Sounds right on 
# For each record:
push(@{$logged_in{$fields[2]}}, $fields[0]);

# Sample usage:
foreach my $loc (sort keys %logged_in) {
   foreach my $user (@{$logged_in{$loc}}) {
      print("$user logged in at location $loc.\n");
   }
}

[download]
or if you want the users only listed at most once per location: 
# For each record:
$logged_in{$fields[2]}{$fields[0]} = 1;

# Sample usage:
foreach my $loc (sort keys %logged_in) {
   foreach my $user (sort keys %{$logged_in{$loc}}) {
      print("$user logged in at location $loc.\n");
   }
}

[download]

In reply to Re: Categorization Problem by ikegami
in thread Categorization Problem by kayak9630

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