Re: Infinite loop with each

each is inherently fragile since it uses an iterator you don't control. That's why I prefer using keys.

for my $key ( keys %some_hash ) {
   my $val = $some_hash{$key};
   ...
}
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For example, each doesn't work with last or die.

>perl -le"%h=qw(a ! b @); for(1..2){ for$k(keys%h){ print ++$i,$k; las
+t } }"
1a
2a

>perl -le"%h=qw(a ! b @); for(1..2){ while($k=each%h){ print ++$i,$k; 
+last } }"
1a
2b  <-- b???
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Update: Added example.

Comment on Re: Infinite loop with each Select or Download Code

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Re^2: Infinite loop with each by dragonchild (Archbishop) on Jun 05, 2008 at 13:32 UTC
I'm going to have to disagree with you categorically. You do control the each iterator. You just don't explicitly create it. It's very simple to reset the iterator, should you choose, with a void keys call. I do not consider this a hack. In many Perls, lazy lists aren't fully implemented in for(keys%h). This means that large lists can grow your RAM unnecessarily. When using a tied datastructure, such as from DBM::Deep, each() is strongly preferred due to the way tie implements hash iteration. TIEHASH provides FIRSTKEY and NEXTKEY($current_key). each() calls the methods each iteration and is generally nice. keys() has to call FIRSTKEY, then NEXTKEY($current_key) successively until it returns undef. This can be extremely costly. My criteria for good software: Does it work? Can someone else come in, make a change, and be reasonably certain no bugs were introduced?	[reply]

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Re^2: Infinite loop with each
by dragonchild (Archbishop) on Jun 05, 2008 at 13:32 UTC

You do control the each iterator. You just don't explicitly create it. It's very simple to reset the iterator, should you choose, with a void keys call. I do not consider this a hack.
In many Perls, lazy lists aren't fully implemented in for(keys%h). This means that large lists can grow your RAM unnecessarily.
When using a tied datastructure, such as from DBM::Deep, each() is strongly preferred due to the way tie implements hash iteration. TIEHASH provides FIRSTKEY and NEXTKEY($current_key). each() calls the methods each iteration and is generally nice. keys() has to call FIRSTKEY, then NEXTKEY($current_key) successively until it returns undef. This can be extremely costly.

My criteria for good software:

Does it work?
Can someone else come in, make a change, and be reasonably certain no bugs were introduced?

[reply]