Working with multi dimensional hashes in Perl?

Question

quick question about pushing to a multi-dimensional hash in Perl. I have the following variables:

%pids #name of hash 
$pid = 24633 #key of the has 
$time 00:0 #time reference 
$line #has a line full of data

I am inputing $pid and $time in from $line. If the key 24633 exists along with reference element 05:3, then it add the line to 05:3 and use 05:3 as a key.

pids{24633}{05:3}

I've tried:

if ($pids{$pid}{$time}){
     @{$pids{$pid}{$time}} -> $line;
}

I've also tried this:

if ($pids{$pid}{$time}){
    push @{$pids{$pid}{$time}}, $line;

But it's keep giving me a "Not a HASH reference" when it tries to do the push. Any suggestions? Thanks!

This is how I'm building the hash:

foreach my $key (keys %pids){
    if ($key =~ $mPID){
    push @messages, $line;
    }
}

Here's the hash structure:

$VAR1 = {   
      '17934' => [
                   '14:3'
                 ],
      '17955' => [
                   '13:3'
                 ],
      '24633' => [
                   '05:3'
                 ],
      '6771' => [
                  '04:1'
                ],
      '7601' => [
                  '06:0'
                ],
};

You have a hash of arrays, but you try to use it as a hash of hashes. Your code is also not relevant to the question, as it does not show how you build your hash. — TLP
– TLP, Commented Apr 13, 2014 at 21:34
Nope, you haven't. You've included how you use the hash to try to build an array, which would with the current code fill the array with undefined values. (Because you use $key, but store $line.) — TLP
– TLP, Commented Apr 13, 2014 at 21:47

Miller · Accepted Answer · 2014-04-13 21:40:56Z

1

Your %pids structure is initialized as a hash of arrays when you're trying to access it as a hash of hash

use strict;
use warnings;

my %pids = (   
    '17934' => [ '14:3' ],
    '17955' => [ '13:3' ],
    '24633' => [ '05:3' ],
    '6771'  => [ '04:1' ],
    '7601'  => [ '06:0' ],
);

print $pids{7601}[0], "\n";  #  Prints 06:0

print $pids{7601}{"06:0"}; # Error

You'll have to figure out why your %pids is a hash of arrays first if you really want it to be a hash of hashes of arrays.

perldsc - Perl Data Structures Cookbook

edited Apr 13, 2014 at 21:40

answered Apr 13, 2014 at 21:35

Miller

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3 Comments

cycloxr Over a year ago

why would I use one over the other? forgive me, still learning. And thanks for the link, lots of great info

Miller Over a year ago

Why would one use a hash over an array? You'd use an array if you wanted an ordered list of values. You'd use a hash if you wanted key, value pair relationships. Only you know your data and therefore which one is needed.

cycloxr Over a year ago

Thank you. I meant a hash of arrays or a hash of hashes

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