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23 Apr 2015 00:32:25 UTC
- Distribution: Scalar-List-Utils
- Module version: 1.42
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- License: perl_5
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- NAME
- SYNOPSIS
- DESCRIPTION
- LIST-REDUCTION FUNCTIONS
- KEY/VALUE PAIR LIST FUNCTIONS
- OTHER FUNCTIONS
- KNOWN BUGS
- SUGGESTED ADDITIONS
- SEE ALSO
- COPYRIGHT
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.
$result = reduce { BLOCK } @list
Reduces
@list
by callingBLOCK
in a scalar context multiple times, setting$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$b
to the next element in the list.Returns the result of the last call to the
BLOCK
. If@list
is empty thenundef
is returned. If@list
only contains one element then that element is returned andBLOCK
is not executed.The following examples all demonstrate how
reduce
could be used to implement the other list-reduction functions in this module. (They are not in fact implemented like this, but instead in a more efficient manner in individual C functions).$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 $_ = $b) } 0, @bar # notall # Note that these implementations do not fully short-circuit
If your algorithm requires that
reduce
produce an identity value, then make sure that you always pass that identity value as the first argument to preventundef
being returned$foo = reduce { $a + $b } 0, @values; # sum with 0 identity value
The remaining list-reduction functions are all specialisations of this generic idea.
any
my $bool = any { BLOCK } @list;
Since version 1.33.
Similar to
grep
in that it evaluatesBLOCK
setting$_
to each element of@list
in turn.any
returns true if any element makes theBLOCK
return a true value. IfBLOCK
never returns true or@list
was empty then it returns false.Many cases of using
grep
in a conditional can be written usingany
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 }
all
my $bool = all { BLOCK } @list;
Since version 1.33.
Similar to "any", except that it requires all elements of the
@list
to make theBLOCK
return true. If any element returns false, then it returns false. If theBLOCK
never returns false or the@list
was empty then it returns true.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 theBLOCK
to return true, andnotall
returns true only if not all of the values do.first
my $val = first { BLOCK } @list;
Similar to
grep
in that it evaluatesBLOCK
setting$_
to each element of@list
in turn.first
returns the first element where the result fromBLOCK
is a true value. IfBLOCK
never returns true or@list
was empty thenundef
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 $str = maxstr @list;
Similar to "max", but treats all the entries in the list as strings and returns the highest string as defined by the
gt
operator. If the list is empty thenundef
is returned.$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 $str = minstr @list;
Similar to "min", but treats all the entries in the list as strings and returns the lowest string as defined by the
lt
operator. If the list is empty thenundef
is returned.$foo = minstr 'A'..'Z' # 'A' $foo = minstr "hello","world" # "hello" $foo = minstr @bar, @baz # whatever
product
my $num = product @list;
Since version 1.35.
Returns the numerical product of all the elements in
@list
. If@list
is empty then1
is returned.$foo = product 1..10 # 3628800 $foo = product 3,9,12 # 324
sum
my $num_or_undef = sum @list;
Returns the numerical sum of all the elements in
@list
. For backwards compatibility, if@list
is empty thenundef
is returned.$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 apairsort
function. Better would be to pass pair-like objects as 2-element array references in$_
, in a style similar to the return value of thepairs
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 methodskey
andvalue
. The following code is equivalent:foreach my $pair ( pairs @KVLIST ) { my $key = $pair->key; my $value = $pair->value; ... }
unpairs
my @kvlist = unpairs @pairs
Since version 1.42.
The inverse function to
pairs
; this function takes a list of ARRAY references containing two elements each, and returns a flattened list of the two values from each of the pairs, in order. This is notionally equivalent tomy @kvlist = map { @{$_}[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
andunpairs
, a higher-order list function can be used to operate on the pairs as single scalars; such as the following near-equivalents of the otherpair*
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 $count = pairgrep { BLOCK } @kvlist;
Since version 1.29.
Similar to perl's
grep
keyword, but interprets the given list as an even-sized list of pairs. It invokes theBLOCK
multiple times, in scalar context, with$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 $count = pairmap { BLOCK } @kvlist;
Since version 1.29.
Similar to perl's
map
keyword, but interprets the given list as an even-sized list of pairs. It invokes theBLOCK
multiple times, in list context, with$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
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.
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
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>.
Module Install Instructions
To install Sub::Util, copy and paste the appropriate command in to your terminal.
cpanm Sub::Util
perl -MCPAN -e shell install Sub::Util
For more information on module installation, please visit the detailed CPAN module installation guide.