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
    

- use an array or hash index instead of a subroutine to extract the key

  slottop 0, 2, [4], [1], [3], [2], [4];
     # ==> [1], [2]
    

- 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:

- 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
  slottop lslottop nslottop islottop uslottop rslottop rlslottop rnslottop
  rislottop ruslottop
  topsort ltopsort ntopsort itopsort utopsort rtopsort rltopsort
  rntopsort ritopsort rutopsort
  keytopsort lkeytopsort nkeytopsort ikeytopsort ukeytopsort
  rkeytopsort rlkeytopsort rnkeytopsort rikeytopsort rukeytopsort
  slottopsort lslottopsort nslottopsort islottopsort uslottopsort
  rslottopsort rlslottopsort rnslottopsort rislottopsort ruslottopsort
  head lhead nhead ihead uhead rhead rlhead rnhead rihead ruhead
  keyhead lkeyhead nkeyhead ikeyhead ukeyhead rkeyhead rlkeyhead
  rnkeyhead rikeyhead rukeyhead
  slothead lslothead nslothead islothead uslothead rslothead rlslothead
  rnslothead rislothead ruslothead
  tail ltail ntail itail utail rtail rltail rntail ritail rutail
  keytail lkeytail nkeytail ikeytail ukeytail rkeytail rlkeytail
  rnkeytail rikeytail rukeytail
  slottail lslottail nslottail islottail uslottail rslottail rlslottail
  rnslottail rislottail ruslottail
  atpos latpos natpos iatpos uatpos ratpos rlatpos rnatpos riatpos
  ruatpos
  keyatpos lkeyatpos nkeyatpos ikeyatpos ukeyatpos rkeyatpos
  rlkeyatpos rnkeyatpos rikeyatpos rukeyatpos
  slotatpos lslotatpos nslotatpos islotatpos uslotatpos rslotatpos
  rlslotatpos rnslotatpos rislotatpos ruslotatpos
  part lpart npart ipart upart rpart rlpart rnpart ripart
  rupart
  keypart lkeypart nkeypart ikeypart ukeypart rkeypart
  rlkeypart rnkeypart rikeypart rukeypart
  slotpart lslotpart nslotpart islotpart uslotpart rslotpart
  rlslotpart rnslotpart rislotpart ruslotpart

SEE ALSO¶

Sort::Key, "sort" in perlfunc.

Sort::Key::Top::PP by Toby Inkster, provides a subset of the API of Sort::Key::Top and is written in pure Perl.

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

COPYRIGHT AND LICENSE¶

Copyright (C) 2006-2008, 2011, 2012, 2014 by Salvador Fandiño (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.