in reply to mildly tricky hash question

Try... 
my %reverse_hash;
foreach my $key ( keys %hash ) {
    push @{$reverse_hash{$hash{$key}}, $key;
}

[download]

Or maybe a little clearer: 

my %reverse_hash;
while ( my ( $key, $value ) = each %hash ) {
    push @{$reverse_hash{$value}}, $key;
}

[download]


Dave

Replies are listed 'Best First'.
Re: Re: mildly tricky hash question
by matija (Priest) on Mar 30, 2004 at 08:43 UTC
    That would give him a hash of arrays - not what he's looking for. You could join all the arrays right after your loop, or you could change it to 
    my %reverse_hash;
foreach my $key ( keys %hash ) {
    $reverse_hash{$key}=$reverse_hash{$key}
                        ? "$reverse_hash{$key},$hash{$key}"
                        : $hash{$key};
}

    [download]
[reply]
[d/l]
      I'm getting an "undefined" error at the print statement when I use that bit of code. Warnings says that line 16 is using an uninitilized value..... what have I done wrong? 
      #!/usr/bin/perl -w
use strict;

my $key;
my %hash = (
             ball1 => 'red',
             ball2 => 'blue',
             ball3 => 'red',
           );
# thanks to http://www.perlmonks.org/index.pl?node_id=340896 for this 
+block of code

my %reverse_hash;
foreach $key ( keys %hash ) {
    $reverse_hash{$key} = $reverse_hash{$key}
                        ? "$reverse_hash{$key},$hash{$key}"
                        : $hash{$key};
}
print "$reverse_hash{red}\n";

      [download]
[reply]
[d/l]

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