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List::Util(3)         User Contributed Perl Documentation        List::Util(3)



NAME

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


SYNOPSIS

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

             max maxstr min minstr product sum sum0

             pairs unpairs pairkeys pairvalues pairfirst pairgrep pairmap

             shuffle uniq uniqint uniqnum uniqstr head tail zip mesh
           );


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 apply a given block of code to a
       list of values.

   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 other scalar-returning list reduction functions are all
       specialisations of this generic idea.

   reductions
           @results = reductions { BLOCK } @list

       Since version 1.54.

       Similar to "reduce" except that it also returns the intermediate values
       along with the final result. As before, $a is set to the first element
       of the given list, and the "BLOCK" is then called once for remaining
       item in the list set into $b, with the result being captured for return
       as well as becoming the new value for $a.

       The returned list will begin with the initial value for $a, followed by
       each return value from the block in order. The final value of the
       result will be identical to what the "reduce" function would have
       returned given the same block and list.

           reduce     { "$a-$b" }  "a".."d"    # "a-b-c-d"
           reductions { "$a-$b" }  "a".."d"    # "a", "a-b", "a-b-c", "a-b-c-d"

   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 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 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

       This function is affected by the $RAND variable.

   sample
           my @items = sample $count, @values

       Since version 1.54.

       Randomly select the given number of elements from the input list. Any
       given position in the input list will be selected at most once.

       If there are fewer than $count items in the list then the function will
       return once all of them have been randomly selected; effectively the
       function behaves similarly to "shuffle".

       This function is affected by the $RAND variable.

   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.

   uniqint
           my @subset = uniqint @values

       Since version 1.55.

       Filters a list of values to remove subsequent duplicates, as judged by
       an integer numerical equality test. Preserves the order of unique
       elements, and retains the first value of any duplicate set. Values in
       the returned list will be coerced into integers.

           my $count = uniqint @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 "uniqint" are well-
       behaved as integers.

   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

   zip
           my @result = zip [1..3], ['a'..'c'];
           # [1, 'a'], [2, 'b'], [3, 'c']

       Since version 1.56.

       Returns a list of array references, composed of elements from the given
       list of array references. Each array in the returned list is composed
       of elements at that corresponding position from each of the given input
       arrays. If any input arrays run out of elements before others, then
       "undef" will be inserted into the result to fill in the gaps.

       The "zip" function is particularly handy for iterating over multiple
       arrays at the same time with a "foreach" loop, taking one element from
       each:

           foreach ( zip \@xs, \@ys, \@zs ) {
               my ($x, $y, $z) = @$_;
               ...
           }

       NOTE to users of List::MoreUtils: This function does not behave the
       same as "List::MoreUtils::zip", but is actually a non-prototyped
       equivalent to "List::MoreUtils::zip_unflatten". This function does not
       apply a prototype, so make sure to invoke it with references to arrays.

       For a function similar to the "zip" function from "List::MoreUtils",
       see mesh.

           my @result = zip_shortest ...

       A variation of the function that differs in how it behaves when given
       input arrays of differing lengths. "zip_shortest" will stop as soon as
       any one of the input arrays run out of elements, discarding any
       remaining unused values from the others.

           my @result = zip_longest ...

       "zip_longest" is an alias to the "zip" function, provided simply to be
       explicit about that behaviour as compared to "zip_shortest".

   mesh
           my @result = mesh [1..3], ['a'..'c'];
           # (1, 'a', 2, 'b', 3, 'c')

       Since version 1.56.

       Returns a list of items collected from elements of the given list of
       array references. Each section of items in the returned list is
       composed of elements at the corresponding position from each of the
       given input arrays. If any input arrays run out of elements before
       others, then "undef" will be inserted into the result to fill in the
       gaps.

       This is similar to zip, except that all of the ranges in the result are
       returned in one long flattened list, instead of being bundled into
       separate arrays.

       Because it returns a flat list of items, the "mesh" function is
       particularly useful for building a hash out of two separate arrays of
       keys and values:

           my %hash = mesh \@keys, \@values;

           my $href = { mesh \@keys, \@values };

       NOTE to users of List::MoreUtils: This function is a non-prototyped
       equivalent to "List::MoreUtils::mesh" or "List::MoreUtils::zip"
       (themselves aliases of each other). This function does not apply a
       prototype, so make sure to invoke it with references to arrays.

           my @result = mesh_shortest ...

           my @result = mesh_longest ...

       These variations are similar to those of zip, in that they differ in
       behaviour when one of the input lists runs out of elements before the
       others.


CONFIGURATION VARIABLES

   $RAND
           local $List::Util::RAND = sub { ... };

       Since version 1.54.

       This package variable is used by code which needs to generate random
       numbers (such as the "shuffle" and "sample" functions). If set to a
       CODE reference it provides an alternative to perl's builtin "rand()"
       function. When a new random number is needed this function will be
       invoked with no arguments and is expected to return a floating-point
       value, of which only the fractional part will be used.


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(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 v5.34.3                      2024-09-20                     List::Util(3)

scalar-list-utils 1.660.0 - Generated Thu Oct 3 08:56:57 CDT 2024
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