List::Util - A selection of general-utility list subroutines
- use List::Util qw(first max maxstr min minstr reduce shuffle sum);
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.
The following set of functions all reduce a list down to a single value.
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.
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.
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.
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.
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
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
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
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
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
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
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
Similar to sum
, except this returns 0 when given an empty list, rather than
undef.
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.
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.
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.
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.
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:
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
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
Returns the values of the input in a random order
- @cards = shuffle 0..51 # 0..51 in a random order
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.
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
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>.