C: Defined behaviour returning wrong result

Question

So I have an exam tomorrow and I encountered this on my questions.

#define mul(x,y) (x * y)
#include <stdio.h>
#include <string.h>

int main()
{
    int x = 3;
    int y = 4;
    int z = 0;
    z = mul(x+1, y+1);
    printf("4 * 5 = %d \n", z);
    return 0;
}

my question is, why is this outputting 8 instead of 20. Because when I replace z= mul(x+1,y+1) with z= mul((x+1),(y+1)) I get the correct answer of 20

it expands to `(x + 1 * y + 1)`, which is probably not what you'd expect, you need some more brackets. — Matthew, Apr 19 '17 at 19:28
Read your last sentence carefully again. See the output of the preprocessor and then think about what you learned at your elementary school math class. — too honest for this site, Apr 19 '17 at 19:29
@EdHeal: Oh, macros do have their usage. Just not where a(n `inline`) function will do as well. — too honest for this site, Apr 19 '17 at 19:30
@Olaf - I agree they sometimes do. But better to avoid them though IMHO — Ed Heal, Apr 19 '17 at 19:31
@EdHeal: I normally keep it with writing understandable/readable code which can also be debugged properly (unless optimisations are unavoidable). If a macro suits better for this goal, I'm fine. If both are similarily suited, the function wins. Until now, no user of my code (that includes myself) complained:-) But I'm quite sure that's what you mean, too. — too honest for this site, Apr 19 '17 at 19:36
@EdHeal: Not often. But my customers and co-workers well know I take a complain serious if there are good arguments. Despite what one might think, I'm very willing to acept if I'm **proved** wrong. (notice the emphasis, though:-) — too honest for this site, Apr 19 '17 at 19:39

Alex Lop. · Answer 1 · 2017-04-19T20:19:59.393

Macro #define mul(x,y) (x * y) actually tells the pre-compiler that if it finds string mul(any pattern X, any pattern Y), it should replace it with (any pattern X * any pattern Y).

So what do you have in your example?

int x = 3;
int y = 4;
int z = 0;
z = mul(x+1, y+1);

After mul is replaced, you get

int x = 3;
int y = 4;
int z = 0;
z = x+1*y+1;

==> z = 3 + 1*4 + 1 = 8

The best practice is to surround each macro parameter by brackets. In your example (to achieve the desired result) it should be:

#define mul(x,y) ((x) * (y))

C: Defined behaviour returning wrong result

1 Answers1