Re: inverting hash / grouping values

[hdb]

It can be done this way

my %ih = map { my $k = $_; $k => [ grep { $h{$_} eq $k } keys %h ] } k
+eys {reverse %h};

but it must be much slower than the simple code you have shown. Using keys and reverse to get the list of the unique values of the hash and then repeatedly grep on the keys of the original hash must be rather slow.

[LanX]

Thanks, but I wasn't clear enough!

I would like to see an elegant solution, that is w/o deeply nested constructs like map within map.

IMHO one of the striking points of chaining simple functions is better readability compared to nested solutions.

[hdb]

Next attempt:

use strict;
use warnings;
use List::Util 'reduce';
use List::MoreUtils 'mesh';
use Data::Dumper;

my %t = ( a => 1, b => 2, c => 1, d => 2, e => 1, f => 2, g => 3 );

my( $k, $v ) = @{
     +reduce { 
       if( $a->[0]->[-1] eq $b->[0]->[0] ) {
         push @{$a->[1]->[-1]}, $b->[1]->[0]->[0];
       } else {
         push @{$a->[0]}, $b->[0]->[0];
         push @{$a->[1]}, [$b->[1]->[0]->[0]];
       }
       $a;
     } 
     map { [ [$t{$_}], [[$_]] ] } sort { $t{$a} <=> $t{$b} } keys %t }
+;

my %i = mesh @$k, @$v;
print Dumper \%i;

[LanX]

Thanks, fascinating ...

...but maybe I was not still not clear enough! ;-)

I'd rather prefer a solution better (readable | self documenting | maintainable) then

  DB<140> \%h
 => { a => 1, b => 2, c => 1, d => 2, e => 3 }

  DB<141> push @{ $h2{$h{$_}} }, $_ for keys %h
 => ""

  DB<142> \%h2
 => { 1 => ["c", "a"], 2 => ["b", "d"], 3 => ["e"] }

so not necessarily a complicated solution only relying on List::Util or List::MoreUtils ...

BUT I know that you love this kind of games ... ;D