Anonymonk beat me to the punch. But, the reason for why foreach (keys) and while (each) behave differently has nothing to do with keys being an iterator or not. (Well, it does, but not really.) It has to do with the difference in behavior between foreach and while. foreach is defined to operate on a list. If you give it a list, then you're good. If, however, you give it a function or keyword, then it has to call that function/keyword and construct a temporary list with the return value(s). Incidentally, this is why the following doesn't DWIM:
my @x = 1 .. 5;
sub get_x { @x }
foreach my $v ( get_x() ) {
$v *= 2;
}
print "@x\n";
while, however, re-evaluates its condition every time. This is why the while-loop goes infinite, but the foreach-loop doesn't.
while (my ($k, $v) = each %hash) {
$hash{ $k . 'a' } = 1;
}
foreach my $k (keys %hash) {
$hash{ $k . 'a' } = 1;
}
Being right, does not endow the right to be rude; politeness costs nothing.
Being unknowing, is not the same as being stupid.
Expressing a contrary opinion, whether to the individual or the group, is more often a sign of deeper thought than of cantankerous belligerence.
Do not mistake your goals as the only goals; your opinion as the only opinion; your confidence as correctness. Saying you know better is not the same as explaining you know better.
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Got benchmarks to back that up?
#!/usr/bin/perl
use strict;
use warnings;
use Benchmark qw 'cmpthese';
our %hash = map {$_, $_} 1 .. 100_000;
cmpthese (-10, {
keys => '$a = 0; for (keys %hash) {$a += $_}',
while => '$b = 0; while ($_ = each %hash) {$b += $_}',
});
__END__
Rate keys while
keys 5.23/s -- -26%
while 7.05/s 35% --
Now, that's for a simple hash. If the hash is tied to a huge DBM file, the results would be far more dramatic.
Why does perl not implement an automatic iterator when the parser notices a simple sort-free for (keys %foo)? That's such a common idiom I would be amazed it wasn't getting special attention.
Because it isn't common idiom, and changing it to an iterator changes the results. Perl has an iterator, and it's called each. You can't change keys to an iterator:
for my $k1 (keys %hash) {
for my $k2 (keys %hash) {
}
}
for my $k (keys %hash) {
%hash = ();
....
}
Changing either of the "simple sort-free for (keys %foo)" to an iterator will break the code. | [reply]
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