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
);
By default "List::Util" does not export any subroutines.
$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".
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
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.
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.
# How many elements are true
sub true { scalar grep { $_ } @_ }
# How many elements are false
sub false { scalar grep { !$_ } @_ }
Recent additions and current maintenance by Paul Evans, <leonerd@leonerd.org.uk>.