FAQ 4.60 How do I sort a hash (optionally by value instead of key)?

Discussion in 'Perl Misc' started by PerlFAQ Server, Jan 3, 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 .

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

    4.60: How do I sort a hash (optionally by value instead of key)?

    (contributed by brian d foy)

    To sort a hash, start with the keys. In this example, we give the list
    of keys to the sort function which then compares them ASCIIbetically
    (which might be affected by your locale settings). The output list has
    the keys in ASCIIbetical order. Once we have the keys, we can go through
    them to create a report which lists the keys in ASCIIbetical order.

    my @keys = sort { $a cmp $b } keys %hash;

    foreach my $key ( @keys )
    {
    printf "%-20s %6d\n", $key, $hash{$key};
    }

    We could get more fancy in the "sort()" block though. Instead of
    comparing the keys, we can compute a value with them and use that value
    as the comparison.

    For instance, to make our report order case-insensitive, we use the "\L"
    sequence in a double-quoted string to make everything lowercase. The
    "sort()" block then compares the lowercased values to determine in which
    order to put the keys.

    my @keys = sort { "\L$a" cmp "\L$b" } keys %hash;

    Note: if the computation is expensive or the hash has many elements, you
    may want to look at the Schwartzian Transform to cache the computation
    results.

    If we want to sort by the hash value instead, we use the hash key to
    look it up. We still get out a list of keys, but this time they are
    ordered by their value.

    my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash;

    From there we can get more complex. If the hash values are the same, we
    can provide a secondary sort on the hash key.

    my @keys = sort {
    $hash{$a} <=> $hash{$b}
    or
    "\L$a" cmp "\L$b"
    } keys %hash;



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

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    PerlFAQ Server, Jan 3, 2011
    #1
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