Re: Using an element in a Hash of Arrays to access the key

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1185923

use strict;
use warnings;

my @array1 = ("one", "two", "three", "four", "five");
my @array2 = ("banana", "pear", "apple");

my %hash = (
    numbers => \@array1,
    fruit => \@array2
);

# make a reverse lookup hash

my %reverse;
for my $key (keys %hash)
  {
  push @{ $reverse{$_} }, $key for @{ $hash{$key} };
  }

# then do direct lookup ( you may get multiple values, or none )

print "banana -> @{ $reverse{'banana'} // [] }\n";
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Re^2: Using an element in a Hash of Arrays to access the key by orangepeel1 (Novice) on Mar 25, 2017 at 17:45 UTC
Thank you, this worked. Could you clarify what the purpose of `// []` is in `print "banana -> @{ $reverse{'banana'} // [] }\n";`?	[reply] [d/l] [select]
Re^3: Using an element in a Hash of Arrays to access the key by tybalt89 (Monsignor) on Mar 25, 2017 at 17:48 UTC
That is there to handle the case when the element you are trying to look up does not exist. Try looking up 'peach' with and without that part of the code.	[reply]