NAME

       Array::Unique - Tie-able array that allows only unique values

SYNOPSIS

        use Array::Unique;
        tie @a, 'Array::Unique';

        Now use @a as a regular array.

DESCRIPTION

       This package lets you create an array which will allow only one occurrence of any value.

       In other words no matter how many times you put in 42 it will keep only the first
       occurrence and the rest will be dropped.

       You use the module via tie and once you tied your array to this module it will behave
       correctly.

       Uniqueness is checked with the 'eq' operator so among other things it is case sensitive.

       As a side effect the module does not allow undef as a value in the array.

EXAMPLES

        use Array::Unique;
        tie @a, 'Array::Unique';

        @a = qw(a b c a d e f);
        push @a, qw(x b z);
        print "@a\n";          # a b c d e f x z

DISCUSSION

       When you are collecting a list of items and you want to make sure there is only one
       occurrence of each item, you have several option:

       1) using an array and extracting the unique elements later
           You might use a regular array to hold this unique set of values and either remove
           duplicates on each update by that keeping the array always unique or remove duplicates
           just before you want to use the uniqueness feature of the array. In either case you
           might run a function you call @a = unique_value(@a);

           The problem with this approach is that you have to implement the unique_value function
           (see later) AND you have to make sure you don't forget to call it. I would say don't
           rely on remembering this.

           There is good discussion about it in the 1st edition of the Perl Cookbook of O'Reilly.
           I have copied the solutions here, you can see further discussion in the book.

           Extracting Unique Elements from a List (Section 4.6 in the Perl Cookbook 1st ed.)

           # Straightforward

            %seen = ();
            @uniq = ();
            foreach $item (@list) [
                unless ($seen{$item}) {
                  # if we get here we have not seen it before
                  $seen{$item} = 1;
                  push (@uniq, $item);
               }
            }

           # Faster

            %seen = ();
            foreach $item (@list) {
              push(@uniq, $item) unless $seen{$item}++;
            }

           # Faster but different

            %seen;
            foreach $item (@list) {
              $seen{$item}++;
            }
            @uniq = keys %seen;

            # Faster and even more different
            %seen;
            @uniq = grep {! $seen{$_}++} @list;

       2) using a hash
           Some people use the keys of a hash to keep the items and put an arbitrary value as the
           values of the hash:

           To build such a list:

            %unique = map { $_ => 1 } qw( one two one two three four! );

           To print it:

            print join ", ", sort keys %unique;

           To add values to it:

            $unique{$_}=1 foreach qw( one after the nine oh nine );

           To remove values:

            delete @unique{ qw(oh nine) };

           To check if a value is there:

            $unique{ $value };        # which is why I like to use "1" as my value

           (thanks to Gaal Yahas for the above examples)

           There are three drawbacks I see:

           1) You type more.
           2) Your reader might not understand at first why did you use hash and what will be the
           values.
           3) You lose the order.

           Usually non of them is critical but when I saw this the 10th time in a code I had to
           understand with 0 documentation I got frustrated.

       3) using Array::Unique
           So I decided to write this module because I got frustrated by my lack of understanding
           what's going on in that code I mentioned.

           In addition I thought it might be interesting to write this and then benchmark it.

           Additionally it is nice to have your name displayed in bright lights all over CPAN ...
           or at least in a module.

           Array::Unique lets you tie an array to hmmm, itself (?)  and makes sure the values of
           the array are always unique.

           Since writing this I am not sure if I really recommend its usage.  I would say stick
           with the hash version and document that the variable is aggregating a unique list of
           values.

       4) Using real SET
           There are modules on CPAN that let you create and maintain SETs.  I have not checked
           any of those but I guess they just as much of an overkill for this functionality as
           Unique::Array.

BUGS

        use Array::Unique;
        tie @a, 'Array::Unique';

        @c = @a = qw(a b c a d e f b);

        @c will contain the same as @a AND two undefs at the end because
        @c you get the same length as the right most list.

TODO

       Test:

       Change size of the array Elements with false values ('', '0', 0)

          splice:
          splice @a;
          splice @a,  3;
          splice @a, -3;
          splice @a,  3,  5;
          splice @a,  3, -5;
          splice @a, -3,  5;
          splice @a, -3, -5;
          splice @a,  ?,  ?, @b;

       Benchmark speed

       Add faster functions that don't check uniqueness so if I know part of the data that comes
       from a unique source then I can speed up the process, In short shoot myself in the leg.

       Enable optional compare with other functions

       Write even better implementations.

AUTHOR

       Gabor Szabo <gabor@pti.co.il>

LICENSE

       Copyright (C) 2002-2008 Gabor Szabo <gabor@pti.co.il> All rights reserved.
       http://www.pti.co.il/

       You may distribute under the terms of either the GNU General Public License or the
       Artistic License, as specified in the Perl README file.

       No WARRANTY whatsoever.

CREDITS

        Thanks for suggestions and bug reports to
        Szabo Balazs (dLux)
        Shlomo Yona
        Gaal Yahas
        Jeff 'japhy' Pinyan
        Werner Weichselberger

VERSION

       Version: 0.08

       Date:    2008 June 04