Linux Certif

Toute la documentation sur la certification Linux LPI

List::Util.3perl

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.