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Little tricks

How do I count the unique elements of an array? by stillflame
Posted: 20 Jan 01

  Hashes automatically prevent you from having two
different keys with the same name.  If you assign different
values to the same element, the second one just rewrites
the first.  Thus, if you map an array onto the keys of a
hash with a value of one more than it's current value, you
get back a hash of elements corresponding to the unique
elements of an array and values equal to the number of
occurances of each.

sub count_unique {
    my @array = @_;
    my %count;
    map { $count{$_}++ } @array;

      #print them out:

    map {print "$_ = ${count{$_}}\n"} sort keys(%count);

      #or just return the hash:

    return %count;
}


 Alternatively, you could want to just get back an array of
the unique elements, not being concerned with the number of
occurances.

sub return_unique {
    my @array = @_;
    my %count;
    map {$count{$_} = 1} @array;
    return sort keys(%count);
}

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