Perl accessing the elements in a hash/hash reference data structure

Question

I have a question I'm hoping you could help with as I am new to hashes and hash reference stuff? I have the following data structure: ``` $VAR1 = { 'http://www.superuser.com/' => { 'difference' => { 'http://www.superuser.com/questions' => '10735', 'http://www.superuser.com/faq' => '13095' }, 'equal' => { 'http://www.superuser.com/ ' => '20892' } }, 'http://www.stackoverflow.com/' => { 'difference' => { 'http://www.stackoverflow.com/faq' => '13015', 'http://www.stackoverflow.com/questions' => '10506' }, 'equal' => { 'http://www.stackoverflow.com/ ' => '33362' } } ``` If I want to access all the URLs in the key `'difference'` so I can then perform some other actions on the URLs, what is the correct or preferred method of accessing those elements? e.g I will end up with the following URLs that I can then do stuff to in a `foreach` loop with: ``` http://www.superuser.com/questions http://www.superuser.com/faq http://www.stackoverflow.com/faq http://www.stackoverflow.com/questions ``` ----…

Accepted Answer

It is not difficult, just take the second level of keys off every key in the variable:

```
my @urls;
for my $key (keys %$VAR1) {
    push @urls, keys %{$VAR1->{$key}{'difference'}};
}
```

If you're struggling with dereferencing, just keep in mind that all values in a hash or array can only be a scalar value. In a multilevel hash or array the levels are just single hashes/arrays stacked on top of each other.

For example, you could do:

```
for my $value (values %$VAR1) {
    push @urls, keys %{$value->{'difference'}};
}
```

Or

```
for my $name (keys %$VAR1) {
    my $site = $VAR1->{$name};
    push @urls, keys %{$site->{'difference'}};
}
```

..taking the route either directly over the value (a reference to a hash) or over a temporary variable, representing the value via the key. There is more to read in perldoc perldata.

Perl accessing the elements in a hash/hash reference data structure

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