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Re: Getting Keys of Hash from Values

by tmoertel (Chaplain)
on Nov 16, 2005 at 21:20 UTC ( #509203=note: print w/replies, xml ) Need Help??


in reply to Getting Keys of Hash from Values

On the subject of inverting hashes, you can invert a hash via reverse like so:
my %hash = ( a => 1, b => 2, c => 3 ); my %inverted_hash = reverse %hash;
To see the results, we can use Data::Dumper:
use Data::Dumper; print Dumper(\(%hash, %inverted_hash)); # Output: # $VAR1 = { # 'c' => 3, # 'a' => 1, # 'b' => 2 # }; # $VAR2 = { # '1' => 'a', # '3' => 'c', # '2' => 'b' # };
As you note, however, not all hashes have one-to-one mappings to their inverted forms. In such a case the inverted form will have fewer key-value pairs than the original hash:
%hash = ( a => 1, b => 2, c => 2 ); %inverted_hash = reverse %hash; print Dumper(\(%hash, %inverted_hash)); # Output: # $VAR1 = { # 'c' => 2, # 'a' => 1, # 'b' => 2 # }; # $VAR2 = { # '1' => 'a', # '2' => 'c' # };
You can use this property to test whether you have lost information during inversion:
print "keys were lost during inversion\n" if keys %inverted_hash < keys %hash;

Cheers,
Tom

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