perl核心模块解析（二） ? ?List

详细文档链接如下

源自：http://blog.csdn.net/jonathanxqs

转自：http://perldoc.perl.org/List/Util.html

List::Util

• NAME
• SYNOPSIS
• DESCRIPTION
• LIST-REDUCTION FUNCTIONS
  • $result = reduce { BLOCK } @list
  • any
  • all
  • none
  • notall
  • first
  • max
  • maxstr
  • min
  • minstr
  • product
  • sum
  • sum0
• KEY/VALUE PAIR LIST FUNCTIONS
  • pairgrep
  • pairfirst
  • pairmap
  • pairs
  • pairkeys
  • pairvalues
• OTHER FUNCTIONS
  • shuffle
• KNOWN BUGS
  • RT #95409
• SUGGESTED ADDITIONS
• SEE ALSO
• COPYRIGHT

NAME

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.

By default?List::Util?does not export any subroutines.

The following set of functions all reduce a list down to a single value.

$result = 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?$b?set to the first two elements of the list,subsequent calls will be done by setting?$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 the?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.

The following examples all demonstrate how?reduce?could be used to implement the other list-reduction functions in this module. (They are not in fact implemented like this,but instead in a more efficient manner in individual C functions).

 $foo = reduce { defined($a) ? $a :

   
   
$code->(local $_ = $b$b  undef } undef, @list # first

   
   

   
   
{ $a > $b ? : $b } 1..10 # max

   
   
$a gt } 'A'..'Z' # maxstr

   
   
$a < # min

   
   
$a lt 'aa'..'zz' # minstr

   
   
$a + 1 .. # sum

   
   
$a . } @bar # concat

   
   
$a || ) 0# any

   
   
$a && 1# all

   
   
$a && !# none

   
   
$a || !# notall

   
   
 # Note that these implementations do not fully short-circuit

  
  

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

@values; # sum with 0 identity value

  
  

The remaining list-reduction functions are all specialisations of this generic idea.

 my $bool = any { BLOCK @list;

  
  

Since version 1.33.

Similar to?grep?in that it evaluates?BLOCK?setting?$_?to each element of?@list?in turn.?any?returns true if any element makes the?BLOCK?return a true value. If?BLOCK?never returns true or?@list?was empty then it returns false.

Many cases of using?grep?in a conditional can be written using?any?instead,as it can short-circuit after the first true result.

 if( { length > 10 @strings {

   
   
# at least one string has more than 10 characters

   
   
 }

  
  

all  Similar to?any,except that it requires all elements of the?@list?to make the?BLOCK?return true. If any element returns false,then it returns false. If the?BLOCK?never returns false or the?@list?was empty then it returns true.
 
  none 
  notall 
 
none ;

   
   
notall any?and?all,but with the return sense inverted.?none?returns true only if no value in the?@list?causes theBLOCK?to return true,and?notall?returns true only if not all of the values do.
 
  first 
 
$val = first first?returns the first element where the result from?BLOCK?is a true value. If?@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
  
  
 
  max 
 
$num = max  Returns the entry in the list with the highest numerical value. If the list is empty then? $foo = max 1..10 # 10
   
   
 $foo = max 3,9,12 # 12
   
   
 $foo = max @bar,@baz # whatever
  
  
 
  maxstr 
 
$str = maxstr  Similar to?max,but treats all the entries in the list as strings and returns the highest string as defined by the?gtoperator. If the list is empty then? $foo = maxstr 'A'..'Z' # 'Z'
   
   
 $foo = maxstr "hello","world" # "world"
   
   
 $foo = maxstr @bar,@baz # whatever
  
  
 
  min 
 
min max?but returns the entry in the list with the lowest numerical value. If the list is empty then? $foo = min 1..10 # 1
   
   
 $foo = min 3,12 # 3
   
   
 $foo = min @bar,@baz # whatever
  
  
 
  minstr 
 
minstr  Similar to?min,but treats all the entries in the list as strings and returns the lowest string as defined by the?ltoperator. If the list is empty then? $foo = minstr 'A'..'Z' # 'A'
   
   
 $foo = minstr "hello","world" # "hello"
   
   
 $foo = minstr @bar,@baz # whatever
  
  
 
  product 
 
product  Since version 1.35.
 
 
 Returns the numerical product of all the elements in?@list?. If?1?is returned.
 
 
$foo = product # 3628800

   
   
3912 # 324

  
  
 
  sum 
 
$num_or_undef = sum  Returns the numerical sum of all the elements in?@list?. For backwards compatibility,if? $foo = sum 1..10 # 55
   
   
 $foo = sum 3,12 # 24
   
   
 $foo = sum @bar,@baz # whatever
  
  
 
  sum0 
 
sum0  Since version 1.26.
 
 
 Similar to?sum,except this returns 0 when given an empty list,rather than?undef.
 
  KEY/VALUE PAIR LIST FUNCTIONS 
 
 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.
 
  pairgrep 
 
@kvlist = pairgrep @kvlist$count =  Since version 1.29.
 
 
 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?$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 = $a =~ m/^[[:upper:]]+$/ @kvlist

  
  
 
 
 As with?grep?aliasing?$_?to list elements,?pairgrep?aliases?$b?to elements of the given list. Any modifications of it by the code block will be visible to the caller.
 
  pairfirst 
 
my $key$val ) = pairfirst $found =  Since version 1.30.
 
 
 Similar to the?first?function,sans-serif; font-size:12.8000001907349px"> Returns the first pair of values from the list for which the?$value pairfirst?aliases?$b?to elements of the given list. Any modifications of it by the code block will be visible to the caller.
 
  pairmap 
 
@list = pairmap map?keyword,in list context,sans-serif; font-size:12.8000001907349px"> 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 = { "The key $a has value $b" map?aliasing?pairmap?aliases?$b?to elements of the given list. Any modifications of it by the code block will be visible to the caller.
 
 
 See?KNOWN BUGS?for a known-bug with?pairmap?,and a workaround.
 
  pairs 
 
@pairs = pairs  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
 
 
@pairs = { [ $a]  It is most convenient to use in a?foreach?loop,for example:
 
 
 foreach $pair ( @KVLIST @$pair ...
   
   
}

  
  
 
 
 Since version?1.39?these ARRAY references are blessed objects,recognising the two methods?key?and?value?. The following code is equivalent:
 
 
$key = $pair->key$value = ->value}

  
  
 
  pairkeys 
 
@keys = pairkeys @keys = $a @kvlist

  
  
 
  pairvalues 
 
@values = pairvalues @values = @kvlist

  
  
 
  OTHER FUNCTIONS 
  shuffle 
 
shuffle  Returns the values of the input in a random order
 
 
@cards = shuffle 0..51 # 0..51 in a random order

  
  
 
  KNOWN BUGS 
  RT #95409 
 
 https://rt.cpan.org/Ticket/Display.html?id=95409
 
 
 If the block of code given to?pairmap?contains lexical variables that are captured by a returned closure,and the closure is executed after the block has been re-used for the next iteration,these lexicals will not see the correct values. For example:
 
 
@subs = $var = "$a is $b" sub { print "$varn" }} one => two 2three 3$_->() for @subs Will incorrectly print
 
 
 three is 3

   
   
3

  
  
 
 
 This is due to the performance optimisation of using?MULTICALL?for the code block,which means that fresh SVs do not get allocated for each call to the block. Instead,the same SV is re-assigned for each iteration,and all the closures will share the value seen on the final iteration.
 
 
 To work around this bug,surround the code with a second set of braces. This creates an inner block that defeats theMULTICALL?logic,and does get fresh SVs allocated each time:
 
 
"$varn"; }

   
   
 This bug only affects closures that are generated by the block but used afterwards. Lexical variables that are only used during the lifetime of the block's execution will take their individual values for each invocation,as normal.
 
  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
 
 
# How many elements are true

   
   

 sub true { scalar grep $_ @_ # How many elements are false

   
   

 sub false { !}

  
  
 
  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.
 
 
 Recent additions and current maintenance by Paul Evans,<leonerd@leonerd.org.uk>.