Operator associavity problem with pre and post increment :(

Question

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Could anyone explain these undefined behaviors (i = i++ + ++i , i = i++, etc…)

#include< stdio.h >

int main()
{
    int i = 1;
    int x = ++i * ++i * ++i;
    printf("%d\n", x);
    printf("%d\n\n",i);

    return 0;
}

Im getting output of 1!! and 4 in gcc. I use ubuntu linux

Well, I'm getting "warning: operation on 'i' may be undefined" when I compile with `gcc -Wall`. You should use -Wall and pay attention to the warnings. — sverre, Jun 07 '11 at 09:59
There's a C++-faq question on this: http://stackoverflow.com/questions/4176328/undefined-behavior-and-sequence-points — Steve Jessop, Jun 07 '11 at 10:57

score 2 · Answer 1 · answered Jun 07 '11 at 09:55

2

Undefined Behaviour this is:

int x = ++i * ++i * ++i;

Don't do it!!!!

answered Jun 07 '11 at 09:55

Tony The Lion

score 2 · Accepted Answer · answered Jun 07 '11 at 09:55

2

The behaviour of your code is undefined since i is modified more than once between sequence points:

int x = ++i * ++i * ++i;

See the FAQ (I urge you to read the entire section 3).

answered Jun 07 '11 at 09:55

NPE

2 Answers2