In C, is { a[i] = a[++i] } equivalent to { a[i] = a[i+1]; i++;}?

Question

In C, is

a[i] = a[++i];

equivalent to

a[i] = a[i+1]; i++;

That is, which side of the assignment is evaluated first and what value of i is used on the left side? Or is this assignment ambiguous?

@Amadan, actually, it is not. From http://stackoverflow.com/a/3914332/434551, "C++03 [Section 5/4] says Between the previous and next sequence point a scalar object shall have its stored value modified at most once by the evaluation of an expression. " Here, `i` is being modified only once. — R Sahu, Jun 11 '15 at 04:52
@RSahu: See [this answer](http://stackoverflow.com/a/2989841/240443), a quote from K&R, with this exact example. The fact that it does not violate one constraint does not mean it can't violate another. — Amadan, Jun 11 '15 at 04:53
It is a duplicate of [this question](http://stackoverflow.com/q/949433/1009479), [the second answer](http://stackoverflow.com/a/4177063/1009479) explained it explicitly. — Yu Hao, Jun 11 '15 at 05:04
@Yu Hao: Thank you, that Q&A clearly resolves my question. The behaviour is undefined. — push2eject, Jun 11 '15 at 05:20

score 3 · Accepted Answer · edited May 23 '17 at 12:05

3

In the same sequence point you are using and incrementing i

a[i] = a[i++];

which will lead to undefined behavior.

a[i] = a[i+1];
i++;

is good.

And to answer your question are they same ? No they are not !! One if well defined and another is not.

Check this answer

edited May 23 '17 at 12:05

Community

answered Jun 11 '15 at 04:48

Gopi

1 Answers1