NAME¶
List::Util - A selection of general-utility list subroutines
SYNOPSIS¶
use List::Util qw(first max maxstr min minstr reduce shuffle sum);
DESCRIPTION¶
"List::Util" contains a selection of subroutines that people have
expressed would be nice to have in the perl core, but the usage would not
really be high enough to warrant the use of a keyword, and the size so small
such that being individual extensions would be wasteful.
By default "List::Util" does not export any subroutines.
LIST-REDUCTION FUNCTIONS¶
The following set of functions all reduce a list down to a single value.
$result = reduce { BLOCK } @list¶
Reduces @list by calling "BLOCK" in a scalar context multiple times,
setting $a and $b each time. The first call will be with $a and $b set to the
first two elements of the list, subsequent calls will be done by setting $a to
the result of the previous call and $b to the next element in the list.
Returns the result of the last call to the "BLOCK". If @list is empty
then "undef" is returned. If @list only contains one element then
that element is returned and "BLOCK" is not executed.
The following examples all demonstrate how "reduce" could be used to
implement the other list-reduction functions in this module. (They are not in
fact implemented like this, but instead in a more efficient manner in
individual C functions).
$foo = reduce { defined($a) ? $a :
$code->(local $_ = $b) ? $b :
undef } undef, @list # first
$foo = reduce { $a > $b ? $a : $b } 1..10 # max
$foo = reduce { $a gt $b ? $a : $b } 'A'..'Z' # maxstr
$foo = reduce { $a < $b ? $a : $b } 1..10 # min
$foo = reduce { $a lt $b ? $a : $b } 'aa'..'zz' # minstr
$foo = reduce { $a + $b } 1 .. 10 # sum
$foo = reduce { $a . $b } @bar # concat
$foo = reduce { $a || $code->(local $_ = $b) } 0, @bar # any
$foo = reduce { $a && $code->(local $_ = $b) } 1, @bar # all
$foo = reduce { $a && !$code->(local $_ = $b) } 1, @bar # none
$foo = reduce { $a || !$code->(local $_ = $b) } 0, @bar # notall
# Note that these implementations do not fully short-circuit
If your algorithm requires that "reduce" produce an identity value,
then make sure that you always pass that identity value as the first argument
to prevent "undef" being returned
$foo = reduce { $a + $b } 0, @values; # sum with 0 identity value
The remaining list-reduction functions are all specialisations of this generic
idea.
$b = any { BLOCK } @list¶
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
}
$b = all { BLOCK } @list¶
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.
$b = none { BLOCK } @list¶
$b = notall { BLOCK } @list¶
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.
$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
$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
$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
$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
$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
$num = product @list¶
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
$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
$num = sum0 @list¶
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.
@kvlist = pairgrep { BLOCK } @kvlist¶
$count = pairgrep { BLOCK } @kvlist¶
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.
( $key, $val ) = pairfirst { BLOCK } @kvlist¶
$found = pairfirst { BLOCK } @kvlist¶
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.
@list = pairmap { BLOCK } @kvlist¶
$count = pairmap { BLOCK } @kvlist¶
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.
@pairs = pairs @kvlist¶
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 ( pairs @KVLIST ) {
my ( $key, $value ) = @$_;
...
}
@keys = pairkeys @kvlist¶
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
@values = pairvalues @kvlist¶
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
OTHER FUNCTIONS¶
@values = shuffle @values¶
Returns the values of the input in a random order
@cards = shuffle 0..51 # 0..51 in a random order
KNOWN BUGS¶
With perl versions prior to 5.005 there are some cases where reduce will return
an incorrect result. This will show up as test 7 of reduce.t failing.
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>.