FAQ 4.60 How do I sort a hash (optionally by value instead of key)?
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From: PerlFAQ Server on 18 Apr 2010 00:00 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 . -------------------------------------------------------------------- 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; -------------------------------------------------------------------- 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. |