Perl Examples: Array of Arrays, Hash of Arrays, Hash of Hashes, Stack

Perl developers should understand how to use complex data structures effectively.

In this article, I picked the top 5 useful posts about complex data structures from perlmonks, and gave simple examples on how to use them.

1. Perl Array of Arrays

The following example defines a sample perl array of arrays.

@tgs = (
    ['article series', 'sed & awk', 'troubleshooting', 'vim', 'bash'],
    ['ebooks', 'linux 101', 'vim 101', 'nagios core', 'bash 101' ]
);

To access a single element, for example, to Access 2nd element from the 1st array, do the following:

$tgs[0][1];

Access all the elements one by one as shown below.

print @$_, "\n" foreach ( @tgs );

Read more: How do I make an array of arrays?

2. Perl Hash of Hashes

The following example defines a sample perl hash of hashes.

%tgs = (
    'articles' =>  {
                       'vim' => '20 awesome articles posted',
                       'awk' => '9 awesome articles posted',
                       'sed' => '10 awesome articles posted'
                   },
    'ebooks'   =>  {
                       'linux 101'    => 'Practical Examples to Build a Strong Foundation in Linux',
                       'nagios core'  => 'Monitor Everything, Be Proactive, and Sleep Well'
                   }
);

To access a single element from hash, do the following.

print $tgs{'ebooks'}{'linux 101'};

Read more: How do I make a hash of hashes?

3. Hash of Arrays

The following example defines a sample perl hash of arrays.

%tgs = (
    'top 5' =>  [ 'Best linux OS', 'Best System Monitoring', 'Best Linux Text editors' ],
    '15 example' => [ 'rpm command', 'crontab command', 'Yum command', 'grep command' ],
);

To access all elements one by one, do the following.

foreach my $key ( keys %tgs )  {
    print "Articles in group $key are: ";
    foreach ( @{$tgs{$key}} )  {
        print $_;
    }
}

Read more: How do I make a hash of arrays?

4. Making a Stack

Making a stack is very simple in Perl using arrays with push & pop function.

First, define an array as shown below.

@array = ( 1, 2, 3, 4, 5 );

Stack operation:

push ( @array, 6 ); # pushes the content at the end of array.
pop ( @array ); # gives the top element 6.

Read more: How do I make a stack?

5. Visualize a Complex Perl Data Structure

When you see a code which builds complex data structure, to visualize it better, use Dumper as shown below.

use Data::Dumper;
print Dumper $ref;

The above code snippet will dump the output in a user friendly way as shown below.

$VAR1 = {
          'a' => [
                   {
                     'A' => 1,
                     'B' => 2
                   },
                   {
                     'D' => [
                              4,
                              5,
                              6
                            ],
                     'C' => [
                              1,
                              2,
                              3
                            ]
                   }
                 ]
        };

From the above output, you can clearly tell which is array, which is hash, and the dependencies, etc.

Read more: How can I visualize my complex data structure?