• Iterating loop

  foreach my $i (...list...) {
     ...
  }
  [download]

  The list is flattened (requiring memory proportional to the size of the list).

• C-style loop

  for (...; ...; ...) {
     ...
  }
  [download]

  Utmost flexibility. Utmost efficiency. Least readable.

• Counting loop

  for my $i ($x..$y) {
     ...
  }
  [download]

  This is a special case of the iterating loop that uses no memory. When the list consists of a range, it is not flattened. Utmost efficiency. Utmost Readability.

• Reverse Iterating loop

  foreach my $i (reverse ...list...) {
     ...
  }
  [download]

  This is a special case of the iterating loop. Instead of actually reversing the list, the list is processed in reverse. You save the cost of the reverse operation, but that's it. The list is still flattened (even if it's just a range), so it still requires memory proportional to the size of the list.

My personal convention is to write counting loops and C-style loops using for, and iterating loops using foreach. That way, the extra cost of flattening the list is not hidden. However, Perl will gladly accept both for and foreach in every case.

Update: Since posting this, I have been told Reverse Iterating loop has been improved such that it no longer flattens the list, but the following snippet shows that it hasn't been "fixed" in the latest stable version of Perl (Perl 5.8.8).

use strict;
use warnings;

print("$]\n");  # 5.008008

# Don't inline these. It will cause the
# memory to be allocated at compile time.
my $min = 0;
my $max = 10_000_000;

for (        $min .. $max) { print(":"); <STDIN>; last; }  # 2.2MB
for (reverse $min .. $max) { print(":"); <STDIN>; last; }  # 239MB
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The person must have been misinterpreted perl586delta. perl586delta only says "a temporary reversed list isn't created". The temporary flattened list is still created.

My personal convention is to write counting loops and C-style loops using for, and iterating loops using foreach. That way, the extra cost of flattening the list is not hidden. However, Perl will gladly accept both for and foreach in every case.

And many people share your pov, while I'm of the "three chars are easier to type no matter what" school of thought - I also think we've "argued" about these issues in the past, but I'm not really sure. However to contribute with something concrete to this thread, it's also interesting if not properly relevant, to note that in Perl 6 the possible/alleged ambiguity will be removed, with C-style for loops being renamed to loop, and for being the preferred kind of loop for most situations, while foreach will be phased out:

pugs> sub postfix:<!> (Int $x) { [*] 1..$x }
undef
pugs> say $_! for =<>;
3
6
4
24
7
5040
10
3628800
undef
pugs>
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Try this on the command line and see what Perl does for yourself. :)

perl -MO=Deparse -e "print for 1..10"
[download]

UPDATE: By the way, if you had simply typed FOR and FOREACH into the simple search box (found at the top of each page) you would have stumbled across this node: For and foreach....

I should also add that my personal preference is to only use FOR. I see no reason to type the extra characters, no matter what kind of loop i am using. I do not agree that FOREACH is more readable than FOR. (You could always say EACH to yourself when reading the code. ;))

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)

The foreach keyword is actually a synonym for the for keyword, so you can use foreach for readability or for for brevity.

Or just RTFM.

The "foreach" keyword is actually a synonym for the "for" keyword, so you can use "foreach" for readability or "for" for brevity. (Or because the Bourne shell is more familiar to you than csh, so writing "for" comes more naturally.) If VAR is omitted, $_ is set to each value.