Perl - Hash of Array: Key look-up by Value

Question

I have a hash of arrays (HoA). I have been processing the values of this HoA using $arrayrefs. However, now I need to retrieve the $key based on the $arrayrefs.

my %a =  ( 1 => "ONE" , 
           2 => "TWO" ,
           3 => " Three", );

my %aa =  ( 4 => [ 'ONE' , 'TWO', 'THREE'], 
            5 => ['one' , 'two', 'three'],
            6 => ['more', 'dos', 'some'],
);

my @array = ('ONE' , 'TWO', 'THREE');
my $array_ref = \@array;

# returns the $key where the $value is 'ONE'
my ($any_match) = grep { $a{$_} eq 'ONE' } keys %a;
print $any_match."\n"; # this returns '1', as expected.. Good!

my ($match) = grep { $aa{$_} eq @$array_ref } keys %aa;
print $match."\n";  # <--- error: says that match is uninitialized

In the last print statement, I would like it to return 4. Does anyone know how to do this?

Eugene Yarmash · Accepted Answer · 2013-03-23 06:54:58Z

3

You can't compare arrays with eq. A simple solution is to turn both arrays into strings and comparing the strings using eq:

my ($match) = grep { join("", @{$aa{$_}}) eq join("", @$array_ref) } keys %aa;

For comparing arrays you could also utilize one of many modules from CPAN, e.g. Array::Compare, List::Compare, etc.

edited Mar 23, 2013 at 6:54

answered Mar 23, 2013 at 6:48

Eugene Yarmash

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Kenosis · Accepted Answer · 2013-03-23 06:54:06Z

2

Always use strict; use warnings;. Add use v5.10; since Perl's (v5.10+) smart matching will be used to compare arrays. Do the following:

my ($match) = grep { @{$aa{$_}} ~~ @$array_ref } keys %aa;

The smart operator ~~ is used here to compare the arrays.

answered Mar 23, 2013 at 6:54

Kenosis

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