How to extract keyvalue pairs with duplicate values

august3 has asked for the wisdom of the Perl Monks concerning the following question:

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•Re: How to extract keyvalue pairs with duplicate values by merlyn (Sage) on Sep 28, 2004 at 14:49 UTC
You could build up a reverse hash with arrayrefs, then count the length of those: `my %forward_hash = (... your hash...); ## accumulate the reverse mapping my %reverse_hash; while (my($k, $v) = each %forward_hash)) { push @{$reverse_hash{$v}}, $k; } ## now see if there are dups: while (my($k, $v) = each %reverse_hash)) { my @vals = @$v; next unless @vals > 1; # skip single values print "value $k is shared by keys @vals\n"; }` [download] -- Randal L. Schwartz, Perl hacker Be sure to read my standard disclaimer if this is a reply.	[reply] [d/l]
Re: How to extract keyvalue pairs with duplicate values by borisz (Canon) on Sep 28, 2004 at 14:53 UTC
This code build a new hash with the values as keys and collect all keys that point to this value. `my %h = ( a => 10, b => 10, c => 'd', e => 'd', f => 'a', ); my %x; while ( my ( $k, $v ) = each %h ) { push @{ $x{$v} }, $k; } __END__ $x = { 'a' => [ 'f' ], '10' => [ 'a', 'b' ], 'd' => [ 'e', 'c' ] };` [download] If you just want to remove the duplicates try: `my %n = reverse %h; %n = reverse %n;` [download] Boris	[reply] [d/l] [select]
Re: How to extract keyvalue pairs with duplicate values by pelagic (Priest) on Sep 28, 2004 at 14:59 UTC
Well Mister merlyn was faster ... anyway: `use strict; my %hash = ( 1 => 'a' , 2 => 'b' , 3 => 'c' , 4 => 'b' , 5 => 'e' , 6 => 'b' , 7 => 'a' , 8 => 'z' ); my %values_with_multiple_keys = (); foreach my $k (keys %hash) { push @{ $values_with_multiple_keys{$hash{$k}} }, $k; } foreach my $m (keys %values_with_multiple_keys) { if (scalar @{$values_with_multiple_keys{$m}} > 1) { print "$m is referenced by: ", join ', ', @{$values_with_multi +ple_keys{$m}}, "\n"; } } ___OUTPUT___ a is referenced by: 7, 1, b is referenced by: 6, 2, 4,` [download] pelagic	[reply] [d/l]
Re^2: How to extract keyvalue pairs with duplicate values by august3 (Acolyte) on Oct 18, 2004 at 23:52 UTC
Hi everybody Thanks for helping me. Augustine	[reply]
Re^3: How to extract keyvalue pairs with duplicate values by pelagic (Priest) on Oct 19, 2004 at 08:45 UTC
Hello Augustine! it's now 3 weeks ago but you are very welcome anyway! pelagic	[reply]
Re: How to extract keyvalue pairs with duplicate values by Limbic~Region (Chancellor) on Sep 28, 2004 at 14:52 UTC
august3, Your wording leaves a fair amount to be desired. I can see at least two interpretations: Given a value, list all corresponding keys Produce a list of values that have multiple keys The first one is easy: `my @keys = grep { $hash{$_} eq 'value' }keys %hash;` [download] The second is just a bit more work: `my %value; push @{ $value{ $hash{$_} } }, $_ for keys %hash; for ( keys %value ) { if ( @{ $value{$_} } > 1 ) { print "$_ :\n"; print "\t$_\n" for @{ $value{$_} }; } }` [download] Cheers - L~R	[reply] [d/l] [select]