All About Iterators

re: All About Iterators

kfarmer — Fri, 23 Mar 2007 12:15:30 GMT

In the LINQ forums way-back, I'd suggested "yield enumerable" instead of "yield foreach", with the same behavior, but I think I like the foreach version better.

re: All About Iterators

Sean — Fri, 23 Mar 2007 14:38:05 GMT

Hey, another great post.

I find that a very large amount of my time is spent iterating over items in a list, so the introduction of templates to C# was definitely a major advantage to me.

Another great article on custom iterators that I highly recommend is one written by Bill Wagner:

http://msdn2.microsoft.com/en-us/vcsharp/bb264519.aspx

And the article that originally introduced me to the idea (and I always carry a copy of the MSDN magazine it was in to use as a reference):

http://msdn.microsoft.com/msdnmag/issues/06/00/c20/default.aspx

By the way, completely offtopic, but I noticed that code in your <pre> tags are causing some font issues with Firefox. I didn't dig into it too much, but the code is extremely small and hard to read. My guess would be that it's inheriting from its parent with a font-style of something like 0.8em.

re: All About Iterators

wesdyer — Fri, 23 Mar 2007 18:18:36 GMT

Sean:

Thanks for the heads up about the size of the code. I guess now it's out that I don't use Firefox, I'll fix it.

re: All About Iterators

wesdyer — Fri, 23 Mar 2007 18:57:23 GMT

I've updated the page based on Sean's feedback.

I guess when I was copying and pasting code from Reflector into Windows Live Writer that it didn't work as well as I would have liked ;). Thanks again Sean.

re: All About Iterators

Sean — Fri, 23 Mar 2007 22:52:24 GMT

Any time! You've helped me a ton. That's why I subscribe to your blog. ;-)

By the way, looks great now.

re: All About Iterators

Christopher Diggins — Sat, 07 Apr 2007 19:56:12 GMT

Hi Wes,

I was wondering how you would compare IEnumerable to implementing an abstract ForEach method in a base class? For example:

http://functional-lists.googlecode.com/svn/trunk/FunctionalList.cs

Which one is likely to have better performance? What reasons would you choose one over the other?

Cheers,

Christopher Diggins

Community Convergence XXIV

Charlie Calvert's Community Blog — Mon, 09 Apr 2007 09:48:46 GMT

Recently my time has been taken up with a series of internal issues involving Beta1, C# Samples and various

re: All About Iterators

wesdyer — Tue, 10 Apr 2007 23:47:25 GMT

Hi Christopher,

Hope everything is going well. I expect that the typical IEnumerable + static Foreach will do better in performance. But try it out and tell me. Specifically, try out the performance of your code against the latest LINQ CTP or Beta 1. Also, you might want to consider making your code generic (add some Ts and Us).

re: All About Iterators

Peter Mullen — Thu, 12 Apr 2007 09:04:26 GMT

Hi Wes,

Your code for MoveNext() assigns this.current from an element in this.list. Your comments claim that MoveNext() only computes enough to realize the next element. But hasn't this.list been filled before the first call to MoveNext()?

thanks,

peter

re: All About Iterators

Peter Mullen — Thu, 12 Apr 2007 09:20:43 GMT

Now I get it. The compiler generates different versions of MoveNext() based on the code that was in the original GetElements() method. Just like you said in the text.

thanks,

re: All About Iterators

Joost Morsink — Wed, 16 May 2007 16:52:38 GMT

When is your next article going to be posted? I'm checking this site daily for almost two months now :)

re: All About Iterators

Fduch — Sun, 17 Jun 2007 12:28:08 GMT

It's fun. I "invented" the concept of iterators some time ago not knowing about this feature.

Will VB ever have it?

Immutability in C# Part Seven: More on Binary Trees

Fabulous Adventures In Coding — Wed, 19 Dec 2007 23:01:20 GMT

Lots of good comments on my previous post. To briefly follow up: One of the downsides of immutable tree

Immutability in C# Part Seven: More on Binary Trees

Noticias externas — Wed, 19 Dec 2007 23:52:43 GMT

Lots of good comments on my previous post. To briefly follow up: One of the downsides of immutable tree

re: All About Iterators

matthew — Thu, 03 Jan 2008 20:25:42 GMT

> Will VB ever have it?

do you mean vb.net or vb6?

re: All About Iterators

richard_deeming — Fri, 11 Jan 2008 23:37:27 GMT

The performance of the Concat method for multiple chained iterators could be greatly improved with a fairly trivial tweak. Simply create a container class to hold all of the concatenated iterators, and iterate over them all in sequence:

foreach (IEnumerable<T> iterator in iterators)

foreach (T item in iterator)

{

yield return item;

}

On my system, using your test code, the built-in Concat method for 10000 items takes 4807629 ticks, whereas my version takes 4472 ticks.