List::UtilsBy(3)

NAME
"List::UtilsBy" - higher-order list utility functions

SYNOPSIS
 use List::UtilsBy qw( nsort_by min_by );

 use File::stat qw( stat );
 my @files_by_age = nsort_by { stat($_)->mtime } @files;

 my $shortest_name = min_by { length } @names;

DESCRIPTION
This module provides a number of list utility functions, all of which take an initial code block to control their behaviour. They are variations on similar core perl or "List::Util" functions of similar names, but which use the block to control their behaviour. For example, the core Perl function "sort" takes a list of values and returns them, sorted into order by their string value. The "sort_by" function sorts them according to the string value returned by the extra function, when given each value.

 my @names_sorted = sort @names;

 my @people_sorted = sort_by { $_->name } @people;

FUNCTIONS

  @vals = sort_by { KEYFUNC } @vals
Returns the list of values sorted according to the string values returned by the "KEYFUNC" block or function. A typical use of this may be to sort objects according to the string value of some accessor, such as

 sort_by { $_->name } @people

The key function is called in scalar context, being passed each value in turn as both $_ and the only argument in the parameters, @_. The values are then sorted according to string comparisons on the values returned.

This is equivalent to

 sort { $a->name cmp $b->name } @people

except that it guarantees the "name" accessor will be executed only once per value.

  @vals = nsort_by { KEYFUNC } @vals
Similar to "sort_by" but compares its key values numerically.

  @vals = rev_sort_by { KEYFUNC } @vals

  @vals = rev_nsort_by { KEYFUNC } @vals
Similar to "sort_by" and "nsort_by" but returns the list in the reverse order. Equivalent to

 @vals = reverse sort_by { KEYFUNC } @vals

except that these functions are slightly more efficient because they avoid the final "reverse" operation.

  $optimal = max_by { KEYFUNC } @vals

  @optimal = max_by { KEYFUNC } @vals
Returns the (first) value from @vals that gives the numerically largest result from the key function.

 my $tallest = max_by { $_->height } @people

 use File::stat qw( stat );
 my $newest = max_by { stat($_)->mtime } @files;

In scalar context, the first maximal value is returned. In list context, a list of all the maximal values is returned. This may be used to obtain positions other than the first, if order is significant.

If called on an empty list, an empty list is returned.

  $optimal = min_by { KEYFUNC } @vals

  @optimal = min_by { KEYFUNC } @vals
Similar to "max_by" but returns values which give the numerically smallest result from the key function.

  @vals = uniq_by { KEYFUNC } @vals
Returns a list of the subset of values for which the key function block returns unique values. The first value yielding a particular key is chosen, subsequent values are rejected.

 my @some_fruit = uniq_by { $_->colour } @fruit;

To select instead the last value per key, reverse the input list. If the order of the results is significant, don't forget to reverse the result as well:

 my @some_fruit = reverse uniq_by { $_->colour } reverse @fruit;

  %parts = partition_by { KEYFUNC } @vals
Returns a hash of ARRAY refs, containing all the original values distributed according to the result of the key function block. Each ARRAY ref will contain all the values which returned the same string from the key function, in their original order.

 my %balls_by_colour = partition_by { $_->colour } @balls;

Because the values returned by the key function are used as hash keys, they ought to either be strings, or at least well-behaved as strings (such as numbers, or object references which overload stringification in a suitable manner).

  %counts = count_by { KEYFUNC } @vals
Returns a hash of integers, giving the number of times the key function block returned a particular result, for each value in the list.

 my %count_of_balls = count_by { $_->colour } @balls;

Because the values returned by the key function are used as hash keys, they ought to either be strings, or at least well-behaved as strings (such as numbers, or object references which overload stringification in a suitable manner).

  @vals = zip_by { ITEMFUNC } \@arr0, \@arr1, \@arr2,...
Returns a list of each of the values returned by the function block, when invoked with values from across each each of the given ARRAY references. Each value in the returned list will be the result of the function having been invoked with arguments at that position, from across each of the arrays given.

 my @transposition = zip_by { [ @_ ] } @matrix;

 my @names = zip_by { "$_[1], $_[0]" } \@firstnames, \@surnames;

 print zip_by { "$_[0] => $_[1]\n" } [ keys %hash ], [ values %hash ];

If some of the arrays are shorter than others, the function will behave as if they had "undef" in the trailing positions. The following two lines are equivalent:

 zip_by { f(@_) } [ 1, 2, 3 ], [ "a", "b" ]
 f( 1, "a" ), f( 2, "b" ), f( 3, undef )

The item function is called by "map", so if it returns a list, the entire list is included in the result. This can be useful for example, for generating a hash from two separate lists of keys and values

 my %nums = zip_by { @_ } [qw( one two three )], [ 1, 2, 3 ];
 # %nums = ( one => 1, two => 2, three => 3 )

(A function having this behaviour is sometimes called "zipWith", e.g. in Haskell, but that name would not fit the naming scheme used by this module).

  @vals = extract_by { SELECTFUNC } @arr
Removes elements from the referenced array on which the selection function returns true, and returns a list containing those elements. This function is similar to "grep", except that it modifies the referenced array to remove the selected values from it, leaving only the unselected ones.

 my @red_balls = extract_by { $_->color eq "red" } @balls;

 # Now there are no red balls in the @balls array

This function modifies a real array, unlike most of the other functions in this module. Because of this, it requires a real array, not just a list.

This function is implemented by invoking "splice()" on the array, not by constructing a new list and assigning it. One result of this is that weak references function will not be disturbed.

 extract_by { !defined $_ } @refs;

will leave weak references weakened in the @refs array, whereas

 @refs = grep { defined $_ } @refs;

will strengthen them all again.

  @vals = weighted_shuffle_by { WEIGHTFUNC } @vals
Returns the list of values shuffled into a random order. The randomisation is not uniform, but weighted by the value returned by the "WEIGHTFUNC". The probabilty of each item being returned first will be distributed with the distribution of the weights, and so on recursively for the remaining items.

  @vals = bundle_by { BLOCKFUNC } $number, @vals
Similar to a regular "map" functional, returns a list of the values returned by "BLOCKFUNC". Values from the input list are given to the block function in bundles of $number.

TODO

•   XS implementations

    These functions are currently all written in pure perl. Some at least, may benefit from having XS implementations to speed up their logic.

•   Merge into List::Util or List::MoreUtils

    This module shouldn't really exist. The functions should instead be part of one of the existing modules that already contain many list utility functions. Having Yet Another List Utilty Module just worsens the problem.

    I have attempted to contact the authors of both of the above modules, to no avail; therefore I decided it best to write and release this code here anyway so that it is at least on CPAN. Once there, we can then see how best to merge it into an existing module.

AUTHOR
Paul Evans <leonerd@leonerd.org.uk>