$VAR1 = {
          'x' => 'cabd'
        };
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The value would be shorter, as there would be less keys after the second iteration.

#!/usr/bin/perl
use warnings;
use strict;

use Data::Dumper;

my %hash = ( a => 11,
             b => 12,
             c => 13,
             d => 14,
           );

for my $key (keys %hash) {
    delete $hash{$_} for qw( a b c d );
    $hash{'x'} .= $key;
}
print Dumper \%hash;
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Hello choroba,

Yes, you’re right: despite the calls to delete, the for loop list doesn’t change, so it must be constructed before the first loop iteration.

Whereas using each I get an infinite loop:

use warnings;
use strict;
use Data::Dump qw(dd pp);

my %hash =
(
    a => 11,
    b => 12,
    c => 13,
    d => 14,
);

while (my ($key, $value) = each %hash)
{
    print "key is $key, value is $value, hash is ", pp(\%hash), "\n";
    delete $hash{$_} for qw( a b c d );
    $hash{x} .= $key;
}

dd \%hash;
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Output:

23:43 >perl 1148_SoPW.pl
key is c, value is 13, hash is { a => 11, b => 12, c => 13, d => 14 }
key is x, value is c, hash is { x => "c" }
key is x, value is cx, hash is { x => "cx" }
key is x, value is cxx, hash is { x => "cxx" }
key is x, value is cxxx, hash is { x => "cxxx" }
key is x, value is cxxxx, hash is { x => "cxxxx" }
key is x, value is cxxxxx, hash is { x => "cxxxxx" }
key is x, value is cxxxxxx, hash is { x => "cxxxxxx" }
key is x, value is cxxxxxxx, hash is { x => "cxxxxxxx" }
key is x, value is cxxxxxxxx, hash is { x => "cxxxxxxxx" }
...
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Which isn’t surprising given the warning in the documentation for each:

If you add or delete a hash's elements while iterating over it, the effect on the iterator is unspecified; for example, entries may be skipped or duplicated--so don't do that. Exception: It is always safe to delete the item most recently returned by each()...

So it appears that sundialsvc4’s concern over memory is justified: for a large hash, the use of keys in a for loop will consume significantly more memory than the equivalent use of each. It seems strange to me that the documentation doesn’t highlight this — or did I just miss it somewhere?

Cheers,

Athanasius <°(((>< contra mundum

Iustus alius egestas vitae, eros Piratica,

The docs clearly state: "Called in list context, returns a list consisting of all the keys of the named hash," (my emphasis!).

And has done for a long time.

---

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority". I'm with torvalds on this

In the absence of evidence, opinion is indistinguishable from prejudice. Agile (and TDD) debunked