Re: HoH problem representing a data structure.

Rodrick Brown wrote:
> On Sun, Jun 8, 2008 at 10:37 AM, Gunnar Hjalmarsson <[email protected]>
> wrote:
> 
>> Rodrick Brown wrote:
>>
>>> #!/usr/bin/perl -w
>>>
>> The -w switch is redundant, since you have "use warnings;".
>>
>>  use strict;
>>> use warnings;
>>> use Data::Dumper;
>>> my $file = '/etc/passwd';
>>> my $hash;
>>> my ($user, $homeDir);
>>> my $count=0;
>>>
>> Why did you declare that variable?
>>
>>  open(my $fh, "<", $file) or die("Fatal error unable to read $file: $!");
>>> while(<$fh>) {
>>>  next if /^#/;
>>>  ($user, $homeDir) = (split /:/,$_)[0,5];
>>>  $hash->{$homeDir} = $user;
>>>
>> You probably want:
>>
>>    push @{ $hash->{$homeDir} }, $user;
>>
>>  }
>>> print Dumper($hash);
>>
> Yes please explain how exactly that line works? I know @{} dereferences an 
> array so it looks like your pushing each user into an anonymous array.

Exactly. The value of a hash element can only be a scalar, and so must represent
a list of items using a reference to an anonymous hash. The syntax

  push @{ $hash->{$homeDir} }, $user;

is concise because of 'autovivifaction', which will initialise the hash element
with an implied

  $hash->{$homeDir} = [];

if it doesn't already exist.

You should take a look at

  perldoc perlref

and

  perldoc perllol


to understand further.

HTH,

Rob