Populating Hash passed to a sub as reference

techman2006 has asked for the wisdom of the Perl Monks concerning the following question:

Hi, I am somewhat novice to Perl world. I was trying to write a sub which takes Hash as reference and populate the hash (passed as reference). But when I try to dump the data of the Hash I didn't get any output. Below is the code which I tried to do so

use strict;
use warnings;
use Data::Dumper;

sub PopulateHash {
    my $hashParm = shift;
    my %hash1 = %$hashParm;
    %hash1 = ( 
                1 => 'a',
                2 => 'b',
                3 => 'c',
                4 => 'd');
    print Dumper(\%hash1);
}

my %hash = ();
PopulateHash(\%hash);
print Dumper(\%hash);
[download]

The output of above code is given below

$VAR1 = {
          '4' => 'd',
          '1' => 'a',
          '3' => 'c',
          '2' => 'b'
        };
$VAR1 = {};
[download]

As this hash will be used at many places so I don't want to have a copy to avoid overheads. Any pointers about where I am missing will be great.

Comment on Populating Hash passed to a sub as reference Select or Download Code

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Re: Populating Hash passed to a sub as reference by Kenosis (Priest) on Oct 24, 2013 at 18:48 UTC
Oh, you're so very close! Try the following, instead: `use strict; use warnings; use Data::Dumper; sub PopulateHash { my $hashParm = shift; %$hashParm = ( 1 => 'a', 2 => 'b', 3 => 'c', 4 => 'd'); print Dumper(\%$hashParm); } my %hash = (); PopulateHash(\%hash); print Dumper(\%hash);` [download] Output: `$VAR1 = { '4' => 'd', '1' => 'a', '3' => 'c', '2' => 'b' }; $VAR1 = { '4' => 'd', '1' => 'a', '3' => 'c', '2' => 'b' };` [download] Just use the dereferenced parm that was sent to populate your hash...	[reply] [d/l] [select]
Re: Populating Hash passed to a sub as reference by GotToBTru (Prior) on Oct 24, 2013 at 19:11 UTC
One item of clarfication: you have `PopulateHash(\%hash);` [download] which passes a reference to %hash to the subroutine PopulateHash. You create a new hash, %hash1, which is a copy of the old, %hash. Both are empty in this case. `my %hash1 = %$hashParm;` [download] The changes in the subroutine are made to the new hash, which is a separate from the original. That's why you don't see the changes in the main program.	[reply] [d/l] [select]
Re: Populating Hash passed to a sub as reference by Anonymous Monk on Oct 25, 2013 at 00:46 UTC
See references quick reference and see the "References" section in Chapter 3 in the free book Modern Perl a loose description of how experienced and effective Perl 5 programmers work....You can learn this too.	[reply]