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List::Util(3pm)                       Perl Programmers Reference Guide                       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. 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;

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@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 v5.12.5                                     2012-11-03                                  List::Util(3pm)