How can I delete keys in hash that don't exist as elements in array (Perl)?

Question

I have an array of key names and need to remove any keys that are not in this list from a hash.

I gather deleting keys in a hash is a Bad Thing while iterating over it, but it does seem to work:

use strict;
use warnings;
use Data::Dumper;

my @array=('item1', 'item3');
my %hash=(item1 => 'test 1', item2 => 'test 2', items3 => 'test 3', item4 => 'test 4');

print(Dumper(\%hash));

foreach (keys %hash)
{
    delete $hash{$_} unless $_ ~~ @array;
}    

print(Dumper(\%hash));

gives the output:

$VAR1 = {
      'item3' => 'test 3',
      'item1' => 'test 1',
      'item2' => 'test 2',
      'item4' => 'test 4'
    };
$VAR1 = {
      'item3' => 'test 3',
      'item1' => 'test 1'
    };

What is a better/cleaner/safer way of doing this?

amon · Accepted Answer · 2014-03-06 11:13:49Z

3

Don't use smartmatch ~~, it's fundamentally broken and will likely be removed or substantially changed in upcoming releases of Perl.

The easiest solution is to build a new hash only containing those elements you're interested in:

my %old_hash = (
    item1 => 'test 1',
    item2 => 'test 2',
    item3 => 'test 3',
    item4 => 'test 4',
);
my @keys = qw/item1 item3/;

my %new_hash;
@new_hash{@keys} = @old_hash{@keys};  # this uses a "hash slice"

If you want to update the original hash, then do %old_hash = %new_hash afterwards. If you don't want to use another hash, you might like to use List::MoreUtils qw/zip/:

# Unfortunately, "zip" uses an idiotic "prototype", which we override
# by calling it like "&zip(...)"
%hash = &zip(\@keys, [@hash{@keys}]);

which has the same effect.

answered Mar 6, 2014 at 11:13

amon

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1 Comment

Tim Jones Over a year ago

Is there any way to do it without a temporary hash to hold the slice??

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