int a = 1; printf("%d, %d\n", (a++), a);

Asked Mar 18 '15 at 11:42

Active Mar 18 '15 at 11:44

Viewed 43 times

#include <stdio.h>

void main(void)
{
    int a = 1;
    printf("%d, %d\n", a++, a);
    a = 1;
    printf("%d, %d\n", (a++), a);
    a = 1;
    printf("%d, %d\n", ++a, a);
}

I wonder Why is this result

1, 2
1, 2
2, 2

rather than

1, 2
2, 2
2, 2

By the way, I use visual studio 2013 to build this.

edited Mar 18 '15 at 11:44

Spikatrix

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asked Mar 18 '15 at 11:42

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1

[Undefined Behavior](http://en.wikipedia.org/wiki/Undefined_behavior) – Spikatrix Mar 18 '15 at 11:43
Undefined and unspecified behavior. – Sourav Ghosh Mar 18 '15 at 11:44
1

See [Why are these constructs (using ++) undefined behavior?](http://stackoverflow.com/q/949433/1708801) but there is a better duplicate someplace – Shafik Yaghmour Mar 18 '15 at 11:44
@ShafikYaghmour A closer dupe : [This](http://stackoverflow.com/q/27877872/2173917). – Sourav Ghosh Mar 18 '15 at 11:46
1

@SouravGhosh But don't use closed questions as duplicates: use one of the threads listed below "frequent", because every "close as duplicate" counts towards the question becoming on of the "frequent" ones. – Lundin Mar 18 '15 at 11:48
1

http://stackoverflow.com/questions/4176328/undefined-behavior-and-sequence-points Could also be the duplicate – Lundin Mar 18 '15 at 11:48
@SouravGhosh it is heavily downvoted which makes it a bad dup for me but the [duplicate is is linked to](http://stackoverflow.com/q/376278/1708801) is good. – Shafik Yaghmour Mar 18 '15 at 11:48
@Lundin crossing tags is not good and there are definitely better C duplciates – Shafik Yaghmour Mar 18 '15 at 11:49
@ShafikYaghmour In this case, there's really not any notable differences between the C and C++ languages. – Lundin Mar 18 '15 at 11:51
@ShafikYaghmour Right. Understood. – Sourav Ghosh Mar 18 '15 at 11:53
@Lundin I did not vote for dupe. Was just suggesting an alternate read. :-) – Sourav Ghosh Mar 18 '15 at 11:53
I get `1,1 1,1 2,2` which makes no sense whether you look at left-to-right evaluation or right-to-left. Undefined behaviour. – Weather Vane Mar 18 '15 at 12:01

int a = 1; printf("%d, %d\n", (a++), a);

0 Answers0