Provided by: liblist-utilsby-perl_0.12-2_all 
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
All functions added since version 0.04 unless otherwise stated, as the original names for
earlier versions were renamed.
sort_by
@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.
One interesting use-case is to sort strings which may have numbers embedded in them
"naturally", rather than lexically.
sort_by { s/(\d+)/sprintf "%09d", $1/eg; $_ } @strings
This sorts strings by generating sort keys which zero-pad the embedded numbers to some
level (9 digits in this case), helping to ensure the lexical sort puts them in the correct
order.
nsort_by
@vals = nsort_by { KEYFUNC } @vals
Similar to "sort_by" but compares its key values numerically.
rev_sort_by
rev_nsort_by
@vals = rev_sort_by { KEYFUNC } @vals
@vals = rev_nsort_by { KEYFUNC } @vals
Since version 0.06.
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.
max_by
$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.
For symmetry with the "nsort_by" function, this is also provided under the name "nmax_by"
since it behaves numerically.
min_by
$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. Also provided as "nmin_by"
minmax_by
( $minimal, $maximal ) = minmax_by { KEYFUNC } @vals
Since version 0.11.
Similar to calling both "min_by" and "max_by" with the same key function on the same list.
This version is more efficient than calling the two other functions individually, as it
has less work to perform overall. In the case of ties, only the first optimal element
found in each case is returned. Also provided as "nminmax_by".
uniq_by
@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;
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).
partition_by
%parts = partition_by { KEYFUNC } @vals
Returns a key/value list of ARRAY refs containing all the original values distributed
according to the result of the key function block. Each value will be an ARRAY ref
containing all the values which returned the 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).
count_by
%counts = count_by { KEYFUNC } @vals
Since version 0.07.
Returns a key/value list of integers, giving the number of times the key function block
returned the key, for each value in the list.
my %count_of_balls = count_by { $_->colour } @balls;
Because the values returned by the key function are used as hash keys, they ought to
either be strings, or at least well-behaved as strings (such as numbers, or object
references which overload stringification in a suitable manner).
zip_by
@vals = zip_by { ITEMFUNC } \@arr0, \@arr1, \@arr2,...
Returns a list of each of the values returned by the function block, when invoked with
values from across each each of the given ARRAY references. Each value in the returned
list will be the result of the function having been invoked with arguments at that
position, from across each of the arrays given.
my @transposition = zip_by { [ @_ ] } @matrix;
my @names = zip_by { "$_[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).
unzip_by
$arr0, $arr1, $arr2, ... = unzip_by { ITEMFUNC } @vals
Since version 0.09.
Returns a list of ARRAY references containing the values returned by the function block,
when invoked for each of the values given in the input list. Each of the returned ARRAY
references will contain the values returned at that corresponding position by the function
block. That is, the first returned ARRAY reference will contain all the values returned in
the first position by the function block, the second will contain all the values from the
second position, and so on.
my ( $firstnames, $lastnames ) = unzip_by { m/^(.*?) (.*)$/ } @names;
If the function returns lists of differing lengths, the result will be padded with "undef"
in the missing elements.
This function is an inverse of "zip_by", if given a corresponding inverse function.
extract_by
@vals = extract_by { SELECTFUNC } @arr
Since version 0.05.
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 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.
extract_first_by
$val = extract_first_by { SELECTFUNC } @arr
Since version 0.10.
A hybrid between "extract_by" and "List::Util::first". Removes the first element from the
referenced array on which the selection function returns true, returning it.
As with "extract_by", this function requires a real array and not just a list, and is also
implemented using "splice" so that weak references are not disturbed.
If this function fails to find a matching element, it will return an empty list in list
context. This allows a caller to distinguish the case between no matching element, and the
first matching element being "undef".
weighted_shuffle_by
@vals = weighted_shuffle_by { WEIGHTFUNC } @vals
Since version 0.07.
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 probability of each item being
returned first will be distributed with the distribution of the weights, and so on
recursively for the remaining items.
bundle_by
@vals = bundle_by { BLOCKFUNC } $number, @vals
Since version 0.07.
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.
If given a list of values whose length does not evenly divide by $number, the final call
will be passed fewer elements than the others.
EXAMPLES
As many of these functions both take and return lists of values, more useful behaviours
can be created by combining them together.
Modal Value
To pick the modal value in a list (i.e. the value that appears most frequently according
to some key function) combine "count_by" and "max_by":
my %counts = count_by { KEYFUNC } ITEMS;
my $modal = max_by { $counts{$_} } keys %counts;
This can be performed in a single step by use of the "pairs" function from List::Util:
use List::Util qw( pairs );
my $modal = ( max_by { $_->value } pairs count_by { KEYFUNC } ITEMS )->key;
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.
Updated 2015/07/16: As I am now the maintainer of List::Util, some amount of
merging/copying should be possible. However, given the latter's key position in the
core perl distribution and head of the "CPAN River" I am keen not to do this
wholesale, but a selected pick of what seems best, by a popular consensus.
• "head" and "tail"-like functions
Consider perhaps
head_before { COND } LIST # excludes terminating element
head_upto { COND } LIST # includes terminating element
tail_since { COND } LIST # includes initiating element
tail_after { COND } LIST # excludes initiating element
(See also <https://rt.cpan.org/Ticket/Display.html?id=105907>).
AUTHOR
Paul Evans <leonerd@leonerd.org.uk>