.h>

int main(){

    printf("C%d\n",(int)(90-(-4.5//**/
    -4.5)));

    return 0;
}

The interesting thing with this program is that when it is compiled and run in C89 mode, it prints C89 and when it is compiled and run in C99 mode, it prints C99. But I am not able to figure out how this program works.

Can you explain how the second argument of printf works in the above program?

Answer 1

C99 allows //-style comments, C89 does not. So, to translate:

C99:

 printf("C%d\n",(int)(90-(-4.5     /*Some  comment stuff*/
                         -4.5)));
// Outputs: 99

C89:

printf("C%d\n",(int)(90-(-4.5/      
                         -4.5)));
/* so  we get 90-1 or 89 */