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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.


LIST-REDUCTION FUNCTIONS

       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 $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 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


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.


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(3), List::MoreUtils(3)


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 v5.24.0                      2016-03-01                   List::Util(3pm)

perl 5.24 - Generated Mon Nov 14 19:21:06 CST 2016
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