Dave Rolsky > List-AllUtils-0.11 > List::AllUtils

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Module Version: 0.11   Source   Latest Release: List-AllUtils-0.12

NAME ^

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

VERSION ^

version 0.11

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. Similar, List::SomeUtils has some small overlap with List::UtilsBy. List::AllUtils always favors the version provided by List::Util, List::SomeUtils or List::UtilsBy in that order.

The docs below come from List::Util 1.31, List::SomeUtils 0.50, and List::UtilsBy 0.10.

LIST-REDUCTION FUNCTIONS ^

The following set of functions all reduce a list down to a single value.

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 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.

    $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

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 remaining list-reduction functions are all specialisations of this generic idea.

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

This function could be implemented using reduce like this

    $foo = reduce { defined($a) ? $a : wanted($b) ? $b : undef } undef, @list

for example wanted() could be defined() which would return the first defined value in @list

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

This function could be implemented using reduce like this

    $foo = reduce { $a > $b ? $a : $b } 1..10

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

This function could be implemented using reduce like this

    $foo = reduce { $a gt $b ? $a : $b } 'A'..'Z'

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

This function could be implemented using reduce like this

    $foo = reduce { $a < $b ? $a : $b } 1..10

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

This function could be implemented using reduce like this

    $foo = reduce { $a lt $b ? $a : $b } 'A'..'Z'

sum LIST

Returns the sum of all the elements in LIST. If LIST is empty then undef is returned.

    $foo = sum 1..10                # 55
    $foo = sum 3,9,12               # 24
    $foo = sum @bar, @baz           # whatever

This function could be implemented using reduce like this

    $foo = reduce { $a + $b } 1..10

sum0 LIST

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.

pairgrep BLOCK KVLIST

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

Similar to grep, 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 BLOCK KVLIST

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

Similar to grep, 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 BLOCK KVLIST

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

Similar to map, 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.

pairs KVLIST

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

    pairmap { [ $a, $b ] } KVLIST

It is most convenient to use in a foreach loop, for example:

    foreach ( pairs @KVLIST ) {
       my ( $key, $value ) = @$_;
       ...
    }

pairkeys KVLIST

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

    pairmap { $a } KVLIST

pairvalues KVLIST

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

    pairmap { $b } KVLIST

OTHER FUNCTIONS ^

shuffle LIST

Returns the elements of LIST in a random order

    @cards = shuffle 0..51      # 0..51 in a random order

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:

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

Applies BLOCK to each item in LIST and returns a list of the values after BLOCK has been applied. In scalar context, the last element is returned. 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 }

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 '', 'S1', A5' and complains about "Use of uninitialized value"
  my @s = distinct '', undef, 'S1', 'A5'
  # returns undef, 'S1', A5' and complains about "Use of uninitialized value"
  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 { $idx[$++ % 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.

*By functions

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

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 probability 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.

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

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 probability 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 through the RT bug tracker (or bug-list-allutils@rt.cpan.org).

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

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 http://www.urth.org/~autarch/fs-donation.html.

AUTHOR ^

Dave Rolsky <autarch@urth.org>

CONTRIBUTORS ^

COPYRIGHT AND LICENCE ^

This software is Copyright (c) 2016 by Dave Rolsky.

This is free software, licensed under:

  The Artistic License 2.0 (GPL Compatible)
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