/ 
List-AllUtils-0.19
River stage three • 145 direct dependents • 543 total dependents
/ List::AllUtils

NAME

List::AllUtils - Combines List::Util, List::SomeUtils and List::UtilsBy in one bite-sized package

VERSION

version 0.19

SYNOPSIS

use List::AllUtils qw( first any );

# _Everything_ from List::Util, List::SomeUtils, and List::UtilsBy
use List::AllUtils qw( :all );

my @numbers = ( 1, 2, 3, 5, 7 );
# or don't import anything
return List::AllUtils::first { $_ > 5 } @numbers;

DESCRIPTION

Are you sick of trying to remember whether a particular helper is defined in List::Util, List::SomeUtils or List::UtilsBy? I sure am. Now you don't have to remember. This module will export all of the functions that either of those three modules defines.

Note that all function documentation has been shamelessly copied from List::Util, List::SomeUtils and List::UtilsBy.

Which One Wins?

Recently, List::Util has started including some of the subs that used to only be in List::SomeUtils. Similarly, List::SomeUtils has some small overlap with List::UtilsBy.

List::AllUtils use to always favors the subroutine provided by List::Util, List::SomeUtils or List::UtilsBy in that order. However, as of List::Util 1.56, it included some functions, mesh and zip with the same name as List::SomeUtils functions, but different behavior.

So going forward, we will always prefer backwards compatibility. This means that mesh and zip will always come from List::SomeUtils. If other incompatible functions are added to List::Util, those will also be skipped in favor of the List::SomeUtils version.

The docs below come from List::Util 1.56, List::SomeUtils 0.58, and List::UtilsBy 0.11.

WHAT IS EXPORTED?

All this module does is load List::Util, List::SomeUtils, and List::UtilsBy, and then re-export everything that they provide. That means that regardless of the documentation below, you will get any subroutine that your installed version provides.

LIST-REDUCTION FUNCTIONS

The following set of functions all apply a given block of code to a list of values.

reduce

$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.

reductions

@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"    # "a", "a-b", "a-b-c", "a-b-c-d"

any

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.

all

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.

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 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.

first

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

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 then undef 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 then undef 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 then 1 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 then undef 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 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;
   ...
}

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 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.

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 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

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 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.

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 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.

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

This function is affected by the $RAND variable.

sample

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.

uniq

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.

uniqint

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.

uniqnum

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.

uniqstr

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

tail

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

List::SomeUtils FUNCTIONS

Junctions

Treatment of an empty list

There are two schools of thought for how to evaluate a junction on an empty list:

  • Reduction to an identity (boolean)

  • Result is undefined (three-valued)

In the first case, the result of the junction applied to the empty list is determined by a mathematical reduction to an identity depending on whether the underlying comparison is "or" or "and". Conceptually:

                "any are true"      "all are true"
                --------------      --------------
2 elements:     A || B || 0         A && B && 1
1 element:      A || 0              A && 1
0 elements:     0                   1

In the second case, three-value logic is desired, in which a junction applied to an empty list returns undef rather than true or false

Junctions with a _u suffix implement three-valued logic. Those without are boolean.

all BLOCK LIST

all_u BLOCK LIST

Returns a true value if all items in LIST meet the criterion given through BLOCK. Sets $_ for each item in LIST in turn:

print "All values are non-negative"
  if all { $_ >= 0 } ($x, $y, $z);

For an empty LIST, all returns true (i.e. no values failed the condition) and all_u returns undef.

Thus, all_u(@list) is equivalent to @list ? all(@list) : undef.

Note: because Perl treats undef as false, you must check the return value of all_u with defined or you will get the opposite result of what you expect.

any BLOCK LIST

any_u BLOCK LIST

Returns a true value if any item in LIST meets the criterion given through BLOCK. Sets $_ for each item in LIST in turn:

print "At least one non-negative value"
  if any { $_ >= 0 } ($x, $y, $z);

For an empty LIST, any returns false and any_u returns undef.

Thus, any_u(@list) is equivalent to @list ? any(@list) : undef.

none BLOCK LIST

none_u BLOCK LIST

Logically the negation of any. Returns a true value if no item in LIST meets the criterion given through BLOCK. Sets $_ for each item in LIST in turn:

print "No non-negative values"
  if none { $_ >= 0 } ($x, $y, $z);

For an empty LIST, none returns true (i.e. no values failed the condition) and none_u returns undef.

Thus, none_u(@list) is equivalent to @list ? none(@list) : undef.

Note: because Perl treats undef as false, you must check the return value of none_u with defined or you will get the opposite result of what you expect.

notall BLOCK LIST

notall_u BLOCK LIST

Logically the negation of all. Returns a true value if not all items in LIST meet the criterion given through BLOCK. Sets $_ for each item in LIST in turn:

print "Not all values are non-negative"
  if notall { $_ >= 0 } ($x, $y, $z);

For an empty LIST, notall returns false and notall_u returns undef.

Thus, notall_u(@list) is equivalent to @list ? notall(@list) : undef.

one BLOCK LIST

one_u BLOCK LIST

Returns a true value if precisely one item in LIST meets the criterion given through BLOCK. Sets $_ for each item in LIST in turn:

print "Precisely one value defined"
    if one { defined($_) } @list;

Returns false otherwise.

For an empty LIST, one returns false and one_u returns undef.

The expression one BLOCK LIST is almost equivalent to 1 == true BLOCK LIST, except for short-cutting. Evaluation of BLOCK will immediately stop at the second true value.

Transformation

apply BLOCK LIST

Makes a copy of the list and then passes each element from the copy to the BLOCK. Any changes or assignments to $_ in the BLOCK will only affect the elements of the new list. However, if $_ is a reference then changes to the referenced value will be seen in both the original and new list.

This function is similar to map but will not modify the elements of the input list:

my @list = (1 .. 4);
my @mult = apply { $_ *= 2 } @list;
print "\@list = @list\n";
print "\@mult = @mult\n";
__END__
@list = 1 2 3 4
@mult = 2 4 6 8

Think of it as syntactic sugar for

for (my @mult = @list) { $_ *= 2 }

Note that you must alter $_ directly inside BLOCK in order for changes to make effect. New value returned from the BLOCK are ignored:

# @new is identical to @list.
my @new = apply { $_ * 2 } @list;

# @new is different from @list
my @new = apply { $_ =* 2 } @list;

insert_after BLOCK VALUE LIST

Inserts VALUE after the first item in LIST for which the criterion in BLOCK is true. Sets $_ for each item in LIST in turn.

my @list = qw/This is a list/;
insert_after { $_ eq "a" } "longer" => @list;
print "@list";
__END__
This is a longer list

insert_after_string STRING VALUE LIST

Inserts VALUE after the first item in LIST which is equal to STRING.

my @list = qw/This is a list/;
insert_after_string "a", "longer" => @list;
print "@list";
__END__
This is a longer list

pairwise BLOCK ARRAY1 ARRAY2

Evaluates BLOCK for each pair of elements in ARRAY1 and ARRAY2 and returns a new list consisting of BLOCK's return values. The two elements are set to $a and $b. Note that those two are aliases to the original value so changing them will modify the input arrays.

@a = (1 .. 5);
@b = (11 .. 15);
@x = pairwise { $a + $b } @a, @b;     # returns 12, 14, 16, 18, 20

# mesh with pairwise
@a = qw/a b c/;
@b = qw/1 2 3/;
@x = pairwise { ($a, $b) } @a, @b;    # returns a, 1, b, 2, c, 3

mesh ARRAY1 ARRAY2 [ ARRAY3 ... ]

zip ARRAY1 ARRAY2 [ ARRAY3 ... ]

Returns a list consisting of the first elements of each array, then the second, then the third, etc, until all arrays are exhausted.

Examples:

@x = qw/a b c d/;
@y = qw/1 2 3 4/;
@z = mesh @x, @y;         # returns a, 1, b, 2, c, 3, d, 4

@a = ('x');
@b = ('1', '2');
@c = qw/zip zap zot/;
@d = mesh @a, @b, @c;   # x, 1, zip, undef, 2, zap, undef, undef, zot

zip is an alias for mesh.

uniq LIST

distinct LIST

Returns a new list by stripping duplicate values in LIST by comparing the values as hash keys, except that undef is considered separate from ''. The order of elements in the returned list is the same as in LIST. In scalar context, returns the number of unique elements in LIST.

my @x = uniq 1, 1, 2, 2, 3, 5, 3, 4; # returns 1 2 3 5 4
my $x = uniq 1, 1, 2, 2, 3, 5, 3, 4; # returns 5
# returns "Mike", "Michael", "Richard", "Rick"
my @n = distinct "Mike", "Michael", "Richard", "Rick", "Michael", "Rick"
# returns '', undef, 'S1', A5'
my @s = distinct '', undef, 'S1', 'A5'
# returns '', undef, 'S1', A5'
my @w = uniq undef, '', 'S1', 'A5'

distinct is an alias for uniq.

RT#49800 can be used to give feedback about this behavior.

singleton

Returns a new list by stripping values in LIST occurring more than once by comparing the values as hash keys, except that undef is considered separate from ''. The order of elements in the returned list is the same as in LIST. In scalar context, returns the number of elements occurring only once in LIST.

my @x = singleton 1,1,2,2,3,4,5 # returns 3 4 5

Partitioning

after BLOCK LIST

Returns a list of the values of LIST after (and not including) the point where BLOCK returns a true value. Sets $_ for each element in LIST in turn.

@x = after { $_ % 5 == 0 } (1..9);    # returns 6, 7, 8, 9

after_incl BLOCK LIST

Same as after but also includes the element for which BLOCK is true.

before BLOCK LIST

Returns a list of values of LIST up to (and not including) the point where BLOCK returns a true value. Sets $_ for each element in LIST in turn.

before_incl BLOCK LIST

Same as before but also includes the element for which BLOCK is true.

part BLOCK LIST

Partitions LIST based on the return value of BLOCK which denotes into which partition the current value is put.

Returns a list of the partitions thusly created. Each partition created is a reference to an array.

my $i = 0;
my @part = part { $i++ % 2 } 1 .. 8;   # returns [1, 3, 5, 7], [2, 4, 6, 8]

You can have a sparse list of partitions as well where non-set partitions will be undef:

my @part = part { 2 } 1 .. 10;            # returns undef, undef, [ 1 .. 10 ]

Be careful with negative values, though:

my @part = part { -1 } 1 .. 10;
__END__
Modification of non-creatable array value attempted, subscript -1 ...

Negative values are only ok when they refer to a partition previously created:

my @idx  = ( 0, 1, -1 );
my $i    = 0;
my @part = part { $idx[$i++ % 3] } 1 .. 8; # [1, 4, 7], [2, 3, 5, 6, 8]

Iteration

each_array ARRAY1 ARRAY2 ...

Creates an array iterator to return the elements of the list of arrays ARRAY1, ARRAY2 throughout ARRAYn in turn. That is, the first time it is called, it returns the first element of each array. The next time, it returns the second elements. And so on, until all elements are exhausted.

This is useful for looping over more than one array at once:

my $ea = each_array(@a, @b, @c);
while ( my ($a, $b, $c) = $ea->() )   { .... }

The iterator returns the empty list when it reached the end of all arrays.

If the iterator is passed an argument of 'index', then it returns the index of the last fetched set of values, as a scalar.

each_arrayref LIST

Like each_array, but the arguments are references to arrays, not the plain arrays.

natatime EXPR, LIST

Creates an array iterator, for looping over an array in chunks of $n items at a time. (n at a time, get it?). An example is probably a better explanation than I could give in words.

Example:

my @x = ('a' .. 'g');
my $it = natatime 3, @x;
while (my @vals = $it->())
{
  print "@vals\n";
}

This prints

a b c
d e f
g

Searching

bsearch BLOCK LIST

Performs a binary search on LIST which must be a sorted list of values. BLOCK must return a negative value if the current element (stored in $_) is smaller, a positive value if it is bigger and zero if it matches.

Returns a boolean value in scalar context. In list context, it returns the element if it was found, otherwise the empty list.

bsearchidx BLOCK LIST

bsearch_index BLOCK LIST

Performs a binary search on LIST which must be a sorted list of values. BLOCK must return a negative value if the current element (stored in $_) is smaller, a positive value if it is bigger and zero if it matches.

Returns the index of found element, otherwise -1.

bsearch_index is an alias for bsearchidx.

firstval BLOCK LIST

first_value BLOCK LIST

Returns the first element in LIST for which BLOCK evaluates to true. Each element of LIST is set to $_ in turn. Returns undef if no such element has been found.

first_value is an alias for firstval.

onlyval BLOCK LIST

only_value BLOCK LIST

Returns the only element in LIST for which BLOCK evaluates to true. Sets $_ for each item in LIST in turn. Returns undef if no such element has been found.

only_value is an alias for onlyval.

lastval BLOCK LIST

last_value BLOCK LIST

Returns the last value in LIST for which BLOCK evaluates to true. Each element of LIST is set to $_ in turn. Returns undef if no such element has been found.

last_value is an alias for lastval.

firstres BLOCK LIST

first_result BLOCK LIST

Returns the result of BLOCK for the first element in LIST for which BLOCK evaluates to true. Each element of LIST is set to $_ in turn. Returns undef if no such element has been found.

first_result is an alias for firstres.

onlyres BLOCK LIST

only_result BLOCK LIST

Returns the result of BLOCK for the first element in LIST for which BLOCK evaluates to true. Sets $_ for each item in LIST in turn. Returns undef if no such element has been found.

only_result is an alias for onlyres.

lastres BLOCK LIST

last_result BLOCK LIST

Returns the result of BLOCK for the last element in LIST for which BLOCK evaluates to true. Each element of LIST is set to $_ in turn. Returns undef if no such element has been found.

last_result is an alias for lastres.

indexes BLOCK LIST

Evaluates BLOCK for each element in LIST (assigned to $_) and returns a list of the indices of those elements for which BLOCK returned a true value. This is just like grep only that it returns indices instead of values:

@x = indexes { $_ % 2 == 0 } (1..10);   # returns 1, 3, 5, 7, 9

firstidx BLOCK LIST

first_index BLOCK LIST

Returns the index of the first element in LIST for which the criterion in BLOCK is true. Sets $_ for each item in LIST in turn:

my @list = (1, 4, 3, 2, 4, 6);
printf "item with index %i in list is 4", firstidx { $_ == 4 } @list;
__END__
item with index 1 in list is 4

Returns -1 if no such item could be found.

first_index is an alias for firstidx.

onlyidx BLOCK LIST

only_index BLOCK LIST

Returns the index of the only element in LIST for which the criterion in BLOCK is true. Sets $_ for each item in LIST in turn:

my @list = (1, 3, 4, 3, 2, 4);
printf "uniqe index of item 2 in list is %i", onlyidx { $_ == 2 } @list;
__END__
unique index of item 2 in list is 4

Returns -1 if either no such item or more than one of these has been found.

only_index is an alias for onlyidx.

lastidx BLOCK LIST

last_index BLOCK LIST

Returns the index of the last element in LIST for which the criterion in BLOCK is true. Sets $_ for each item in LIST in turn:

my @list = (1, 4, 3, 2, 4, 6);
printf "item with index %i in list is 4", lastidx { $_ == 4 } @list;
__END__
item with index 4 in list is 4

Returns -1 if no such item could be found.

last_index is an alias for lastidx.

Sorting

sort_by BLOCK LIST

Returns the list of values sorted according to the string values returned by the KEYFUNC block or function. A typical use of this may be to sort objects according to the string value of some accessor, such as

sort_by { $_->name } @people

The key function is called in scalar context, being passed each value in turn as both $_ and the only argument in the parameters, @_. The values are then sorted according to string comparisons on the values returned. This is equivalent to

sort { $a->name cmp $b->name } @people

except that it guarantees the name accessor will be executed only once per value. One interesting use-case is to sort strings which may have numbers embedded in them "naturally", rather than lexically.

sort_by { s/(\d+)/sprintf "%09d", $1/eg; $_ } @strings

This sorts strings by generating sort keys which zero-pad the embedded numbers to some level (9 digits in this case), helping to ensure the lexical sort puts them in the correct order.

nsort_by BLOCK LIST

Similar to sort_by but compares its key values numerically.

Counting and calculation

true BLOCK LIST

Counts the number of elements in LIST for which the criterion in BLOCK is true. Sets $_ for each item in LIST in turn:

printf "%i item(s) are defined", true { defined($_) } @list;

false BLOCK LIST

Counts the number of elements in LIST for which the criterion in BLOCK is false. Sets $_ for each item in LIST in turn:

printf "%i item(s) are not defined", false { defined($_) } @list;

minmax LIST

Calculates the minimum and maximum of LIST and returns a two element list with the first element being the minimum and the second the maximum. Returns the empty list if LIST was empty.

The minmax algorithm differs from a naive iteration over the list where each element is compared to two values being the so far calculated min and max value in that it only requires 3n/2 - 2 comparisons. Thus it is the most efficient possible algorithm.

However, the Perl implementation of it has some overhead simply due to the fact that there are more lines of Perl code involved. Therefore, LIST needs to be fairly big in order for minmax to win over a naive implementation. This limitation does not apply to the XS version.

mode LIST

Calculates the most common items in the list and returns them as a list. This is effectively done by string comparisons, so references will be stringified. If they implement string overloading, this will be used.

If more than one item appears the same number of times in the list, all such items will be returned. For example, the mode of a unique list is the list itself.

This function returns a list in list context. In scalar context it returns a count indicating the number of modes in the list.

List::UtilsBy FUNCTIONS

All functions added since version 0.04 unless otherwise stated, as the original names for earlier versions were renamed.

sort_by

@vals = sort_by { KEYFUNC } @vals

Returns the list of values sorted according to the string values returned by the KEYFUNC block or function. A typical use of this may be to sort objects according to the string value of some accessor, such as

sort_by { $_->name } @people

The key function is called in scalar context, being passed each value in turn as both $_ and the only argument in the parameters, @_. The values are then sorted according to string comparisons on the values returned.

This is equivalent to

sort { $a->name cmp $b->name } @people

except that it guarantees the name accessor will be executed only once per value.

One interesting use-case is to sort strings which may have numbers embedded in them "naturally", rather than lexically.

sort_by { s/(\d+)/sprintf "%09d", $1/eg; $_ } @strings

This sorts strings by generating sort keys which zero-pad the embedded numbers to some level (9 digits in this case), helping to ensure the lexical sort puts them in the correct order.

nsort_by

@vals = nsort_by { KEYFUNC } @vals

Similar to "sort_by" but compares its key values numerically.

rev_sort_by

rev_nsort_by

@vals = rev_sort_by { KEYFUNC } @vals

@vals = rev_nsort_by { KEYFUNC } @vals

Since version 0.06.

Similar to "sort_by" and "nsort_by" but returns the list in the reverse order. Equivalent to

@vals = reverse sort_by { KEYFUNC } @vals

except that these functions are slightly more efficient because they avoid the final reverse operation.

max_by

$optimal = max_by { KEYFUNC } @vals

@optimal = max_by { KEYFUNC } @vals

Returns the (first) value from @vals that gives the numerically largest result from the key function.

my $tallest = max_by { $_->height } @people

use File::stat qw( stat );
my $newest = max_by { stat($_)->mtime } @files;

In scalar context, the first maximal value is returned. In list context, a list of all the maximal values is returned. This may be used to obtain positions other than the first, if order is significant.

If called on an empty list, an empty list is returned.

For symmetry with the "nsort_by" function, this is also provided under the name nmax_by since it behaves numerically.

min_by

$optimal = min_by { KEYFUNC } @vals

@optimal = min_by { KEYFUNC } @vals

Similar to "max_by" but returns values which give the numerically smallest result from the key function. Also provided as nmin_by

minmax_by

( $minimal, $maximal ) = minmax_by { KEYFUNC } @vals

Since version 0.11.

Similar to calling both "min_by" and "max_by" with the same key function on the same list. This version is more efficient than calling the two other functions individually, as it has less work to perform overall. In the case of ties, only the first optimal element found in each case is returned. Also provided as nminmax_by.

uniq_by

@vals = uniq_by { KEYFUNC } @vals

Returns a list of the subset of values for which the key function block returns unique values. The first value yielding a particular key is chosen, subsequent values are rejected.

my @some_fruit = uniq_by { $_->colour } @fruit;

To select instead the last value per key, reverse the input list. If the order of the results is significant, don't forget to reverse the result as well:

my @some_fruit = reverse uniq_by { $_->colour } reverse @fruit;

Because the values returned by the key function are used as hash keys, they ought to either be strings, or at least well-behaved as strings (such as numbers, or object references which overload stringification in a suitable manner).

partition_by

%parts = partition_by { KEYFUNC } @vals

Returns a key/value list of ARRAY refs containing all the original values distributed according to the result of the key function block. Each value will be an ARRAY ref containing all the values which returned the string from the key function, in their original order.

my %balls_by_colour = partition_by { $_->colour } @balls;

Because the values returned by the key function are used as hash keys, they ought to either be strings, or at least well-behaved as strings (such as numbers, or object references which overload stringification in a suitable manner).

count_by

%counts = count_by { KEYFUNC } @vals

Since version 0.07.

Returns a key/value list of integers, giving the number of times the key function block returned the key, for each value in the list.

my %count_of_balls = count_by { $_->colour } @balls;

Because the values returned by the key function are used as hash keys, they ought to either be strings, or at least well-behaved as strings (such as numbers, or object references which overload stringification in a suitable manner).

zip_by

@vals = zip_by { ITEMFUNC } \@arr0, \@arr1, \@arr2,...

Returns a list of each of the values returned by the function block, when invoked with values from across each each of the given ARRAY references. Each value in the returned list will be the result of the function having been invoked with arguments at that position, from across each of the arrays given.

my @transposition = zip_by { [ @_ ] } @matrix;

my @names = zip_by { "$_[1], $_[0]" } \@firstnames, \@surnames;

print zip_by { "$_[0] => $_[1]\n" } [ keys %hash ], [ values %hash ];

If some of the arrays are shorter than others, the function will behave as if they had undef in the trailing positions. The following two lines are equivalent:

zip_by { f(@_) } [ 1, 2, 3 ], [ "a", "b" ]
f( 1, "a" ), f( 2, "b" ), f( 3, undef )

The item function is called by map, so if it returns a list, the entire list is included in the result. This can be useful for example, for generating a hash from two separate lists of keys and values

my %nums = zip_by { @_ } [qw( one two three )], [ 1, 2, 3 ];
# %nums = ( one => 1, two => 2, three => 3 )

(A function having this behaviour is sometimes called zipWith, e.g. in Haskell, but that name would not fit the naming scheme used by this module).

unzip_by

$arr0, $arr1, $arr2, ... = unzip_by { ITEMFUNC } @vals

Since version 0.09.

Returns a list of ARRAY references containing the values returned by the function block, when invoked for each of the values given in the input list. Each of the returned ARRAY references will contain the values returned at that corresponding position by the function block. That is, the first returned ARRAY reference will contain all the values returned in the first position by the function block, the second will contain all the values from the second position, and so on.

my ( $firstnames, $lastnames ) = unzip_by { m/^(.*?) (.*)$/ } @names;

If the function returns lists of differing lengths, the result will be padded with undef in the missing elements.

This function is an inverse of "zip_by", if given a corresponding inverse function.

extract_by

@vals = extract_by { SELECTFUNC } @arr

Since version 0.05.

Removes elements from the referenced array on which the selection function returns true, and returns a list containing those elements. This function is similar to grep, except that it modifies the referenced array to remove the selected values from it, leaving only the unselected ones.

my @red_balls = extract_by { $_->color eq "red" } @balls;

# Now there are no red balls in the @balls array

This function modifies a real array, unlike most of the other functions in this module. Because of this, it requires a real array, not just a list.

This function is implemented by invoking splice on the array, not by constructing a new list and assigning it. One result of this is that weak references will not be disturbed.

extract_by { !defined $_ } @refs;

will leave weak references weakened in the @refs array, whereas

@refs = grep { defined $_ } @refs;

will strengthen them all again.

extract_first_by

$val = extract_first_by { SELECTFUNC } @arr

Since version 0.10.

A hybrid between "extract_by" and List::Util::first. Removes the first element from the referenced array on which the selection function returns true, returning it.

As with "extract_by", this function requires a real array and not just a list, and is also implemented using splice so that weak references are not disturbed.

If this function fails to find a matching element, it will return an empty list in list context. This allows a caller to distinguish the case between no matching element, and the first matching element being undef.

weighted_shuffle_by

@vals = weighted_shuffle_by { WEIGHTFUNC } @vals

Since version 0.07.

Returns the list of values shuffled into a random order. The randomisation is not uniform, but weighted by the value returned by the WEIGHTFUNC. The probabilty of each item being returned first will be distributed with the distribution of the weights, and so on recursively for the remaining items.

bundle_by

@vals = bundle_by { BLOCKFUNC } $number, @vals

Since version 0.07.

Similar to a regular map functional, returns a list of the values returned by BLOCKFUNC. Values from the input list are given to the block function in bundles of $number.

If given a list of values whose length does not evenly divide by $number, the final call will be passed fewer elements than the others.

EXPORTS

This module exports nothing by default. You can import functions by name, or get everything with the :all tag.

SEE ALSO

List::Util, List::SomeUtils and List::UtilsBy, obviously.

Also see Util::Any, which unifies many more util modules, and also lets you rename functions as part of the import.

BUGS

Please report any bugs or feature requests to bug-list-allutils@rt.cpan.org, or through the web interface at http://rt.cpan.org. I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.

Bugs may be submitted at https://github.com/houseabsolute/List-AllUtils/issues.

I am also usually active on IRC as 'autarch' on irc://irc.perl.org.

SOURCE

The source code repository for List-AllUtils can be found at https://github.com/houseabsolute/List-AllUtils.

DONATIONS

If you'd like to thank me for the work I've done on this module, please consider making a "donation" to me via PayPal. I spend a lot of free time creating free software, and would appreciate any support you'd care to offer.

Please note that I am not suggesting that you must do this in order for me to continue working on this particular software. I will continue to do so, inasmuch as I have in the past, for as long as it interests me.

Similarly, a donation made in this way will probably not make me work on this software much more, unless I get so many donations that I can consider working on free software full time (let's all have a chuckle at that together).

To donate, log into PayPal and send money to autarch@urth.org, or use the button at https://www.urth.org/fs-donation.html.

AUTHOR

Dave Rolsky <autarch@urth.org>

CONTRIBUTORS

  • Andy Jack <github@veracity.ca>

  • Dave Jacoby <jacoby.david@gmail.com>

  • Karen Etheridge <ether@cpan.org>

  • Olaf Alders <olaf@wundersolutions.com>

  • Ricardo Signes <rjbs@cpan.org>

  • Yanick Champoux <yanick@babyl.dyndns.org>

COPYRIGHT AND LICENSE

This software is Copyright (c) 2021 by Dave Rolsky.

This is free software, licensed under:

The Artistic License 2.0 (GPL Compatible)

The full text of the license can be found in the LICENSE file included with this distribution.