List::Util - A selection of general-utility list subroutines
use List::Util qw(first max maxstr min minstr reduce shuffle sum);
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.
List::Util
By default List::Util does not export any subroutines.
The following set of functions all reduce a list down to a single value.
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.
$a
$b
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.
undef
$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
reduce
$foo = reduce { $a + $b } 0, @values; # sum with 0 identity value
The remaining list-reduction functions are all specialisations of this generic idea.
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.
grep
$_
first
$foo = first { defined($_) } @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
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
$foo = reduce { $a > $b ? $a : $b } 1..10
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.
max
gt
$foo = maxstr 'A'..'Z' # 'Z' $foo = maxstr "hello","world" # "world" $foo = maxstr @bar, @baz # whatever
$foo = reduce { $a gt $b ? $a : $b } 'A'..'Z'
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
$foo = reduce { $a < $b ? $a : $b } 1..10
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.
min
lt
$foo = minstr 'A'..'Z' # 'A' $foo = minstr "hello","world" # "hello" $foo = minstr @bar, @baz # whatever
$foo = reduce { $a lt $b ? $a : $b } 'A'..'Z'
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
$foo = reduce { $a + $b } 1..10
Similar to sum, except this returns 0 when given an empty list, rather than undef.
sum
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.
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
Similar to grep, 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.
pairgrep
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
Similar to grep, 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.
pairfirst
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.
map
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
Similar to map, 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.
pairmap
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
pairmap { [ $a, $b ] } KVLIST
It is most convenient to use in a foreach loop, for example:
foreach
foreach ( pairs @KVLIST ) { my ( $key, $value ) = @$_; ... }
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
pairmap { $a } 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
pairmap { $b } KVLIST
Returns the elements of LIST in a random order
@cards = shuffle 0..51 # 0..51 in a random order
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.
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 { $_ } @_ } # How many elements are false sub false { scalar grep { !$_ } @_ }
Scalar::Util, List::MoreUtils
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>.
To install List::Util, copy and paste the appropriate command in to your terminal.
cpanm
cpanm List::Util
CPAN shell
perl -MCPAN -e shell install List::Util
For more information on module installation, please visit the detailed CPAN module installation guide.