#define , why in this example the answer is 10, but not 15?

Question

#include <stdio.h>
# define prod(i,j) i*j
int main() {
int x=3, y=4;
printf("%d", prod(x+2,y-1));

return 0;
}

Answer 1

Because macro function behave weirdly, what actually happens is the preprocessor replaces prod(x+2,y-1) with x+2*y-1 so the equation becomes 3+2*4-1 which is equal to 10 and not 15 if you want it to work just add parentheses prod((x+2),(y-1)) this should fix it

Answer 2

3+2*4-1=10
write:
# define prod(i,j) (i)*(j)
or
printf("%d", prod((x+2),(y-1)));
then you get (3+2)*(4-1)=15