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
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 = $pair->value;
...
}
unpairs
my @kvlist = unpairs @pairs
Since version 1.42.
The inverse function to "pairs"; this function takes a list of "ARRAY"
references containing two elements each, and returns a flattened list
of the two values from each of the pairs, in order. This is notionally
equivalent to
my @kvlist = map { @{$_}[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 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 v5.26.3 2017-09-08 List::Util(3)
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