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Sorting keys of hash table by values

by iman_saleh (Novice)
on Jan 31, 2008 at 21:25 UTC ( [id://665449] : perlquestion . print w/replies, xml ) Need Help??

iman_saleh has asked for the wisdom of the Perl Monks concerning the following question:

Hi all, I want to sort the keys of a hash table according to the values these keys are pointing to in descending order, I only found that using:
@sortedKeys = sort { $h{$a} <=> $h{$b} } keys %h;
does that but the sorting is ascending. Can anyone help me find the way to make it sort descending instead? Thanks,

Replies are listed 'Best First'.
Re: Sorting keys of hash table by values
by samtregar (Abbot) on Jan 31, 2008 at 21:28 UTC
    @sortedKeys = sort { $h{$b} <=> $h{$a} } keys %h;


Re: Sorting keys of hash table by values
by ikegami (Patriarch) on Jan 31, 2008 at 21:53 UTC
    You guys missed
    @sortedKeys = sort { -( $h{$a} <=> $h{$b} ) } keys %h;
Re: Sorting keys of hash table by values
by kyle (Abbot) on Jan 31, 2008 at 21:38 UTC

      Bad. reverse means extra operations and making a second list that is the same size as the original.

      The first reply is the best, as it does not involve any extra operations.


        I thought you might say that (and by "you", I mean "somebody").

        use Benchmark qw( cmpthese ); use List::Util qw( shuffle ); my @set = shuffle 1 .. 1_000_000; cmpthese( 100, { 'sort' => sub { my @x = sort { $b <=> $a } @set; return; } +, 'rsort' => sub { my @x = reverse sort { $a <=> $b } @set; +return; }, } ); __END__ Rate sort rsort sort 1.01/s -- -2% rsort 1.03/s 2% --

        (As I recall, 2% is within the margin of error for Benchmark.)

        In perl (the implementation of Perl), there's a special case for reverse sort so that it doesn't have the performance penalty you might otherwise expect it to have. That being the case, the main difference between reverse sort { $a cmp $b } and sort { $b cmp $a } is how they read to the programmer. I think that it's far more obvious what's going on when you reverse sort especially as the expressions involved become more complicated. It could be pretty easy to lose the $a and $b in a big block. Even if it were not optimized, I think you'd have to have a pretty long list before the performance penalty outweighs the maintainability benefit.

        This also means there's no performance penalty for reverse sort { $b <=> $a }, but that's just rude.

        Did you try it?
        I thought that too but testing samtregar and kyles versions took the same time. Ikegamis version is about 6% slower.