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List::Uniq - extract the unique elements of a list


  use List::Uniq ':all';

  @uniq = uniq(@list);

  $list = [ qw|foo bar baz foo| ];
  $uniq = uniq($list);


List::Uniq extracts the unique elements of a list. This is a commonly re-written (or at least re-looked-up) idiom in Perl programs.


uniq( { OPTIONS }, ele1, ele2, ..., eleN )

uniq() takes a list of elements and returns the unique elements of the list. Each element may be a scalar value or a reference to a list.

If the first element is a hash reference it is taken to be a set of options that alter the way in which the unique filter is applied. The keys of the option set are:

  • sort

    If set to a true value, the unique elements of the list will be returned sorted. Perl's default sort will be used unless the compare option is also passed.

    sort defaults to false.

  • flatten

    If set to a true value, array references in the list will be recursively flattened, such that

      ( 'foo', [ [ 'bar' ] ], [ [ [ 'baz', 'quux' ] ] ] )


      ( 'foo', 'bar', 'baz', 'quux' )

    flatten defaults to true.

  • compare

    A code reference that will be used to sort the elements of the list if the sort option is set. Passing a non-coderef will cause uniq to throw an exception.

    The code ref will be passed a pair of list elements to be compared and should return the same values as the cmp operator.

    Using a custom sort slows things down because the sort routine will be outside of the List::Uniq package. This requires that the pairs to be compared be passed as parameters to the sort routine, not set as package globals (see "sort" in perlfunc). If speed is a concern, you are better off sorting the return of uniq yourself.

The return value is a list of the unique elements if called in list context or a reference to a list of unique elements if called in scalar context.



Nothing by default.

Optionally the uniq function.

Everything with the :all tag.


If you want to unique a list as you insert into it, see Array::Unique by Gabor Szabo.

This module was written out of a need to unique an array that was auto-vivified and thus not easily tied to Array::Unique.


James FitzGibbon <>


The idioms used to unique lists are taken from recipe 4.7 in the Perl Cookbook, 2e., published by O'Reilly and Associates and from the Perl FAQ section 5.4.

I pretty much just glued it together in a way that I find easy to use. Hopefully you do too.


Copyright (c) 2004-2008 Primus Telecommunications Canada Inc.

Copyright (c) 2008-2010 James FitzGibbon

All Rights Reserved.

This library is free software; you may use it under the same terms as perl itself.