Re: Optimal use of foreach

If your app depends on the speed difference of a loop on an array you
should consider tossing more hardware at the issue. Hardware is far
cheaper than a week or so of programmer time tweaking foreach or for next
loops :-)

John Warner


> -----Original Message-----
> From: Discussion of advanced .NET topics. [mailto:ADVANCED-
> [email protected]] On Behalf Of Sébastien Lorion
> Sent: Wednesday, January 28, 2009 11:23 AM
> To: [email protected]
> Subject: Re: [ADVANCED-DOTNET] Optimal use of foreach
> 
> Well, I said that from memory, so I guess I remembered that bit from
what I
> read (as you point out in your blog). Out of curiosity, I popped an old
> benchmark I made and ya, the results are the same on all .NET versions
:)
> Anyway, if your perf depends on the speed difference of a loop construct
on
> an array, you need really some serious speed ...
> 
> Sébastien
> On Wed, Jan 28, 2009 at 10:20 AM, Greg Young
> <[email protected]>wrote:
> 
> > >for loops are usually used in inner loops where you don't want to
create
> > >millions of new IEnumerator<T>. They are also slightly faster on
arrays,
> > but
> > >the vast majority of the time, foreach is fine and makes the code
clearer.
> >
> > They are not slightly faster on arrays.
> >
> > Please see:
> > http://codebetter.com/blogs/gregyoung/archive/2006/06/12/146372.aspx
> > which addresses this point in particular (I believe the originating
> > point of this misconception was a few people at MS) and
> > http://codebetter.com/blogs/gregyoung/archive/2006/06/11/146343.aspx
> > for a more in depth look at how for loops work (its old and there are
> > some minor revisions to the JIT but most still holds true).
> >
> > Cheers,
> >
> > Greg
> >
> > On Tue, Jan 27, 2009 at 11:51 PM, Sébastien Lorion
> > <[email protected]> wrote:
> > > When you use foreach, it actually calls
> myDictionary.Keys.GetEnumerator()
> > > which returns a new instance of IEnumerator<T>. That enumerator is
then
> > used
> > > during the loop and Keys is never called again. Open up Reflector
and
> > take a
> > > look at the code, you will better understand what's going on this
way ;)
> > >
> > > for loops are usually used in inner loops where you don't want to
create
> > > millions of new IEnumerator<T>. They are also slightly faster on
arrays,
> > but
> > > the vast majority of the time, foreach is fine and makes the code
> > clearer.
> > >
> > > Sébastien
> > > On Tue, Jan 27, 2009 at 8:43 PM, Mont Rothstein <
> > [email protected]>wrote:
> > >
> > >> When using foreach if the collection is a property will it be
called on
> > >> each
> > >> pass or only once?
> > >>
> > >> Can I do:
> > >>
> > >> foreach (string key in myDictionary.Keys)
> > >> {
> > >> }
> > >>
> > >> or will that call Keys each time, therefore requiring that I do:
> > >>
> > >> Dictionary<string, string>.KeysCollections keys;
> > >>
> > >> keys = myDictionary.Keys;
> > >>
> > >> foreach (string key in keys)
> > >> {
> > >> }
> > >>
> > >> Thanks,
> > >> -Mont
> > >>
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> >
> >
> > --
> > It is the mark of an educated mind to be able to entertain a thought
> > without accepting it.
> >
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