Re: Performance: Iterating a vector in a loop..


> But he was saying its slow because .begin() and .end() are called every
> loop.
>
> I know that the function is called every iteration, but was under the
> impression that STL is highly optimised in dealing with things like this,
> so it shouldnt make too much difference.
>
> Can anyone shed any light on this and explain whether or not it really has
> any speed hits when doing that, or, if it would be better to re-write the
> code like this (which i feel is less legible).
>
> vector<MyType>::iterator itLoop, itBegin = vec.begin(), itEnd= vec.end();
>
> for (itLoop = itBegin ; itLoop != itEnd; itLoop++)
> {
> itLoop->DoSomething();
> }
>
> Thanks for any info.

I dont think that begin() is required more then once.
But the fact that you only call end() once is logicaly an improvement.

and im not sure if

while( itLoop != itEnd )
{
(++itLoop)->DoSomething();
}

would be faster either. At this point if the vector is very large,
you may want to set up a test to verify these things yourself.





.

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