Why is this program giving unexpected output?

Question

int main()
{
    int x = 3, z ;
    z = x / + + x ;
    printf ("x = %dz = %d", x , z );
    return 0;
}

I thought the output will be x=4 z=0 or x=4 z=1. But I'm getting x=3 z=1.

You might want to read [this question and its accepted answer](http://stackoverflow.com/questions/4176328/undefined-behavior-and-sequence-points). — Some programmer dude, Sep 02 '13 at 08:43
@JoachimPileborg : Thanks for the link, but I think the question has some rather basic syntactic mistake. — Don't You Worry Child, Sep 02 '13 at 08:44
@nishant Yes, but when that's fixed it's still undefined behavior as described in my link. — Some programmer dude, Sep 02 '13 at 08:46
@JoachimPileborg : Right, but after giving two answer possibilities, I think user itself is expecting a UB. :-) — Don't You Worry Child, Sep 02 '13 at 08:50

score 3 · Answer 1 · answered Sep 02 '13 at 08:42

3

Try removing spaces between ++ (increment operator). Use ++x or ++ x. Compiler may be interpreting it as +(+x), i.e. unary + operator.

answered Sep 02 '13 at 08:42

6

Ironically, that means in its current form the behaviour is perfectly defined. – Medinoc Sep 02 '13 at 08:51
@Medinoc : Right, may be here it is, but I think it may happen that on seeing a string of `/ + +` some compiler report Compile Error. – Don't You Worry Child Sep 02 '13 at 08:53
@nishant Why would "some compilers report Compile Error"? That's a perfectly valid sequence of operators. – Sep 02 '13 at 09:09
@H2CO3 : I didn't tried running it, but `/ + +` lacks operands between the two `+`. I thought some compiler may interpret it this way and report error. Is it so that it will always be interpreted as `+(+x)`? – Don't You Worry Child Sep 02 '13 at 09:13
@nishant It's absolutely unambiguous. One `/` operator then two unary `+`. Any compiler rejecting it is wrong. – Sep 02 '13 at 09:14
@H2CO3 : Thanks for the heads up!! – Don't You Worry Child Sep 02 '13 at 09:15

Gangadhar · Answer 2 · 2013-09-02T09:16:06.767

1

remove spaces in between two pluses

z = x / ++ x ;  //will gives z value as 1 always

//except when x=-1 (Floating point exception )

this Might have Undefined Behaviour because Lack of sequence point.

rather than above if you could try like this.

int x = 3, z=3;
printf ("x = %dz = %d", x , z );

z/=(++x); // z/=++x; is also same.  
printf ("x = %dz = %d", x , z );

edited Sep 02 '13 at 09:16

answered Sep 02 '13 at 08:43

Gangadhar

2 Answers2