Provided by: libscalar-list-utils-perl_1.52-1_amd64 bug

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 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
           }

       Note: Due to XS issues the block passed may be able to access the outer @_ directly. This
       is not intentional and will break under debugger.

   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.

       Note: Due to XS issues the block passed may be able to access the outer @_ directly. This
       is not intentional and will break under debugger.

   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.

       Note: Due to XS issues the block passed may be able to access the outer @_ directly. This
       is not intentional and will break under debugger.

   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;
              ...
           }

       Since version 1.51 they also have a "TO_JSON" method to ease serialisation.

   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.

   head
           my @values = head $size, @list;

       Since version 1.50.

       Returns the first $size elements from @list. If $size is negative, returns all but the
       last $size elements from @list.

           @result = head 2, qw( foo bar baz );
           # foo, bar

           @result = head -2, qw( foo bar baz );
           # foo

   tail
           my @values = tail $size, @list;

       Since version 1.50.

       Returns the last $size elements from @list. If $size is negative, returns all but the
       first $size elements from @list.

           @result = tail 2, qw( foo bar baz );
           # bar, baz

           @result = tail -2, qw( foo bar baz );
           # baz

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>.