Comments on: Enumeration isn’t cheap

By: Bill

Bill — Wed, 10 May 2006 19:26:42 +0000

for (int i…
{
int x = ((SomeClass)list[i]).Val;
}

By: Bill

Bill — Wed, 10 May 2006 19:24:57 +0000

Hmm…looks like my sample was clipped. I was trying to emphasize the following amendment to the for (int i…loop:

int length = list.Count;
for (int i = 0; i

By: Bill

Bill — Wed, 10 May 2006 19:22:34 +0000

Another thing worth noting here is that in more typical usage of an ArrayList’s indexer, a cast is required. When the above sample is modified in the following fashion, results converge significantly to:

Enumerator: 328.1376 Indexing: 125.0048

private void button1_Click(object sender, System.EventArgs e)
{
ArrayList list = new ArrayList();
for (int i = 0; i

By: Luke Flemmer

Luke Flemmer — Sun, 30 Apr 2006 17:21:41 +0000

There are some interesting imporvements in 2.0, made possible through the introduction of generic types. Specifcally, the boxing/unboxing costs associated with value types in a collection, or even just the cost of the downcast from Object, can now be avoided. There is quite a nice article on it here: http://msdn.microsoft.com/msdnmag/issues/04/05/C20/

By: Joe Morrison

Joe Morrison — Sat, 29 Apr 2006 22:34:42 +0000

Did either of you try this with CLR 2.0? I did, and the Enumerator version varied between 1.5-2x worse than the index based version. Nothing as bad as 5-10x worse though. Looks like Microsoft has been working on this.

By: Luke Flemmer

Luke Flemmer — Mon, 10 Apr 2006 20:17:41 +0000

Yes. I’m certainly not advancing this is as the canonical testing suite… Another thing it doesn’t demonstrate is that the cost doesn’t just exist for large collections. Iterations over small collections that are called frequently can actually be worse, since you incurr the cost of creating the Enumerator, and then later GC’ing it.

By: Damien Morton

Damien Morton — Mon, 10 Apr 2006 19:35:09 +0000

Umm, I was trying to show that you should accumulate all of the hashcodes your grab, i.e. x += o.GetHashCode()

By: Damien Morton

Damien Morton — Mon, 10 Apr 2006 19:33:35 +0000

When doing these kinds of things, I also try to ensure that the code I am testing in the inner loop actually _does_ something, otherwise it can be optimised away.

x = 0;
int t0 = Environment.TickCount;
for (int i = 0; i

By: Luke Flemmer

Luke Flemmer — Mon, 10 Apr 2006 18:40:07 +0000

Good point. Reversing them the ratio falls to about 7x worse for the enumerator.

By: Damien Morton

Damien Morton — Mon, 10 Apr 2006 18:34:32 +0000

I would check this again, with the order of your tests reversed. I believe that Object.GetHashCode() is more expensive the first time it is called.

Certainly, in comparing enumerators with indexing, you will find differences, but the magnitude of the differences you are finding seems out of whack.