Why the compiler can't autoconvert foreach to for

RE: Why the compiler can't autoconvert foreach to for

kpako@yahoo.com (Dare Obasanjo) — Wed, 21 Apr 2004 17:01:00 GMT

My favorite movie subject all at once period was Antz and A Bug's Life. The Deep Impact/Armageddon period was also interesting.

re: Why the compiler can't autoconvert foreach to for

Eric Lippert — Wed, 21 Apr 2004 17:29:00 GMT

On a related topic, the way JScript/JScript .NET handle this is a little weird. I wrote a couple of articles on that a while back:

http://blogs.msdn.com/ericlippert/archive/2003/09/22/53063.aspx

http://blogs.msdn.com/ericlippert/archive/2003/10/01/53134.aspx

Remember 1988, when all those terrible mind-body-switch movies came out at the same time? That was weird.

RE: Why the compiler can't autoconvert foreach to for

nospamplease75@yahoo.com (Haacked) — Wed, 21 Apr 2004 18:13:00 GMT

To prove your point, I had written an enumerator called PageRangeEnumerator. You give it a string like "1-4,7,18+" and it would only enumerate over those pages.

re: Why the compiler can't autoconvert foreach to for

Clinton Pierce — Wed, 21 Apr 2004 20:01:00 GMT

Larry Wall talks about optimizing for the Programmer as well as for Runtime speed. I'll give up quite a few CPU cycles to maintain foreach(...) over a corresponding for(...) loop anytime.

re: Why the compiler can't autoconvert foreach to for

David Kafrissen — Wed, 21 Apr 2004 21:55:00 GMT

You have described why the compiler cannot optimize one form into another, but you did not describe why one form is less efficient than the other.

re: Why the compiler can't autoconvert foreach to for

Raymond Chen — Wed, 21 Apr 2004 21:59:00 GMT

My topic is "Why the compiler can't autoconvert foreach to for", not "Which one is faster". That's a topic for somebody else (try the links I provided).

re: Why the compiler can't autoconvert foreach to for

JFo — Sun, 25 Apr 2004 01:20:00 GMT

It turns out that sometimes a foreach can be faster than a for loop - some people have been converting their loops from this:

foreach (Control c in this.Controls) {
// ...
}

to this:

for (int i = 0; i < this.Controls.Count; i++) {
// ...
}

Since you're now accessing the "Controls" property on every iteration instead of using the enumerator it significantly slows down the loop.

re: Why the compiler can't autoconvert foreach to for

indranil banerjee — Tue, 27 Apr 2004 22:07:00 GMT

but this just begs the question: Why were they designed to be semantically different?

Sure, "depending on the circumstances, it may be better for the program to crash than to produce incorrect results". And sometimes the other way around. The language designer can never know. So why the inconsistency?

I would choose the more permissive of the two choices, apply it consistently to both keywords and leave error handling to the programmer.

BTW, JFo a really smart foreach would generate this :-

int end = this.Controls.Count
for (int i = 0; i < end; ++i) {
// ...
}

thats what happens in STL

re: Why the compiler can't autoconvert foreach to for

Raymond Chen — Tue, 27 Apr 2004 22:51:00 GMT

They are different because "foreach" is a generalized enumerator whereas "for" works only for integer-indexed objects. One is generalized and one is specialized.

re: For = Foreach

Nick's Delphi Blog — Wed, 05 May 2004 19:34:00 GMT

re: Why the compiler can't autoconvert foreach to for

Raymond Chen — Mon, 05 Jul 2004 19:03:00 GMT

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