FAQ 4.52 How do I sort an array by (anything)?


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This is an excerpt from the latest version perlfaq4.pod, which
comes with the standard Perl distribution. These postings aim to
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to review and update the answers. The latest version of the complete
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4.52: How do I sort an array by (anything)?

Supply a comparison function to sort() (described in "sort" in

@list = sort { $a <=> $b } @list;

The default sort function is cmp, string comparison, which would sort
"(1, 2, 10)" into "(1, 10, 2)". "<=>", used above, is the numerical
comparison operator.

If you have a complicated function needed to pull out the part you want
to sort on, then don't do it inside the sort function. Pull it out
first, because the sort BLOCK can be called many times for the same
element. Here's an example of how to pull out the first word after the
first number on each item, and then sort those words case-insensitively.

@idx = ();
for (@data) {
($item) = /\d+\s*(\S+)/;
push @idx, uc($item);
@sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];

which could also be written this way, using a trick that's come to be
known as the Schwartzian Transform:

@sorted = map { $_->[0] }
sort { $a->[1] cmp $b->[1] }
map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;

If you need to sort on several fields, the following paradigm is useful.

@sorted = sort {
field1($a) <=> field1($b) ||
field2($a) cmp field2($b) ||
field3($a) cmp field3($b)
} @data;

This can be conveniently combined with precalculation of keys as given

See the sort article in the "Far More Than You Ever Wanted To Know"
collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for more
about this approach.

See also the question later in perlfaq4 on sorting hashes.


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