Sort::Key::Top(3pm)					User Contributed Perl Documentation				       Sort::Key::Top(3pm)

NAME

       Sort::Key::Top - select and sort top n elements

SYNOPSIS

	 use Sort::Key::Top (nkeytop top);

	 # select 5 first numbers by absolute value:
	 @top = nkeytop { abs $_ } 5 => 1, 2, 7, 5, 5, 1, 78, 0, -2, -8, 2;
		# ==> @top = (1, 2, 1, 0, -2)

	 # select 5 first numbers by absolute value and sort accordingly:
	 @top = nkeytopsort { abs $_ } 5 => 1, 2, 7, 5, 5, 1, 78, 0, -2, -8, 2;
		# ==> @top = (0, 1, 1, 2, -2)

	 # select 5 first words by lexicographic order:
	 @a = qw(cat fish bird leon penguin horse rat elephant squirrel dog);
	 @top = top 5 => @a;
		# ==> @top = qw(cat fish bird elephant dog);

DESCRIPTION

       The functions available from this module select the top n elements from a list using several common orderings and custom key extraction
       procedures.

       They are all variations around

	 keytopsort { CALC_KEY($_) } $n => @data;

       In array context, this function calculates the ordering key for every element in @data using the expression inside the block. Then it
       selects and orders the $n elements with the lower keys when compared lexicographically.

       It is equivalent to the pure Perl expression:

	 (sort { CALC_KEY($a) cmp CALC_KEY($b) } @data)[0 .. $n-1];

       If $n is negative, the last $n elements from the bottom are selected:

	 topsort 3 => qw(foo doom me bar doz hello);
	      # ==> ('bar', 'doz', 'doom')

	 topsort -3 => qw(foo doom me bar doz hello);
	      # ==> ('foo', 'hello', 'me')

	 top 3 => qw(foo doom me bar doz hello);
	      # ==> ('doom', 'bar', 'doz')

	 top -3 => qw(foo doom me bar doz hello);
	      # ==> ('foo', 'me', 'hello')

       In scalar context, the value returned by the functions on this module is the cutoff value allowing to select nth element from the array.
       For instance:

	 # n = 5;
	 scalar(topsort 5 => @data) eq (sort @data)[4]	  # true

	 # n = -5;
	 scalar(topsort -5 => @data) eq (sort @data)[-5]  # true

       Note that on scalar context, the "sort" variations (see below) are usually the right choice:

	 scalar topsort 3 => qw(me foo doz doom me bar hello); # ==> 'doz'

	 scalar top 3 => qw(me foo doz doom me bar hello); # ==> 'bar'

       Note also, that the index is 1-based (it starts at one instead of at zero). The "atpos" set of functions explained below do the same and
       are 0-based.

       Variations allow to:

       - use the own values as the ordering keys
	     topsort 5 => qw(a b ab t uu g h aa aac);
		# ==> a aa aac ab b

       - return the selected values in the original order
	     top 5 => qw(a b ab t uu g h aa aac);
		# ==> a b ab aa aac

       - use a different ordering
	   For instance comparing the keys as numbers, using the locale configuration or in reverse order:

	     rnkeytop { length $_ } 3 => qw(a ab aa aac b t uu g h);
		# ==> ab aa aac

	     rnkeytopsort { length $_ } 3 => qw(a ab aa aac b t uu g h);
		# ==> aac ab aa

	   A prefix is used to indicate the required ordering:

	   (no prefix)
	       lexicographical ascending order

	   r   lexicographical descending order

	   l   lexicographical ascending order obeying locale configuration

	   r   lexicographical descending order obeying locale configuration

	   n   numerical ascending order

	   rn  numerical descending order

	   i   numerical ascending order but converting the keys to integers first

	   ri  numerical descending order but converting the keys to integers first

	   u   numerical ascending order but converting the keys to unsigned integers first

	   ru  numerical descending order but converting the keys to unsigned integers first

       - select the head element from the list sorted
	     nhead 6, 7, 3, 8, 9, 9;
		 # ==> 3

	     nkeyhead { length $_ } qw(a ab aa aac b t uu uiyii)
		 # ==> 'a'

       - select the tail element from the list sorted
	     tail qw(a ab aa aac b t uu uiyii);
		 # ==> 'uu'

	     nkeytail { length $_ } qw(a ab aa aac b t uu uiyii)
		 # ==> 'uiyii'

       - select the element at position n from the list sorted
	     atpos 3, qw(a ab aa aac b t uu uiyii);
		 # ==> 'ab';

	     rnkeyatpos { abs $_ } 2 => -0.3, 1.1, 4, 0.1, 0.9, -2;
		 # ==> 1.1

	     rnkeyatpos { abs $_ } -2 => -0.3, 1.1, 4, 0.1, 0.9, -2;
		 # ==> -0.3

	   Note that for the atpos set of functions indexes start at zero.

       - return a list composed by the elements with the first n ordered keys and then the remaining ones.
	     ikeypart { length $_ } 3 => qw(a bbbb cc ddddd g fd);
		 # ==> a cc g bbbb ddddd fd

       - return two arrays references, the first array containing the elements with the first n ordered keys and the second with the rest.
	     keypartref { length $_ } 3 => qw(a bbbb cc ddddd g fd);
		 # ==> [a cc g] [bbbb ddddd fd]

       The full list of available functions is:

	 top ltop ntop itop utop rtop rltop rntop ritop rutop

	 keytop lkeytop nkeytop ikeytop ukeytop rkeytop rlkeytop rnkeytop
	 rikeytop rukeytop

	 topsort ltopsort ntopsort itopsort utopsort rtopsort rltopsort
	 rntopsort ritopsort rutopsort

	 keytopsort lkeytopsort nkeytopsort ikeytopsort ukeytopsort
	 rkeytopsort rlkeytopsort rnkeytopsort rikeytopsort rukeytopsort

	 head lhead nhead ihead uhead rhead rlhead rnhead rihead ruhead
	 keyhead lkeyhead nkeyhead ikeyhead ukeyhead rkeyhead rlkeyhead
	 rnkeyhead rikeyhead rukeyhead

	 tail ltail ntail itail utail rtail rltail rntail ritail rutail
	 keytail lkeytail nkeytail ikeytail ukeytail rkeytail rlkeytail
	 rnkeytail rikeytail rukeytail

	 atpos latpos natpos iatpos uatpos ratpos rlatpos rnatpos riatpos
	 ruatpos keyatpos lkeyatpos nkeyatpos ikeyatpos ukeyatpos rkeyatpos
	 rlkeyatpos rnkeyatpos rikeyatpos rukeyatpos

	 part lpart npart ipart upart rpart rlpart rnpart ripart
	 rupart

	 keypart lkeypart nkeypart ikeypart ukeypart rkeypart
	 rlkeypart rnkeypart rikeypart rukeypart

SEE ALSO

       Sort::Key, "sort" in perlfunc.

       The Wikipedia article about selection algorithms <http://en.wikipedia.org/wiki/Selection_algorithm>.

COPYRIGHT AND LICENSE

       Copyright (C) 2006-2008, 2011 by Salvador Fandin~o (sfandino@yahoo.com).

       This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself, either Perl version 5.8.8 or,
       at your option, any later version of Perl 5 you may have available.

perl v5.14.2							    2011-06-17						       Sort::Key::Top(3pm)