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List::Util(3pm) User Contributed Perl Documentation List::Util(3pm)
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 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 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
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, List::MoreUtils
COPYRIGHT
Copyright (c) 1997-2007 Graham Barr <gbarr AT 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 AT leonerd.uk>.
perl v5.34.0 2022-02-20 List::Util(3pm)
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