Which One Wins?

"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.

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".

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.

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.

head

    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

Junctions

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

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

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

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

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

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

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.

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.