FAQ 4.59 How do I sort a hash (optionally by value instead of key)?

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4.59: 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;

    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

    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 <=> $hash } 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 <=> $hash
                    "\L$a" cmp "\L$b"
                    } keys %hash;


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