1 # You may distribute under the terms of either the GNU General Public License
2 # or the Artistic License (the same terms as Perl itself)
4 # (C) Paul Evans, 2009-2012 -- leonerd@leonerd.org.uk
11 our $VERSION = '0.09';
13 use Exporter 'import';
41 C<List::UtilsBy> - higher-order list utility functions
45 use List::UtilsBy qw( nsort_by min_by );
47 use File::stat qw( stat );
48 my @files_by_age = nsort_by { stat($_)->mtime } @files;
50 my $shortest_name = min_by { length } @names;
54 This module provides a number of list utility functions, all of which take an
55 initial code block to control their behaviour. They are variations on similar
56 core perl or C<List::Util> functions of similar names, but which use the block
57 to control their behaviour. For example, the core Perl function C<sort> takes
58 a list of values and returns them, sorted into order by their string value.
59 The C<sort_by> function sorts them according to the string value returned by
60 the extra function, when given each value.
62 my @names_sorted = sort @names;
64 my @people_sorted = sort_by { $_->name } @people;
72 =head2 @vals = sort_by { KEYFUNC } @vals
74 Returns the list of values sorted according to the string values returned by
75 the C<KEYFUNC> block or function. A typical use of this may be to sort objects
76 according to the string value of some accessor, such as
78 sort_by { $_->name } @people
80 The key function is called in scalar context, being passed each value in turn
81 as both C<$_> and the only argument in the parameters, C<@_>. The values are
82 then sorted according to string comparisons on the values returned.
86 sort { $a->name cmp $b->name } @people
88 except that it guarantees the C<name> accessor will be executed only once per
91 One interesting use-case is to sort strings which may have numbers embedded in
92 them "naturally", rather than lexically.
94 sort_by { s/(\d+)/sprintf "%09d", $1/eg; $_ } @strings
96 This sorts strings by generating sort keys which zero-pad the embedded numbers
97 to some level (9 digits in this case), helping to ensure the lexical sort puts
98 them in the correct order.
106 my @keys = map { local $_ = $_; scalar $keygen->( $_ ) } @_;
107 return @_[ sort { $keys[$a] cmp $keys[$b] } 0 .. $#_ ];
110 =head2 @vals = nsort_by { KEYFUNC } @vals
112 Similar to C<sort_by> but compares its key values numerically.
120 my @keys = map { local $_ = $_; scalar $keygen->( $_ ) } @_;
121 return @_[ sort { $keys[$a] <=> $keys[$b] } 0 .. $#_ ];
124 =head2 @vals = rev_sort_by { KEYFUNC } @vals
126 =head2 @vals = rev_nsort_by { KEYFUNC } @vals
128 Similar to C<sort_by> and C<nsort_by> but returns the list in the reverse
131 @vals = reverse sort_by { KEYFUNC } @vals
133 except that these functions are slightly more efficient because they avoid
134 the final C<reverse> operation.
142 my @keys = map { local $_ = $_; scalar $keygen->( $_ ) } @_;
143 return @_[ sort { $keys[$b] cmp $keys[$a] } 0 .. $#_ ];
150 my @keys = map { local $_ = $_; scalar $keygen->( $_ ) } @_;
151 return @_[ sort { $keys[$b] <=> $keys[$a] } 0 .. $#_ ];
154 =head2 $optimal = max_by { KEYFUNC } @vals
156 =head2 @optimal = max_by { KEYFUNC } @vals
158 Returns the (first) value from C<@vals> that gives the numerically largest
159 result from the key function.
161 my $tallest = max_by { $_->height } @people
163 use File::stat qw( stat );
164 my $newest = max_by { stat($_)->mtime } @files;
166 In scalar context, the first maximal value is returned. In list context, a
167 list of all the maximal values is returned. This may be used to obtain
168 positions other than the first, if order is significant.
170 If called on an empty list, an empty list is returned.
172 For symmetry with the C<nsort_by> function, this is also provided under the
173 name C<nmax_by> since it behaves numerically.
185 my @maximal = $_ = shift @_;
186 my $max = $code->( $_ );
189 my $this = $code->( $_ );
194 elsif( wantarray and $this == $max ) {
199 return wantarray ? @maximal : $maximal[0];
204 =head2 $optimal = min_by { KEYFUNC } @vals
206 =head2 @optimal = min_by { KEYFUNC } @vals
208 Similar to C<max_by> but returns values which give the numerically smallest
209 result from the key function. Also provided as C<nmin_by>
221 my @minimal = $_ = shift @_;
222 my $min = $code->( $_ );
225 my $this = $code->( $_ );
230 elsif( wantarray and $this == $min ) {
235 return wantarray ? @minimal : $minimal[0];
240 =head2 @vals = uniq_by { KEYFUNC } @vals
242 Returns a list of the subset of values for which the key function block
243 returns unique values. The first value yielding a particular key is chosen,
244 subsequent values are rejected.
246 my @some_fruit = uniq_by { $_->colour } @fruit;
248 To select instead the last value per key, reverse the input list. If the order
249 of the results is significant, don't forget to reverse the result as well:
251 my @some_fruit = reverse uniq_by { $_->colour } reverse @fruit;
261 my $key = $code->( local $_ = $_ );
266 =head2 %parts = partition_by { KEYFUNC } @vals
268 Returns a key/value list of ARRAY refs containing all the original values
269 distributed according to the result of the key function block. Each value will
270 be an ARRAY ref containing all the values which returned the string from the
271 key function, in their original order.
273 my %balls_by_colour = partition_by { $_->colour } @balls;
275 Because the values returned by the key function are used as hash keys, they
276 ought to either be strings, or at least well-behaved as strings (such as
277 numbers, or object references which overload stringification in a suitable
287 push @{ $parts{ $code->( local $_ = $_ ) } }, $_ for @_;
292 =head2 %counts = count_by { KEYFUNC } @vals
294 Returns a key/value list of integers, giving the number of times the key
295 function block returned the key, for each value in the list.
297 my %count_of_balls = count_by { $_->colour } @balls;
299 Because the values returned by the key function are used as hash keys, they
300 ought to either be strings, or at least well-behaved as strings (such as
301 numbers, or object references which overload stringification in a suitable
311 $counts{ $code->( local $_ = $_ ) }++ for @_;
316 =head2 @vals = zip_by { ITEMFUNC } \@arr0, \@arr1, \@arr2,...
318 Returns a list of each of the values returned by the function block, when
319 invoked with values from across each each of the given ARRAY references. Each
320 value in the returned list will be the result of the function having been
321 invoked with arguments at that position, from across each of the arrays given.
323 my @transposition = zip_by { [ @_ ] } @matrix;
325 my @names = zip_by { "$_[1], $_[0]" } \@firstnames, \@surnames;
327 print zip_by { "$_[0] => $_[1]\n" } [ keys %hash ], [ values %hash ];
329 If some of the arrays are shorter than others, the function will behave as if
330 they had C<undef> in the trailing positions. The following two lines are
333 zip_by { f(@_) } [ 1, 2, 3 ], [ "a", "b" ]
334 f( 1, "a" ), f( 2, "b" ), f( 3, undef )
336 The item function is called by C<map>, so if it returns a list, the entire
337 list is included in the result. This can be useful for example, for generating
338 a hash from two separate lists of keys and values
340 my %nums = zip_by { @_ } [qw( one two three )], [ 1, 2, 3 ];
341 # %nums = ( one => 1, two => 2, three => 3 )
343 (A function having this behaviour is sometimes called C<zipWith>, e.g. in
344 Haskell, but that name would not fit the naming scheme used by this module).
355 scalar @$_ > $len and $len = scalar @$_ for @_;
359 $code->( map { $_[$_][$idx] } 0 .. $#_ )
363 =head2 $arr0, $arr1, $arr2, ... = unzip_by { ITEMFUNC } @vals
365 Returns a list of ARRAY references containing the values returned by the
366 function block, when invoked for each of the values given in the input list.
367 Each of the returned ARRAY references will contain the values returned at that
368 corresponding position by the function block. That is, the first returned
369 ARRAY reference will contain all the values returned in the first position by
370 the function block, the second will contain all the values from the second
373 my ( $firstnames, $lastnames ) = unzip_by { m/^(.*?) (.*)$/ } @names;
375 If the function returns lists of differing lengths, the result will be padded
376 with C<undef> in the missing elements.
378 This function is an inverse of C<zip_by>, if given a corresponding inverse
388 foreach my $idx ( 0 .. $#_ ) {
389 my @slice = $code->( local $_ = $_[$idx] );
390 $#slice = $#ret if @slice < @ret;
391 $ret[$_][$idx] = $slice[$_] for 0 .. $#slice;
397 =head2 @vals = extract_by { SELECTFUNC } @arr
399 Removes elements from the referenced array on which the selection function
400 returns true, and returns a list containing those elements. This function is
401 similar to C<grep>, except that it modifies the referenced array to remove the
402 selected values from it, leaving only the unselected ones.
404 my @red_balls = extract_by { $_->color eq "red" } @balls;
406 # Now there are no red balls in the @balls array
408 This function modifies a real array, unlike most of the other functions in this
409 module. Because of this, it requires a real array, not just a list.
411 This function is implemented by invoking C<splice()> on the array, not by
412 constructing a new list and assigning it. One result of this is that weak
413 references will not be disturbed.
415 extract_by { !defined $_ } @refs;
417 will leave weak references weakened in the C<@refs> array, whereas
419 @refs = grep { defined $_ } @refs;
421 will strengthen them all again.
431 for( my $idx = 0; $idx < scalar @$arrref; ) {
432 if( $code->( local $_ = $arrref->[$idx] ) ) {
433 push @ret, splice @$arrref, $idx, 1, ();
443 =head2 @vals = weighted_shuffle_by { WEIGHTFUNC } @vals
445 Returns the list of values shuffled into a random order. The randomisation is
446 not uniform, but weighted by the value returned by the C<WEIGHTFUNC>. The
447 probabilty of each item being returned first will be distributed with the
448 distribution of the weights, and so on recursively for the remaining items.
452 sub weighted_shuffle_by(&@)
457 my @weights = map { $code->( local $_ = $_ ) } @vals;
461 my $total = 0; $total += $_ for @weights;
462 my $select = int rand $total;
464 while( $select >= $weights[$idx] ) {
465 $select -= $weights[$idx++];
468 push @ret, splice @vals, $idx, 1, ();
469 splice @weights, $idx, 1, ();
472 push @ret, @vals if @vals;
477 =head2 @vals = bundle_by { BLOCKFUNC } $number, @vals
479 Similar to a regular C<map> functional, returns a list of the values returned
480 by C<BLOCKFUNC>. Values from the input list are given to the block function in
481 bundles of C<$number>.
483 If given a list of values whose length does not evenly divide by C<$number>,
484 the final call will be passed fewer elements than the others.
494 for( my ( $pos, $next ) = ( 0, $n ); $pos < @_; $pos = $next, $next += $n ) {
495 $next = @_ if $next > @_;
496 push @ret, $code->( @_[$pos .. $next-1] );
505 =item * XS implementations
507 These functions are currently all written in pure perl. Some at least, may
508 benefit from having XS implementations to speed up their logic.
510 =item * Merge into L<List::Util> or L<List::MoreUtils>
512 This module shouldn't really exist. The functions should instead be part of
513 one of the existing modules that already contain many list utility functions.
514 Having Yet Another List Utilty Module just worsens the problem.
516 I have attempted to contact the authors of both of the above modules, to no
517 avail; therefore I decided it best to write and release this code here anyway
518 so that it is at least on CPAN. Once there, we can then see how best to merge
519 it into an existing module.
525 Paul Evans <leonerd@leonerd.org.uk>