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

Discussion in 'Perl Misc' started by PerlFAQ Server, Jan 29, 2011.

  1. 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 .

    --------------------------------------------------------------------

    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;
    }



    --------------------------------------------------------------------

    The perlfaq-workers, a group of volunteers, maintain the perlfaq. They
    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.

    If you'd like to help maintain the perlfaq, see the details in
    perlfaq.pod.
    PerlFAQ Server, Jan 29, 2011
    #1
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