How can multiple hash values be retrieved using perl? - perl

Question

How can multiple hash values be retrieved? I tried using
use Hash::MultiValue and get_all(). It throws an error saying "Can't call method "get_all" on an undefined value" . Which is the better option to implement this functionality of multiple values for a particular key ? The value of the key is the file that is being opened.
use warnings;
use List::MoreUtils qw(firstidx);
use Hash::MultiValue;
my $key_in;
…
open ($FR, "<$i") or die "couldn't open list";
while($line=<$FR>){
if($line =~ /search_pattern/){
my $idx = firstidx { $_ eq 'hash_key' } #tags;
my $key= #tags[$idx+1];
$hash{$key}= Hash::MultiValue->new($key=>'$i');
}
close($FR);
for my $key_in ( sort keys %hash ) {
#key_in = $hash->get_all('$key_in');
print "$key_in = $hash{$key_in}\n";
}
my $key_in = <STDIN>;
if (exists($hash{$key_in})){
$hash_value = $hash{$key_in};
}else{
exit;
}

Answer 1

I think you want an array reference for the value. You can then treat that as an array. This is the sort of stuff we show you in Intermediate Perl:
$hash{$key} = [];
push #{ $hash{$key} }, $some_value;
my #values = #{ $hash{$key} };
With Perl v5.24, you can use postfix dereferencing to make it a bit prettier:
use v5.24;
$hash{$key} = [];
push $hash{$key}->#*, 'foo';
push $hash{$key}->#*, 'bar';
my #values = $hash{$key}->#*;
And, since Perl automatically takes an undefined value and turns it into the reference structure you need (auto vivification), you don't need to initialize an undefined value:
use v5.24;
push $hash{$key}->#*, 'foo';
push $hash{$key}->#*, 'bar';
my #values = $hash{$key}->#*;
Get all the keys of a hash:
my #keys = keys %hash;
Get all of the values (in the order of the corresponding keys if you haven't changed the hash since you called keys):
my #values = values %keys;
Get some values with a hash slice:
my #some_values = #hash{#some_keys};
Get some keys and values (key-value slice):
use v5.20;
my %smaller_hash = %hash{#some_keys}

Answer 2

Here is an example of how you can use get_all() from Hash::MultiValue to retrive multiple hash values for a given key:
use strict;
use warnings;
use Data::Dumper qw(Dumper);
use Hash::MultiValue;
my $hash = Hash::MultiValue->new();
$hash->add(tag1 => 'file1');
$hash->add(tag1 => 'file2');
$hash->add(tag2 => 'file3');
my #foo = $hash->get_all('tag1');
print(Dumper(\#foo));
Output:
$VAR1 = [
'file1',
'file2'
];