Manpages - List_Util.3perl

$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 other scalar-returning list reduction functions are all specialisations of this generic idea.

@results = reductions { BLOCK } @list

Since version 1.54.

Similar to reduce except that it also returns the intermediate values along with the final result. As before, $a is set to the first element of the given list, and the BLOCK is then called once for remaining item in the list set into $b, with the result being captured for return as well as becoming the new value for $a.

The returned list will begin with the initial value for $a, followed by each return value from the block in order. The final value of the result will be identical to what the reduce function would have returned given the same block and list.

reduce { “$a-$b” } “a”..“d” # “a-b-c-d” reductions { “$a-$b” } “a”..“d”

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.

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; … }

Since version 1.51 they also have a TO_JSON method to ease serialisation.

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 { $a =~ m/^+$/ } @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 { $a =~ m/^+$/ } @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

This function is affected by the $RAND variable.

my @items = sample $count, @values

Since version 1.54.

Randomly select the given number of elements from the input list. Any given position in the input list will be selected at most once.

If there are fewer than $count items in the list then the function will return once all of them have been randomly selected; effectively the function behaves similarly to shuffle.

This function is affected by the $RAND variable.

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 = uniqint @values

Since version 1.55.

Filters a list of values to remove subsequent duplicates, as judged by an integer numerical equality test. Preserves the order of unique elements, and retains the first value of any duplicate set. Values in the returned list will be coerced into integers.

my $count = uniqint @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 uniqint are well-behaved as integers.

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;

Since version 1.50.

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;

Since version 1.50.

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

local $List::Util::RAND = sub { … };

Since version 1.54.

This package variable is used by code which needs to generate random numbers (such as the shuffle and sample functions). If set to a CODE reference it provides an alternative to perl’s builtin rand() function. When a new random number is needed this function will be invoked with no arguments and is expected to return a floating-point value, of which only the fractional part will be used.

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.

Due 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.