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List::Util

Section: Perl Programmers Reference Guide (3)
Updated: 2001-09-21
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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 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

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

 

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 { !$_ } @_ }

 

COPYRIGHT

Copyright (c) 1997-2005 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.


 

Index

NAME
SYNOPSIS
DESCRIPTION
KNOWN BUGS
SUGGESTED ADDITIONS
COPYRIGHT




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