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List::Util(3) User Contributed Perl Documentation List::Util(3)
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. The subroutines defined are
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
This function could be implemented using "reduce" like this
$foo = reduce { defined($a) ? $a : wanted($b) ? $b : undef } undef, @list
for example wanted() could be defined() which would return the first defined value in
@list
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
This function could be implemented using "reduce" like this
$foo = reduce { $a > $b ? $a : $b } 1..10
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
This function could be implemented using "reduce" like this
$foo = reduce { $a gt $b ? $a : $b } 'A'..'Z'
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
This function could be implemented using "reduce" like this
$foo = reduce { $a < $b ? $a : $b } 1..10
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
This function could be implemented using "reduce" like this
$foo = reduce { $a lt $b ? $a : $b } 'A'..'Z'
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 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.
$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
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
shuffle LIST
Returns the elements of LIST in a random order
@cards = shuffle 0..51 # 0..51 in a random order
sum LIST
Returns the sum of all the elements in LIST. If LIST is empty then "undef" is
returned.
$foo = sum 1..10 # 55
$foo = sum 3,9,12 # 24
$foo = sum @bar, @baz # whatever
This function could be implemented using "reduce" like this
$foo = reduce { $a + $b } 1..10
If your algorithm requires that "sum" produce an identity of 0, then make sure that
you always pass 0 as the first argument to prevent "undef" being returned
$foo = sum 0, @values;
sum0 LIST
Similar to "sum", except this returns 0 when given an empty list, rather than "undef".
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
# One argument is true
sub any { $_ && return 1 for @_; 0 }
# All arguments are true
sub all { $_ || return 0 for @_; 1 }
# All arguments are false
sub none { $_ && return 0 for @_; 1 }
# One argument is false
sub notall { $_ || return 1 for @_; 0 }
# 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 AT 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.
perl v5.16.3 2012-12-27 List::Util(3)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
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