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

I have a big hash. Some of the values in the hash are the same how do I do to add all the keys for the values that are the same?

An example of what I mean would be:

%hash=(10=>1, 20=>2, 30=>1 40=>2);

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After the procedure it should look:

%hash=(40=>1, 60=>2);

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If I want to check if a key exist in a hash I do..

if (exists ($hash{$key}))
{
do..
}

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This i very confusing to me. To $hash{$key} should be the same as the value. Could someone explain.

And how do I check if a value exists? I can not reverse the hash since it has many of the same values.

Thank you for your help.

Code tags added by GrandFather

Replies are listed 'Best First'.
Re: Confusion due to hash
by McDarren (Abbot) on Jan 31, 2006 at 00:26 UTC
    Okay, well the approach I would take is to create a new hash where the keys/values of your existing hash are reversed.

    You simply iterate through you existing hash and build up the new hash as you go, like so: 

    foreach my $key (keys %hash) {
    $newhash{$hash{$key}} += $key;
}

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    Using your sample data, that will give you something that looks like: 
    %newhash = (
             '1' => 40,
             '2' => 60
           );

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    Does this help?

    Cheers,
    Darren :)

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Re: Confusion due to hash
by GrandFather (Saint) on Jan 31, 2006 at 00:57 UTC

    I'm not quite sure where you are going with this, but it seems to me that you need two hashes: 

    use strict;
use warnings;
use Data::Dumper;

my %hash= (10=>1, 20=>2, 30=>1, 40=>2);
my %rhash;

map {$rhash{$hash{$_}} += $_} keys %hash;

print Dumper (\%hash);
print Dumper (\%rhash);

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    Prints:

    $VAR1 = '%hash';
$VAR2 = {
          '40' => 2,
          '30' => 1,
          '10' => 1,
          '20' => 2
        };
$VAR1 = '%rhash';
$VAR2 = {
          '1' => 40,
          '2' => 60
        };

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    DWIM is Perl's answer to Gödel
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      reverse is useful to get the output in the desired format: 
      use strict; use warnings;
use Data::Dumper;

my %input = (10=>1, 20=>2, 30=>1, 40=>2);
my %output;

map { $output{ $input{ $_ } } += $_ } keys %input;

%output = reverse %output;

print Dumper \%output;

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      Prints: 
      $VAR1 = {
          '60' => '2',
          '40' => '1'
        };

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      Update: I paid to much attention to the requested output, and not enough to the note that he couldn't reverse the hash
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        Except that OP says

        And how do I check if a value exists? I can not reverse the hash since it has many of the same values.

        in which case leaving %rhash in its original form allows the lookup that OP wants - no reversing required.

        DWIM is Perl's answer to Gödel
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Re: Confusion due to hash
by martin (Friar) on Jan 31, 2006 at 03:21 UTC
    You did not state what should happen in a case like this: 
    %hash = (10 => 1, 20 => 1, 30 => 2);

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    Can you elaborate?

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