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Re: Difference between for and foreach
by Joost (Canon) on Sep 17, 2007 at 14:54 UTC
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foreach is 4 characters longer to type (IOW there is no difference)
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Re: Difference between for and foreach
by n8g (Sexton) on Sep 17, 2007 at 14:49 UTC
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Re: Difference between for and foreach
by johngg (Canon) on Sep 17, 2007 at 15:10 UTC
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There are two types of for loop in Perl, the C-style one
for ($x = 1; $x <= 10; $x ++)
{
...
}
and the other sort
for $x ( 1 .. 10 )
{
...
}
Both can be written with foreach instead of for as the former is just a synonym for the latter. See the sections on loops in perlsyn.
Cheers, JohnGG | [reply]
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Re: Difference between for and foreach
by Hercynium (Hermit) on Sep 17, 2007 at 15:42 UTC
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I'm pretty certain that Joost is correct at least as far as the specifications of how each construct is supposed to work.
I'd like to add that it's worth paying attention to the aliasing behavior in certain situations, where the loop iterator variable ($thing in this example)
for my $thing (@things) { # do stuff to $thing # }
is not a copy of the current element of @things but more like a reference (a magic one that does not need to be de-referenced). The side effects have bitten me in the past, before I learned better :)
Also, I've just been reading Perl Best Practices by TheDamian and chapter 6 (around pg. 100) goes in-depth on some of the techincal *and* psychological implications of different ways of using for and it's brethren. | [reply]
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I've been caught out in the past by expecting to be able to rely on changes to the loop variable being preserved once the loop has terminated. For example, perhaps you want to know at which iteration a foreach loop terminated early using a last. However, the loop variable is localised inside the loop so that it reverts to it's old value after the loop.
$ perl -Mstrict -Mwarnings -le '
> my $x = 0;
> print $x;
> for $x ( 1 .. 10 )
> {
> my $y = rand;
> print qq{$x -- $y};
> last if $y > 0.85;
> }
> print $x;'
0
1 -- 0.820319728766666
2 -- 0.070987764836417
3 -- 0.632845876776752
4 -- 0.195428179899814
5 -- 0.847997411282524
6 -- 0.353572570937089
7 -- 0.865375393672675
0
$
You have to preserve the loop variable from inside the loop if you need to access it afterwards.
Cheers, JohnGG | [reply]
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Re: Difference between for and foreach
by eyepopslikeamosquito (Archbishop) on Sep 17, 2007 at 21:07 UTC
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Notice that foreach has been removed from Perl 6. As specified in S04:
There is no foreach statement any more. It's always spelled for in Perl 6, so it always takes a list as an argument...
And the Perl 5 C-style for loop is now spelled loop. Quoting S04 again:
The loop statement is the C-style for loop in disguise:
loop ($i = 0; $i < 10; $i++) {
...
}
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Nah, perl 6 is just one step closer to Lisp/Scheme.
;-)
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Re: Difference between for and foreach
by andreas1234567 (Vicar) on Sep 17, 2007 at 18:31 UTC
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Time to revisit for vs foreach from the year 2000.
perlsyn says:
The foreach keyword is actually a synonym for the for keyword, so you can use foreach for readability or for for brevity.
Update: I now see the link to for vs foreach was already mentioned by n8g above.
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That comment is about the C-style for loops ( e.g. for ( $i = 0; $i < $j; $i++) versus iterating directly over a list ( e.g. for my $item (@array)).
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Regarding the context, I would say "the equivalen for loop" means the C-style loop. The documentation should probably be improved there.
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