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.NET Forum / Languages / C# / February 2008Which is faster for arrays - foreach or for ( i... ) ?
Hi, Given the following declaration: object[] a = new object[SOME_BIG_NUMBER]; which code is faster? 1. foreach ( object o in a ) DoSomethingWith(o); 2. int iSize = a.Length; int i; foreach ( i = 0; i < iSize; i++ ) DoSomethingWith(a[i]); And why? Regards, DoB > foreach ( i = 0; i < iSize; i++ ) Sorry, there was a typo here, of course ;-). should be "for". DoB > Given the following declaration: > [quoted text clipped - 13 lines] > > And why? In general, "for i" used with an array is faster than "foreach", the reason being that "for i" only needs to increment and compare an integer and do an indexing operation for every iteration, which are fast operations, while "foreach" needs to call into IEnumerator to move to the next element and then retrieve the current one. These are two method calls, in addition to whatever is inside the methods, so it has a bigger overhead. >> Given the following declaration: >> [quoted text clipped - 21 lines] > two method calls, in addition to whatever is inside the methods, so > it has a bigger overhead. arrays are a special case for the foreach keyword, the compiler automatically avoids using IEnumerator > Given the following declaration: > > object[] a = new object[SOME_BIG_NUMBER]; > > which code is faster? I wouldn't concentrate on which code is faster until I've proved it matters - at which point I'd have numbers to compare if I made changes. I'd concentrate on which is more readable, and simpler - in which case "foreach" wins in most situations. Don't micro-optimise until you know you've got a problem - and if you know you've got a problem, that should include hard figures, which will make it easy to test changes. Jon > Don't micro-optimise until you know you've got a problem - and if you > know you've got a problem, that should include hard figures, which > will make it easy to test changes. I actually came across a situation in which I wanted to change "foreach" to "for" for some reasons (not performance-related) and before making a change I wanted to know what was an impact on performance. DoB > > Don't micro-optimise until you know you've got a problem - and if you > > know you've got a problem, that should include hard figures, which [quoted text clipped - 3 lines] > "for" for some reasons (not performance-related) and before making a change > I wanted to know what was an impact on performance. Miniscule either way, unless you're really, really not doing anything in the loop. If you have performance concerns, measuring them is crucial. I wouldn't try to micro-optimise the for loop, btw. Just do: for (int i=0; i < myArray.Length; i++) { } instead of using the temporary variable. I seem to recall that this is actually faster in some situations, but it's certainly more readable. Jon > I wouldn't try to micro-optimise the for loop, btw. Just do: > [quoted text clipped - 4 lines] > instead of using the temporary variable. I seem to recall that this is > actually faster in some situations, but it's certainly more readable. I've made some tests and... yes... you are absolutely right - this is a bit faster. Why? Optimisation? DoB. > > I wouldn't try to micro-optimise the for loop, btw. Just do: > > [quoted text clipped - 8 lines] > faster. > Why? Optimisation? The JIT compiler recognises the pattern, and can remove the array bounds check within the loop, as I understand it.  SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet World class .NET training in the UK: http://iterativetraining.co.uk >> I've made some tests and... yes... you are absolutely right - this is a >> bit [quoted text clipped - 3 lines] > The JIT compiler recognises the pattern, and can remove the array > bounds check within the loop, as I understand it. What is interesting, I tested in debug mode. Looks like it is optimised, too. DoB Just my 2 cents.. I find the Array.ForEach<> most readable, but maybe not the most efficient: object[] arr = new object[SOME_BIG_NUMBER]; Array.ForEach<object>(arr, new Action<object>(DoSomething)); >> Given the following declaration: >> [quoted text clipped - 14 lines] > > Jon > Just my 2 cents.. I find the Array.ForEach<> most readable, but maybe not > the most efficient: Definitely not efficient. LP, Dejan > > Just my 2 cents.. I find the Array.ForEach<> most readable, but maybe not > > the most efficient: > > Definitely not efficient. Well, to the extent that there's a delegate call each time. As ever, it's unlikely to be significant in most code. SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet World class .NET training in the UK: http://iterativetraining.co.uk Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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