List::Util(3pm)

NAME
List::Util - A selection of general-utility list subroutines

SYNOPSIS
    use List::Util qw(first max maxstr min minstr reduce shuffle sum);

DESCRIPTION
"List::Util" contains a selection of subroutines that people have expressed would be nice to have in the perl core, but the usage would not really be high enough to warrant the use of a keyword, and the size so small such that being individual extensions would be wasteful.

By default "List::Util" does not export any subroutines.

LIST-REDUCTION FUNCTIONS
The following set of functions all reduce a list down to a single value.

  $result = reduce { BLOCK } @list
Reduces @list by calling "BLOCK" in a scalar context multiple times, setting $a and $b each time. The first call will be with $a and $b set to the first two elements of the list, subsequent calls will be done by setting $a to the result of the previous call and $b to the next element in the list.

Returns the result of the last call to the "BLOCK". If @list is empty then "undef" is returned. If @list only contains one element then that element is returned and "BLOCK" is not executed.

The following examples all demonstrate how "reduce" could be used to implement the other list-reduction functions in this module. (They are not in fact implemented like this, but instead in a more efficient manner in individual C functions).

    $foo = reduce { defined($a)            ? $a :
                    $code->(local $_ = $b) ? $b :
                                             undef } undef, @list # first

    $foo = reduce { $a > $b ? $a : $b } 1..10       # max
    $foo = reduce { $a gt $b ? $a : $b } 'A'..'Z'   # maxstr
    $foo = reduce { $a < $b ? $a : $b } 1..10       # min
    $foo = reduce { $a lt $b ? $a : $b } 'aa'..'zz' # minstr
    $foo = reduce { $a + $b } 1 .. 10               # sum
    $foo = reduce { $a . $b } @bar                  # concat

    $foo = reduce { $a || $code->(local $_ = $b) } 0, @bar   # any
    $foo = reduce { $a && $code->(local $_ = $b) } 1, @bar   # all
    $foo = reduce { $a && !$code->(local $_ = $b) } 1, @bar  # none
    $foo = reduce { $a || !$code->(local $_ = $b) } 0, @bar  # notall
       # Note that these implementations do not fully short-circuit

If your algorithm requires that "reduce" produce an identity value, then make sure that you always pass that identity value as the first argument to prevent "undef" being returned

  $foo = reduce { $a + $b } 0, @values;             # sum with 0 identity value

The remaining list-reduction functions are all specialisations of this generic idea.

  $b = any { BLOCK } @list
Similar to "grep" in that it evaluates "BLOCK" setting $_ to each element of @list in turn. "any" returns true if any element makes the "BLOCK" return a true value. If "BLOCK" never returns true or @list was empty then it returns false.

Many cases of using "grep" in a conditional can be written using "any" instead, as it can short-circuit after the first true result.

    if( any { length > 10 } @strings ) {
        # at least one string has more than 10 characters
    }

  $b = all { BLOCK } @list
Similar to "any", except that it requires all elements of the @list to make the "BLOCK" return true. If any element returns false, then it returns false. If the "BLOCK" never returns false or the @list was empty then it returns true.

  $b = none { BLOCK } @list

  $b = notall { BLOCK } @list
Similar to "any" and "all", but with the return sense inverted. "none" returns true only if no value in the LIST causes the BLOCK to return true, and "notall" returns true only if not all of the values do.

  $val = first { BLOCK } @list
Similar to "grep" in that it evaluates "BLOCK" setting $_ to each element of @list in turn. "first" returns the first element where the result from "BLOCK" is a true value. If "BLOCK" never returns true or @list was empty then "undef" is returned.

    $foo = first { defined($_) } @list    # first defined value in @list
    $foo = first { $_ > $value } @list    # first value in @list which
                                          # is greater than $value

  $num = max @list
Returns the entry in the list with the highest numerical value. If the list is empty then "undef" is returned.

    $foo = max 1..10                # 10
    $foo = max 3,9,12               # 12
    $foo = max @bar, @baz           # whatever

  $str = maxstr @list
Similar to "max", but treats all the entries in the list as strings and returns the highest string as defined by the "gt" operator. If the list is empty then "undef" is returned.

    $foo = maxstr 'A'..'Z'          # 'Z'
    $foo = maxstr "hello","world"   # "world"
    $foo = maxstr @bar, @baz        # whatever

  $num = min @list
Similar to "max" but returns the entry in the list with the lowest numerical value. If the list is empty then "undef" is returned.

    $foo = min 1..10                # 1
    $foo = min 3,9,12               # 3
    $foo = min @bar, @baz           # whatever

  $str = minstr @list
Similar to "min", but treats all the entries in the list as strings and returns the lowest string as defined by the "lt" operator. If the list is empty then "undef" is returned.

    $foo = minstr 'A'..'Z'          # 'A'
    $foo = minstr "hello","world"   # "hello"
    $foo = minstr @bar, @baz        # whatever

  $num = product @list
Returns the numerical product of all the elements in @list. If @list is empty then 1 is returned.

    $foo = product 1..10            # 3628800
    $foo = product 3,9,12           # 324

  $num_or_undef = sum @list
Returns the numerical sum of all the elements in @list. For backwards compatibility, if @list is empty then "undef" is returned.

    $foo = sum 1..10                # 55
    $foo = sum 3,9,12               # 24
    $foo = sum @bar, @baz           # whatever

  $num = sum0 @list
Similar to "sum", except this returns 0 when given an empty list, rather than "undef".

KEY/VALUE PAIR LIST FUNCTIONS
The following set of functions, all inspired by List::Pairwise, consume an even-sized list of pairs. The pairs may be key/value associations from a hash, or just a list of values. The functions will all preserve the original ordering of the pairs, and will not be confused by multiple pairs having the same "key" value - nor even do they require that the first of each pair be a plain string.

  @kvlist = pairgrep { BLOCK } @kvlist

  $count = pairgrep { BLOCK } @kvlist
Similar to perl's "grep" keyword, but interprets the given list as an even-sized list of pairs. It invokes the "BLOCK" multiple times, in scalar context, with $a and $b set to successive pairs of values from the @kvlist.

Returns an even-sized list of those pairs for which the "BLOCK" returned true in list context, or the count of the number of pairs in scalar context. (Note, therefore, in scalar context that it returns a number half the size of the count of items it would have returned in list context).

    @subset = pairgrep { $a =~ m/^[[:upper:]]+$/ } @kvlist

As with "grep" aliasing $_ to list elements, "pairgrep" aliases $a and $b to elements of the given list. Any modifications of it by the code block will be visible to the caller.

  ( $key, $val ) = pairfirst { BLOCK } @kvlist

  $found = pairfirst { BLOCK } @kvlist
Similar to the "first" function, but interprets the given list as an even-sized list of pairs. It invokes the "BLOCK" multiple times, in scalar context, with $a and $b set to successive pairs of values from the @kvlist.

Returns the first pair of values from the list for which the "BLOCK" returned true in list context, or an empty list of no such pair was found. In scalar context it returns a simple boolean value, rather than either the key or the value found.

    ( $key, $value ) = pairfirst { $a =~ m/^[[:upper:]]+$/ } @kvlist

As with "grep" aliasing $_ to list elements, "pairfirst" aliases $a and $b to elements of the given list. Any modifications of it by the code block will be visible to the caller.

  @list = pairmap { BLOCK } @kvlist

  $count = pairmap { BLOCK } @kvlist
Similar to perl's "map" keyword, but interprets the given list as an even-sized list of pairs. It invokes the "BLOCK" multiple times, in list context, with $a and $b set to successive pairs of values from the @kvlist.

Returns the concatenation of all the values returned by the "BLOCK" in list context, or the count of the number of items that would have been returned in scalar context.

    @result = pairmap { "The key $a has value $b" } @kvlist

As with "map" aliasing $_ to list elements, "pairmap" aliases $a and $b to elements of the given list. Any modifications of it by the code block will be visible to the caller.

  @pairs = pairs @kvlist
A convenient shortcut to operating on even-sized lists of pairs, this function returns a list of ARRAY references, each containing two items from the given list. It is a more efficient version of

    @pairs = pairmap { [ $a, $b ] } @kvlist

It is most convenient to use in a "foreach" loop, for example:

    foreach ( pairs @KVLIST ) {
       my ( $key, $value ) = @$_;
       ...
    }

  @keys = pairkeys @kvlist
A convenient shortcut to operating on even-sized lists of pairs, this function returns a list of the the first values of each of the pairs in the given list. It is a more efficient version of

    @keys = pairmap { $a } @kvlist

  @values = pairvalues @kvlist
A convenient shortcut to operating on even-sized lists of pairs, this function returns a list of the the second values of each of the pairs in the given list. It is a more efficient version of

    @values = pairmap { $b } @kvlist

OTHER FUNCTIONS

  @values = shuffle @values
Returns the values of the input in a random order

    @cards = shuffle 0..51      # 0..51 in a random order

KNOWN BUGS
With perl versions prior to 5.005 there are some cases where reduce will return an incorrect result. This will show up as test 7 of reduce.t failing.

SUGGESTED ADDITIONS
The following are additions that have been requested, but I have been reluctant to add due to them being very simple to implement in perl

  # How many elements are true

  sub true { scalar grep { $_ } @_ }

  # How many elements are false

  sub false { scalar grep { !$_ } @_ }

SEE ALSO
Scalar::Util, List::MoreUtils

COPYRIGHT
Copyright (c) 1997-2007 Graham Barr <gbarr@pobox.com>. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

Recent additions and current maintenance by Paul Evans, <leonerd@leonerd.org.uk>.