Unexpected output printf statement

Question

Why outputs of i and j in the following two printf()s are different?

#include <cstdio>
#define MAX(x,y) (x)>(y)?(x):(y)
int main()
{
    int i=10,j=5,k=0;
    k==MAX(i++,++j);
    printf("%d %d %d\n",i,j,k);
    i=10,j=5,k=0;
    k=MAX(i++,++j);
    printf("%d %d %d\n",i,j,k);
    return 0;
}

I think only k should differ, but it is showing different output for all, i, j and k, in printf() statement. What is the reason?

Output :

11 7 0
12 6 11

Answer 1

k==MAX(i++,++j); is translated to:

  k==(i++) > (++j) ? (i++) : (++j);
/* 2       1       3  : Precedance */

In your case i = 10 and j = 5, so the comparision would be 10 v/s 6 which would be true or 1 with side effect i becomes 11 and j becomes 6. (Recall that: Macros are not functions)

As k = 0 and not equal to 1, k==(i++) > (++j) would be false and ++j would be executed. Causing j to be incremented to 7.

In latter case, there is no modification to the value of variables.

When can I expect same output in both case? Even if you change the macro with a function, your arguments have the side effects, which would cause the output 11 6 for i j, so you are changing the variables using ++ operator, don't expect both statements to have the same values.

Answer 2

• Let's analyse the output from first printf()

  1. Assuming you meant = instead of == in your code,

     k==MAX(i++,++j);
     

     gets translated to

      k = (i++) > (++j) ? (i++): (++j)
     

     which gets evaluated changing all the values of i, j and k.

  2. Even if you meant ==, as mentioned in the answer by Mr. Mohit Jain, you'll face similar issue with precedence.

  Have a look at operator precedence for better understanding.

• Let's analyse the output from second printf()

In the second printf() statement, you're re-setting the values of i, j, k and not using the MACRO. So, those values will get printed. Simple.

So, TL;DR, before first printf(), the MACRO changes all the three values for i, j, k. So, your output for first and second printf() differs for all the three variable values.

---

FWIW, if you meant the MACRO to behave as a ternary conditional expression , you need to change your MACRO definition to

#define MAX(x,y) ( (x)>(y) ? (x) : (y) )

To avoid issues with operator precedence after MACRO expansion.

Answer 3

You have side-effects.

MAX(i++,++j)

is expanded to

(i++)>(j++)?(i++):(j++)

And i is modified twice in the macro call.

Moreover, you have to add parens to your macro definition:

#define MAX(x,y) ((x)>(y)?(x):(y))

Answer 4

i++ and ++j both increment i and j, so you change their values before printing.

Answer 5

The macro is a red herring. OP did not ask about k but "Why output of i and j in following two printf are different?" The answer is because i and j have been modified between setting their values and printing, in the first case, but not in the second case.