Can someone explain why a value gets incremented when "++" is used for comparison?

Question

This may already be a question out there, but with the lack of specific key terms, it's a bit hard to search for. Just looking for more insight on this topic.

Right now, I'm working in C++ and wondering why my value is replaced with an incremented value when I compare using "++".

So here, we print 14 times (numbers 1-14).

int i = 0, x = 0;
while (x < 30) {
    x++;
    if (13 < i++) break;
    cout << i << endl;
}

Here, we print 30 zeros.

int i = 0, x = 0;
while (x < 30) {
    x++;
    if (13 < i+1) break;
    cout << i << endl;
}

And this just plain doesn't work. (I wanted to try because i++ = i=i+1).

int i = 0, x = 0;
while (x < 30) {
    x++;
    if (13 < i=i+1) break;
    cout << i << endl;
}

`++i` = `i = i + 1`, the comparison evaluates to: `if (13 < i); ... ; i = i + 1` — atmostmediocre, Apr 07 '18 at 18:44
Order of operations is important here, you're missing some parentheses. — miradulo, Apr 07 '18 at 18:44
The third piece of code really isn't the core of my question. — Bella, Apr 07 '18 at 18:55

score 3 · Answer 1 · answered Apr 07 '18 at 18:46

3

Expression i++ has both a value and a side effect. The value is that of i, and the side effect is that i is incremented (after having taken its value as expression result).

Expression i+1, in contrast, does not have a side effect, it only has a value (which is the value of i plus 1) but leaves the value of i as it is.

That's why.

answered Apr 07 '18 at 18:46

Stephan Lechner

34,891
4
35
58

Thank you! Using "side effect" to explain makes a lot of sense. – Bella Apr 07 '18 at 18:53
@Bella: You're welcome. Maybe you can accept the answer (the green check) if it helped. – Stephan Lechner Apr 07 '18 at 21:36

score 1 · Answer 2 · answered Apr 07 '18 at 19:16

And this just plain doesn't work. (I wanted to try because i++ = i=i+1).

The reason it doesn't work is because of operator precedence in this expression:

 if (13 < i=i+1) break;

It is evaluated by compiler as

if ((13 < i)=(i+1)) break;

and assignment to bool fails. What you need is to add correct parentheses for it to work:

int i = 0, x = 0;
while (x < 30) {
    x++;
    if (13 < (i=i+1)) break;
    cout << i << endl;
}

https://ideone.com/0NSNwU

Can someone explain why a value gets incremented when "++" is used for comparison?

2 Answers2