List::UtilsBy(3) User Contributed Perl Documentation 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.
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.
@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.
For symmetry with the "nsort_by" function, this is also provided under
the name "nmax_by" since it behaves numerically.
$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"
@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 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).
%counts = count_by { KEYFUNC } @vals
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 { $_->colour } @balls;
Because the values returned by the key function are used as hash keys,
they ought to either be strings, or at least well-behaved as strings
(such as numbers, or object references which overload stringification
in a suitable manner).
@vals = zip_by { ITEMFUNC } \@arr0, \@arr1, \@arr2,...
Returns a list of each of the values returned by the function block,
when invoked with values from across each each of the given ARRAY
references. Each value in the returned list will be the result of the
function having been invoked with arguments at that position, from
across each of the arrays given.
my @transposition = zip_by { [ @_ ] } @matrix;
my @names = zip_by { "$_[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).
$arr0, $arr1, $arr2, ... = unzip_by { ITEMFUNC } @vals
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.
@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 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.
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.
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>
perl v5.14.2 2012-03-04 List::UtilsBy(3)
