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>