| List::Util - A selection of general-utility list subroutines |
List::Util - A selection of general-utility list subroutines
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
);
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.
$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.
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.
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.
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.
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
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
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
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
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
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
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
my $num = sum0 @list;
Since version 1.26.
Similar to sum, except this returns 0 when given an empty list, rather
than undef.
The following set of functions, all inspired by the List::Pairwise manpage, 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;
...
}
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 = $pair->value;
...
}
my @kvlist = unpairs @pairs
Since version 1.42.
The inverse function to pairs; this function takes a list of ARRAY
references containing two elements each, and returns a flattened list of the
two values from each of the pairs, in order. This is notionally equivalent to
my @kvlist = map { @{$_}[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
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
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
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.
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.
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.
my @values = shuffle @values;
Returns the values of the input in a random order
@cards = shuffle 0..51 # 0..51 in a random order
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.
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.
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.
my @values = head $size, @list;
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
my @values = tail $size, @list;
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
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 bignumsDue 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.
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 { $_ } @_ }
# How many elements are false
sub false { scalar grep { !$_ } @_ }
the Scalar::Util manpage, the List::MoreUtils manpage
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>.
| List::Util - A selection of general-utility list subroutines |