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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>
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