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.\" ========================================================================
.\"
.IX Title "Sort::Key::Top 3pm"
.TH Sort::Key::Top 3pm "2020-11-27" "perl v5.32.0" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH "NAME"
Sort::Key::Top \- select and sort top n elements
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&  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 $_ } 5 => 1, 2, 7, 5, 5, 1, 78, 0, \-2, \-8, 2;
\&         # ==> @top = (0, 1, 1, 2, \-2)
\&
\&  # select 5 first words by lexicographic order:
\&  @a = qw(cat fish bird leon penguin horse rat elephant squirrel dog);
\&  @top = top 5 => @a;
\&         # ==> @top = qw(cat fish bird elephant dog);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
The functions available from this module select the top n elements from a list
using several common orderings and custom key extraction procedures.
.PP
They are all variations around
.PP
.Vb 1
\&  keytopsort { CALC_KEY($_) } $n => @data;
.Ve
.PP
In array context, this function calculates the ordering key for every
element in \f(CW@data\fR using the expression inside the block. Then it
selects and orders the \f(CW$n\fR elements with the lower keys when
compared lexicographically.
.PP
It is equivalent to the pure Perl expression:
.PP
.Vb 1
\&  (sort { CALC_KEY($a) cmp CALC_KEY($b) } @data)[0 .. $n\-1];
.Ve
.PP
If \f(CW$n\fR is negative, the last \f(CW$n\fR elements from the bottom are selected:
.PP
.Vb 2
\&  topsort 3 => qw(foo doom me bar doz hello);
\&       # ==> (\*(Aqbar\*(Aq, \*(Aqdoz\*(Aq, \*(Aqdoom\*(Aq)
\&
\&  topsort \-3 => qw(foo doom me bar doz hello);
\&       # ==> (\*(Aqfoo\*(Aq, \*(Aqhello\*(Aq, \*(Aqme\*(Aq)
\&
\&  top 3 => qw(foo doom me bar doz hello);
\&       # ==> (\*(Aqdoom\*(Aq, \*(Aqbar\*(Aq, \*(Aqdoz\*(Aq)
\&
\&  top \-3 => qw(foo doom me bar doz hello);
\&       # ==> (\*(Aqfoo\*(Aq, \*(Aqme\*(Aq, \*(Aqhello\*(Aq)
.Ve
.PP
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:
.PP
.Vb 2
\&  # n = 5;
\&  scalar(topsort 5 => @data) eq (sort @data)[4]    # true
\&
\&  # n = \-5;
\&  scalar(topsort \-5 => @data) eq (sort @data)[\-5]  # true
.Ve
.PP
Note that on scalar context, the \f(CW\*(C`sort\*(C'\fR variations (see below) are
usually the right choice:
.PP
.Vb 1
\&  scalar topsort 3 => qw(me foo doz doom me bar hello); # ==> \*(Aqdoz\*(Aq
\&
\&  scalar top 3 => qw(me foo doz doom me bar hello); # ==> \*(Aqbar\*(Aq
.Ve
.PP
Note also, that the index is 1\-based (it starts at one instead of at
zero). The \f(CW\*(C`atpos\*(C'\fR set of functions explained below do the same and
are 0\-based.
.PP
Variations allow to:
.IP "\- use the own values as the ordering keys" 4
.IX Item "- use the own values as the ordering keys"
.Vb 2
\&  topsort 5 => qw(a b ab t uu g h aa aac);
\&     # ==> a aa aac ab b
.Ve
.IP "\- use an array or hash index instead of a subroutine to extract the key" 4
.IX Item "- use an array or hash index instead of a subroutine to extract the key"
.Vb 2
\&  slottop 0, 2, [4], [1], [3], [2], [4];
\&     # ==> [1], [2]
.Ve
.IP "\- return the selected values in the original order" 4
.IX Item "- return the selected values in the original order"
.Vb 2
\&  top 5 => qw(a b ab t uu g h aa aac);
\&     # ==> a b ab aa aac
.Ve
.IP "\- use a different ordering" 4
.IX Item "- use a different ordering"
For instance comparing the keys as numbers, using the locale
configuration or in reverse order:
.Sp
.Vb 2
\&  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
.Ve
.Sp
A prefix is used to indicate the required ordering:
.RS 4
.IP "(no prefix)" 4
.IX Item "(no prefix)"
lexicographical ascending order
.IP "r" 4
.IX Item "r"
lexicographical descending order
.IP "l" 4
.IX Item "l"
lexicographical ascending order obeying locale configuration
.IP "r" 4
.IX Item "r"
lexicographical descending order obeying locale configuration
.IP "n" 4
.IX Item "n"
numerical ascending order
.IP "rn" 4
.IX Item "rn"
numerical descending order
.IP "i" 4
.IX Item "i"
numerical ascending order but converting the keys to integers first
.IP "ri" 4
.IX Item "ri"
numerical descending order but converting the keys to integers first
.IP "u" 4
.IX Item "u"
numerical ascending order but converting the keys to unsigned integers first
.IP "ru" 4
.IX Item "ru"
numerical descending order but converting the keys to unsigned integers first
.RE
.RS 4
.RE
.IP "\- select the head element from the list sorted" 4
.IX Item "- select the head element from the list sorted"
.Vb 2
\&  nhead 6, 7, 3, 8, 9, 9;
\&      # ==> 3
\&
\&  nkeyhead { length $_ } qw(a ab aa aac b t uu uiyii)
\&      # ==> \*(Aqa\*(Aq
.Ve
.IP "\- select the tail element from the list sorted" 4
.IX Item "- select the tail element from the list sorted"
.Vb 2
\&  tail qw(a ab aa aac b t uu uiyii);
\&      # ==> \*(Aquu\*(Aq
\&
\&  nkeytail { length $_ } qw(a ab aa aac b t uu uiyii)
\&      # ==> \*(Aquiyii\*(Aq
.Ve
.IP "\- select the element at position n from the list sorted" 4
.IX Item "- select the element at position n from the list sorted"
.Vb 2
\&  atpos 3, qw(a ab aa aac b t uu uiyii);
\&      # ==> \*(Aqab\*(Aq;
\&
\&  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
.Ve
.Sp
Note that for the \f(CW\*(C`atpos\*(C'\fR set of functions indexes start at zero.
.IP "\- return a list composed by the elements with the first n ordered keys and then the remaining ones." 4
.IX Item "- return a list composed by the elements with the first n ordered keys and then the remaining ones."
.Vb 2
\&  ikeypart { length $_ } 3 => qw(a bbbb cc ddddd g fd);
\&      # ==> a cc g bbbb ddddd fd
.Ve
.IP "\- return two arrays references, the first array containing the elements with the first n ordered keys and the second with the rest." 4
.IX Item "- return two arrays references, the first array containing the elements with the first n ordered keys and the second with the rest."
.Vb 2
\&  keypartref { length $_ } 3 => qw(a bbbb cc ddddd g fd);
\&      # ==> [a cc g] [bbbb ddddd fd]
.Ve
.PP
The full list of available functions is:
.PP
.Vb 1
\&  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
.Ve
.SH "SEE ALSO"
.IX Header "SEE ALSO"
Sort::Key, \*(L"sort\*(R" in perlfunc.
.PP
Sort::Key::Top::PP by Toby Inkster, provides a subset of the \s-1API\s0 of
Sort::Key::Top and is written in pure Perl.
.PP
The Wikipedia article about selection algorithms
<http://en.wikipedia.org/wiki/Selection_algorithm>.
.SH "COPYRIGHT AND LICENSE"
.IX Header "COPYRIGHT AND LICENSE"
Copyright (C) 2006\-2008, 2011, 2012, 2014 by Salvador Fandiño
(sfandino@yahoo.com).
.PP
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.