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From: PerlFAQ Server on 10 Aug 2010 06:00
This is an excerpt from the latest version perlfaq4.pod, which
comes with the standard Perl distribution. These postings aim to
reduce the number of repeated questions as well as allow the community
to review and update the answers. The latest version of the complete
perlfaq is at http://faq.perl.org .

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4.58: How do I look up a hash element by value?

Create a reverse hash:

%by_value = reverse %by_key;
$key = $by_value{$value};

That's not particularly efficient. It would be more space-efficient to
use:

while (($key, $value) = each %by_key) {
$by_value{$value} = $key;
}

If your hash could have repeated values, the methods above will only
find one of the associated keys. This may or may not worry you. If it
does worry you, you can always reverse the hash into a hash of arrays
instead:

while (($key, $value) = each %by_key) {
push @{$key_list_by_value{$value}}, $key;
}



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are not necessarily experts in every domain where Perl might show up,
so please include as much information as possible and relevant in any
corrections. The perlfaq-workers also don't have access to every
operating system or platform, so please include relevant details for
corrections to examples that do not work on particular platforms.
Working code is greatly appreciated.

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perlfaq.pod.
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