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List::Util(3)	       Perl Programmers	Reference Guide		 List::Util(3)

NAME
       List::Util - A selection	of general-utility list	subroutines

SYNOPSIS
	   use List::Util qw(
	     reduce any	all none notall	first

	     max maxstr	min minstr product sum sum0

	     pairs unpairs pairkeys pairvalues pairfirst pairgrep pairmap

	     shuffle uniq uniqnum uniqstr
	   );

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.

LIST-REDUCTION FUNCTIONS
       The following set of functions all reduce a list	down to	a single
       value.

   reduce
	   $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	$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 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

	   $foo	= reduce { $a >	$b ? $a	: $b } 1..10	   # max
	   $foo	= reduce { $a gt $b ? $a : $b }	'A'..'Z'   # maxstr
	   $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

	   $foo	= reduce { $a || $code->(local $_ = $b)	} 0, @bar   # any
	   $foo	= reduce { $a && $code->(local $_ = $b)	} 1, @bar   # all
	   $foo	= reduce { $a && !$code->(local	$_ = $b) } 1, @bar  # none
	   $foo	= reduce { $a || !$code->(local	$_ = $b) } 0, @bar  # 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

	 $foo =	reduce { $a + $b } 0, @values;		   # sum with 0	identity value

       The above example code blocks also suggest how to use "reduce" to build
       a more efficient	combined version of one	of these basic functions and a
       "map" block. For	example, to find the total length of the all the
       strings in a list, we could use

	   $total = sum	map { length } @strings;

       However,	this produces a	list of	temporary integer values as long as
       the original list of strings, only to reduce it down to a single	value
       again. We can compute the same result more efficiently by using
       "reduce"	with a code block that accumulates lengths by writing this
       instead as:

	   $total = reduce { $a	+ length $b } 0, @strings

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

   any
	   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( any { length > 10 } @strings ) {
	       # at least one string has more than 10 characters
	   }

   all
	   my $bool = all { BLOCK } @list;

       Since version 1.33.

       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
	   my $bool = none { BLOCK } @list;

	   my $bool = notall { BLOCK } @list;

       Since version 1.33.

       Similar to "any"	and "all", but with the	return sense inverted. "none"
       returns true only if no value in	the @list causes the "BLOCK" to	return
       true, and "notall" returns true only if not all of the values do.

   first
	   my $val = 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

   max
	   my $num = 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

   maxstr
	   my $str = 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

   min
	   my $num = 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

   minstr
	   my $str = 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

   product
	   my $num = product @list;

       Since version 1.35.

       Returns the numerical product of	all the	elements in @list. If @list is
       empty then 1 is returned.

	   $foo	= product 1..10		   # 3628800
	   $foo	= product 3,9,12	   # 324

   sum
	   my $num_or_undef = sum @list;

       Returns the numerical sum of all	the elements in	@list. For backwards
       compatibility, if @list is empty	then "undef" is	returned.

	   $foo	= sum 1..10		   # 55
	   $foo	= sum 3,9,12		   # 24
	   $foo	= sum @bar, @baz	   # whatever

   sum0
	   my $num = sum0 @list;

       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.

       NOTE: At	the time of writing, the following "pair*" functions that take
       a block do not modify the value of $_ within the	block, and instead
       operate using the $a and	$b globals instead. This has turned out	to be
       a poor design, as it precludes the ability to provide a "pairsort"
       function. Better	would be to pass pair-like objects as 2-element	array
       references in $_, in a style similar to the return value	of the "pairs"
       function. At some future	version	this behaviour may be added.

       Until then, users are alerted NOT to rely on the	value of $_ remaining
       unmodified between the outside and the inside of	the control block. In
       particular, the following example is UNSAFE:

	my @kvlist = ...

	foreach	(qw( some keys here )) {
	   my @items = pairgrep	{ $a eq	$_ } @kvlist;
	   ...
	}

       Instead,	write this using a lexical variable:

	foreach	my $key	(qw( some keys here )) {
	   my @items = pairgrep	{ $a eq	$key } @kvlist;
	   ...
	}

   pairs
	   my @pairs = pairs @kvlist;

       Since version 1.29.

       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 = pairmap { [	$a, $b ] } @kvlist

       It is most convenient to	use in a "foreach" loop, for example:

	   foreach my $pair ( pairs @kvlist ) {
	      my ( $key, $value	) = @$pair;
	      ...
	   }

       Since version 1.39 these	"ARRAY"	references are blessed objects,
       recognising the two methods "key" and "value". The following code is
       equivalent:

	   foreach my $pair ( pairs @kvlist ) {
	      my $key	= $pair->key;
	      my $value	= $pair->value;
	      ...
	   }

   unpairs
	   my @kvlist =	unpairs	@pairs

       Since version 1.42.

       The inverse function to "pairs";	this function takes a list of "ARRAY"
       references containing two elements each,	and returns a flattened	list
       of the two values from each of the pairs, in order. This	is notionally
       equivalent to

	   my @kvlist =	map { @{$_}[0,1] } @pairs

       except that it is implemented more efficiently internally.
       Specifically, for any input item	it will	extract	exactly	two values for
       the output list;	using "undef" if the input array references are	short.

       Between "pairs" and "unpairs", a	higher-order list function can be used
       to operate on the pairs as single scalars; such as the following	near-
       equivalents of the other	"pair*"	higher-order functions:

	   @kvlist = unpairs grep { FUNC } pairs @kvlist
	   # Like pairgrep, but	takes $_ instead of $a and $b

	   @kvlist = unpairs map { FUNC	} pairs	@kvlist
	   # Like pairmap, but takes $_	instead	of $a and $b

       Note however that these versions	will not behave	as nicely in scalar
       context.

       Finally,	this technique can be used to implement	a sort on a keyvalue
       pair list; e.g.:

	   @kvlist = unpairs sort { $a->key cmp	$b->key	} pairs	@kvlist

   pairkeys
	   my @keys = pairkeys @kvlist;

       Since version 1.29.

       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

	   @keys = pairmap { $a	} @kvlist

   pairvalues
	   my @values =	pairvalues @kvlist;

       Since version 1.29.

       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

	   @values = pairmap { $b } @kvlist

   pairgrep
	   my @kvlist =	pairgrep { BLOCK } @kvlist;

	   my $count = pairgrep	{ BLOCK	} @kvlist;

       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 $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

       As with "grep" aliasing $_ to list elements, "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.

   pairfirst
	   my (	$key, $val ) = pairfirst { BLOCK } @kvlist;

	   my $found = pairfirst { BLOCK } @kvlist;

       Since version 1.30.

       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

       As with "grep" aliasing $_ to list elements, "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.

   pairmap
	   my @list = pairmap {	BLOCK }	@kvlist;

	   my $count = pairmap { BLOCK } @kvlist;

       Since version 1.29.

       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.

       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

       As with "map" aliasing $_ to list elements, "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.

       See "KNOWN BUGS"	for a known-bug	with "pairmap",	and a workaround.

OTHER FUNCTIONS
   shuffle
	   my @values =	shuffle	@values;

       Returns the values of the input in a random order

	   @cards = shuffle 0..51      # 0..51 in a random order

   uniq
	   my @subset =	uniq @values

       Since version 1.45.

       Filters a list of values	to remove subsequent duplicates, as judged by
       a DWIM-ish string equality or "undef" test. Preserves the order of
       unique elements,	and retains the	first value of any duplicate set.

	   my $count = uniq @values

       In scalar context, returns the number of	elements that would have been
       returned	as a list.

       The "undef" value is treated by this function as	distinct from the
       empty string, and no warning will be produced. It is left as-is in the
       returned	list. Subsequent "undef" values	are still considered identical
       to the first, and will be removed.

   uniqnum
	   my @subset =	uniqnum	@values

       Since version 1.44.

       Filters a list of values	to remove subsequent duplicates, as judged by
       a numerical equality test. Preserves the	order of unique	elements, and
       retains the first value of any duplicate	set.

	   my $count = uniqnum @values

       In scalar context, returns the number of	elements that would have been
       returned	as a list.

       Note that "undef" is treated much as other numerical operations treat
       it; it compares equal to	zero but additionally produces a warning if
       such warnings are enabled ("use warnings	'uninitialized';"). In
       addition, an "undef" in the returned list is coerced into a numerical
       zero, so	that the entire	list of	values returned	by "uniqnum" are well-
       behaved as numbers.

       Note also that multiple IEEE "NaN" values are treated as	duplicates of
       each other, regardless of any differences in their payloads, and
       despite the fact	that "0+'NaN' == 0+'NaN'" yields false.

   uniqstr
	   my @subset =	uniqstr	@values

       Since version 1.45.

       Filters a list of values	to remove subsequent duplicates, as judged by
       a string	equality test. Preserves the order of unique elements, and
       retains the first value of any duplicate	set.

	   my $count = uniqstr @values

       In scalar context, returns the number of	elements that would have been
       returned	as a list.

       Note that "undef" is treated much as other string operations treat it;
       it compares equal to the	empty string but additionally produces a
       warning if such warnings	are enabled ("use warnings 'uninitialized';").
       In addition, an "undef" in the returned list is coerced into an empty
       string, so that the entire list of values returned by "uniqstr" are
       well-behaved as strings.

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:

	my @subs = pairmap {
	   my $var = "$a is $b";
	   sub { print "$var\n"	};
	} one => 1, two	=> 2, three => 3;

	$_->() for @subs;

       Will incorrectly	print

	three is 3
	three is 3
	three is 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	the "MULTICALL"	logic, and
       does get	fresh SVs allocated each time:

	my @subs = pairmap {
	   {
	      my $var =	"$a is $b";
	      sub { print "$var\n"; }
	   }
	} one => 1, two	=> 2, three => 3;

       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.

   uniqnum() on	oversized bignums
       Due to the way that "uniqnum()" compares	numbers, it cannot distinguish
       differences between bignums (especially bigints)	that are too large to
       fit in the native platform types. For example,

	my $x =	Math::BigInt->new( "1" x 100 );
	my $y =	$x + 1;

	say for	uniqnum( $x, $y	);

       Will print just the value of $x,	believing that $y is a numerically-
       equivalent value. This bug does not affect "uniqstr()", which will
       correctly observe that the two values stringify to different strings.

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 { 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.

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

perl v5.26.0			  2017-04-19			 List::Util(3)

NAME | SYNOPSIS | DESCRIPTION | LIST-REDUCTION FUNCTIONS | KEY/VALUE PAIR LIST FUNCTIONS | OTHER FUNCTIONS | KNOWN BUGS | SUGGESTED ADDITIONS | SEE ALSO | COPYRIGHT

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