http://www.perlmonks.org?node_id=981335


in reply to Uniq and sort in perl

Any time you need to get a unique list in Perl you should think of using a hash. Say you defined a hash called %seen outside the while loop and changed the print line to read... 
print "$vm, $filer, \n" if ! $seen{"$vm:$filler"}++;

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This will print the entry the first time and increment the seen flag for that pair, now subsequent identical results will encounter a set flag and not print as a result

print "Good ",qw(night morning afternoon evening)[(localtime)[2]/6]," fellow monks."

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Re^2: Uniq and sort in perl
by slayedbylucifer (Scribe) on Jul 12, 2012 at 09:22 UTC

    Utilitarian, I can't thank you enough. Your solution is **exactly** what I was looking for. Thank you very Much !!

    But, unfortunately i am not able to decode or understand your oneliner. Could you guide me to some documentation for the same.

    Thanks Again for the lightning fast response.

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[reply]
      What is happening in that one liner: 
      print "$vm, $filer, \n" if ! $seen{"$vm:$filler"}++

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      It's quite an elegant piece of code, with a lot going on. So let's break it down:
      • The hash called '%seen' is being checked - to see if it contains a key called "$vm:$filler". The value of this is being tested for being 'false'. (because there's 'if !' which is 'if not') - which is literally 'if not seen'
      • So if $seen{"$vm:filler"} is _not_ 'true' - we haven't seen it, and so we print it.
      • And then that ++ kicks in, to post increment, 'adding one' to $seen{"$vm:$filler"}' - so the first time we try doing that, it's going to be undefined (and thus false) triggering a print. Second time, it'll be 'non zero' and so evaluate as 'true'.
      For bonus points, you can probably do: 
      foreach my $key ( keys %seen )
{
  print "$key was seen $seen{$key} times\n";
}

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      Does that make it any clearer?
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