Decrease varibale until a condition is met (in bodyless while loop )

Question

I am looking for something like this:

int nVal = 130;
while(nVal % 6 != 0)
  nVal--;

in a while loop without body:

int nVal = 130;
while(nVal-- % 6 != 0)

Of course that way, the -- operator is called once too often. Any suggestions here or should I go with option #1?

`while(--nVal % 6 != 0)`.. But why? There's nothing wrong with a `while`-body. — Kninnug, Sep 03 '15 at 15:02
Why do you need a bodyless `while`? Concentrate on writing clean code and let the compiler do optimisations like this one.. — Jabberwocky, Sep 03 '15 at 15:04
This code is equivalent to `nVal=126`. And it what would be produced by a good compiler. — Eugene Sh., Sep 03 '15 at 15:06

score 5 · Answer 1 · answered Sep 03 '15 at 15:03

Use the first alternative as it is clearer and correct. It is never a good idea to show as many operators as possible into a single line. Particularly not the ++ and -- operators since those include a side effect, and are therefore notoriously dangerous to use together with other operators.

CIsForCookies · Answer 2 · 2015-09-03T15:08:09.033

2

You can do this: while((nVal-- -1) % 6 != 0) but, as Lundin said, the first example is clearer

edited Sep 03 '15 at 15:08

answered Sep 03 '15 at 15:04

CIsForCookies

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It sure is, but @tzippy wanted to have a bodyless while loop, so I provided – CIsForCookies Sep 03 '15 at 15:06

score 0 · Answer 3 · answered Sep 03 '15 at 15:03

0

After the optimization it should be the same code in asm. Probably it could be better to use --nVal instead of nVal-- but with -o3 it does not matter I think.

answered Sep 03 '15 at 15:03

DenSumy

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I don't think the optimizer would be allowed to produce the same assembly for prefix/postfix in this case because they aren't semantically identical – awksp Sep 03 '15 at 15:04
2nd option decreases the value even when the condition evaluates to 0, so they are not equivalent. – Lundin Sep 03 '15 at 15:04
If the optimizer is smart enough it will be reduced to `nVal=126`. – Eugene Sh. Sep 03 '15 at 15:05
@EugeneSh. Are loop bodies evaluated during optimization? I thought that this kind of thing normally wouldn't be done because in the majority of cases the code cannot be evaluated at compile time – awksp Sep 03 '15 at 15:08
@awksp You can't know for sure. Optimizers are getting better (worse?) in this type of things. They can even do strange things like [this](http://stackoverflow.com/questions/30262801/what-is-my-best-approach-to-determining-compiler-behaviour-for-empty-infinite-lo). – Eugene Sh. Sep 03 '15 at 15:10
I've looked at the asm code after Visual studio (don't remember version) there where too loops with ++i and i++ and optimized code were the same in both cases. – DenSumy Sep 04 '15 at 11:41

Decrease varibale until a condition is met (in bodyless while loop )

3 Answers3