# module Iterator(T)

##
Overview

An `Iterator` allows processing sequences lazily, as opposed to `Enumerable` which processes sequences eagerly and produces an `Array` in most of its methods.

As an example, let's compute the first three numbers in the range `1..10_000_000` that are even, multiplied by three. One way to do this is:

    (1..10_000_000).select(&.even?).map { |x| x * 3 }.first(3) # => [6, 12, 18]

The above works, but creates many intermediate arrays: one for the `#select` call, one for the `#map` call and one for the `#first` call. A more efficient way is to invoke `Range#each` without a block, which gives us an `Iterator` so we can process the operations lazily:

    (1..10_000_000).each.select(&.even?).map { |x| x * 3 }.first(3) # => #< Iterator(T)::First...

`Iterator` redefines many of `Enumerable`'s method in a lazy way, returning iterators instead of arrays.

At the end of the call chain we get back a new iterator: we need to consume it, either using `#each` or `Enumerable#to_a`:

    (1..10_000_000).each.select(&.even?).map { |x| x * 3 }.first(3).to_a # => [6, 12, 18]

Because iterators only go forward, when using methods that consume it entirely or partially – `to_a`, `any?`, `count`, `none?`, `one?` and `size` – subsequent calls will give a different result as there will be less elements to consume.

    iter = (0...100).each
    iter.size # => 100
    iter.size # => 0

###
Iterating step-by-step

An iterator returns its next element on the method `#next`. Its return type is a union of the iterator's element type and the `Stop` type: `T | Iterator::Stop`. The stop type is a sentinel value which indicates that the iterator has reached its end. It usually needs to be handled and filtered out in order to use the element value for anything useful. Unlike `Nil` it's not an implicitly falsey type.

    iter = (1..5).each

    # Unfiltered elements contain `Iterator::Stop` type
    iter.next + iter.next # Error: expected argument #1 to 'Int32#+' to be Int32, not (Int32 | Iterator::Stop)

    # Type filtering eliminates the stop type
    a = iter.next
    b = iter.next
    unless a.is_a?(Iterator::Stop) || b.is_a?(Iterator::Stop)
      a + b # => 3
    end

`Iterator::Stop` is only present in the return type of `#next`. All other methods remove it from their return types.

Iterators can be used to build a loop.

    iter = (1..5).each
    sum = 0
    while !(elem = iter.next).is_a?(Iterator::Stop)
      sum += elem
    end
    sum # => 15

###
Implementing an Iterator

To implement an `Iterator` you need to define a `#next` method that must return the next element in the sequence or `Iterator::Stop::INSTANCE`, which signals the end of the sequence (you can invoke `#stop` inside an iterator as a shortcut).

For example, this is an iterator that returns a sequence of `N` zeros:

    class Zeros
      include Iterator(Int32)

      def initialize(@size : Int32)
        @produced = 0
      end

      def next
        if @produced < @size
          @produced += 1
          0
        else
          stop
        end
      end
    end

    zeros = Zeros.new(5)
    zeros.to_a # => [0, 0, 0, 0, 0]

The standard library provides iterators for many classes, like `Array`, `Hash`, `Range`, `String` and `IO`. Usually to get an iterator you invoke a method that would usually yield elements to a block, but without passing a block: `Array#each`, `Array#each_index`, `Hash#each`, `String#each_char`, `IO#each_line`, etc.

##
Included Modules

- Enumerable(T)

##
Direct including types

- Steppable::StepIterator(T, L, B)

##
Defined in:

iterator.cr
json/from_json.cr
json/to_json.cr
yaml/to_yaml.cr

##
Constructors

- .new(pull : JSON::PullParser)

  Creates a new iterator which iterates over a JSON array.

##
Class Method Summary

- .chain(iters : Iterator(Iter)) forall Iter

  The same as `#chain`, but have better performance when the quantity of iterators to chain is large (usually greater than 4) or undetermined.

- .chain(iters : Iterable(Iter)) forall Iter

  the same as `.chain(Iterator(Iter))`

- .empty

  Returns an empty iterator.

- .from_json(string_or_io)

  Reads the content of a JSON array into an iterator in a lazy way.

- .of(element : T)
- .of(&block : -> T)
- .stop

  Shortcut for `Iterator::Stop::INSTANCE`, to signal that there are no more elements in an iterator.

##
Instance Method Summary

- #accumulate(initial : U) forall U

  Returns an iterator that returns initial and its prefix sums with the original iterator's elements.

- #accumulate

  Returns an iterator that returns the prefix sums of the original iterator's elements.

- #accumulate(initial : U, &block : U, T -> U) forall U

  Returns an iterator that accumulates initial with the original iterator's elements by the given block.

- #accumulate(&block : T, T -> T)

  Returns an iterator that accumulates the original iterator's elements by the given block.

- #chain(other : Iterator(U)) forall U

  Returns an iterator that returns elements from the original iterator until it is exhausted and then returns the elements of the second iterator.

- #chunk(reuse = false, &block : T -> U) forall T, U

  Returns an Iterator that enumerates over the items, chunking them together based on the return value of the block.

- #chunk_while(reuse : Bool | Array(T) = false, &block : T, T -> B) forall B

  Returns an iterator for each chunked elements where elements are kept in a given chunk as long as the block's value over a pair of elements is truthy.

- #compact_map(&func : T -> _)

  Returns an iterator that applies the given function to the element and then returns it unless it is `nil`.

- #cons(n : Int, reuse = false)

  Returns an iterator that returns consecutive chunks of the size n.

- #cons_pair : Iterator(Tuple(T, T))

  Returns an iterator that returns consecutive pairs of adjacent items.

- #cycle(n : Int)

  Returns an iterator that repeatedly returns the elements of the original iterator starting back at the beginning when the end was reached, but only n times.

- #cycle

  Returns an iterator that repeatedly returns the elements of the original iterator forever starting back at the beginning when the end was reached.

- #each
- #each(& : T -> ) : Nil

  Calls the given block once for each element, passing that element as a parameter.

- #each_slice(n, reuse = false)

  Returns an iterator that then returns slices of n elements of the initial iterator.

- #first(n : Int)

  Returns an iterator that only returns the first n elements of the initial iterator.

- #flat_map(&func : T -> _)

  Returns a new iterator with the concatenated results of running the block once for every element in the collection.

- #flatten

  Returns an iterator that flattens nested iterators and arrays into a single iterator whose type is the union of the simple types of all of the nested iterators and arrays (and their nested iterators and arrays, and so on).

- #in_groups_of(size : Int, filled_up_with = nil, reuse = false)

  Returns an iterator that chunks the iterator's elements in arrays of size filling up the remaining elements if no element remains with `nil` or a given optional parameter.

- #map(&func : T -> U) forall U

  Returns an iterator that applies the given block to the next element and returns the result.

- #next

  Returns the next element in this iterator, or `Iterator::Stop::INSTANCE` if there are no more elements.

- #reject(&func : T -> U) forall U

  Returns an iterator that only returns elements for which the passed in block returns a falsey value.

- #reject(type : U.class) forall U

  Returns an iterator that only returns elements that are not of the given type.

- #reject(pattern)

  Returns an iterator that only returns elements where `pattern === element` does not hold.

- #select(&func : T -> U) forall U

  Returns an iterator that only returns elements for which the passed in block returns a truthy value.

- #select(type : U.class) forall U

  Returns an iterator that only returns elements of the given type.

- #select(pattern)

  Returns an iterator that only returns elements where `pattern === element`.

- #skip(n : Int)

  Returns an iterator that skips the first n elements and only returns the elements after that.

- #skip_while(&func : T -> U) forall U

  Returns an iterator that only starts to return elements once the given block has returned falsey value for one element.

- #slice(n : Int, reuse = false)

  Alias of `#each_slice`.

- #slice_after(reuse : Bool | Array(T) = false, &block : T -> B) forall B

  Returns an iterator over chunks of elements, where each chunk ends right after the given block's value is truthy.

- #slice_after(pattern, reuse : Bool | Array(T) = false)

  Returns an iterator over chunks of elements, where each chunk ends right after the given pattern is matched with `pattern === element`.

- #slice_before(reuse : Bool | Array(T) = false, &block : T -> B) forall B

  Returns an iterator over chunks of elements, where each chunk ends right before the given block's value is truthy.

- #slice_before(pattern, reuse : Bool | Array(T) = false)

  Returns an iterator over chunks of elements, where each chunk ends right before the given pattern is matched with `pattern === element`.

- #slice_when(reuse : Bool | Array(T) = false, &block : T, T -> B) forall B

  Returns an iterator for each chunked elements where the ends of chunks are defined by the block, when the block's value over a pair of elements is truthy.

- #step(n : Int)

  Returns an iterator that only returns every nth element, starting with the first.

- #stop

  Shortcut for `Iterator::Stop::INSTANCE`, to signal that there are no more elements in an iterator.

- #take_while(&func : T -> U) forall U

  Returns an iterator that returns elements while the given block returns a truthy value.

- #tap(&block : T -> )

  Returns an iterator that calls the given block with the next element of the iterator when calling `#next`, still returning the original element.

- #to_json(json : JSON::Builder)

  Converts the content of an iterator into a JSON array in lazy way.

- #to_yaml(yaml : YAML::Nodes::Builder)

  Converts the content of an iterator to YAML.

- #uniq

  Returns an iterator that only returns unique values of the original iterator.

- #uniq(&func : T -> U) forall U

  Returns an iterator that only returns unique values of the original iterator.

- #with_index(offset : Int = 0)

  Returns an iterator that returns a `Tuple` of the element and its index.

- #with_index(offset : Int = 0, &)

  Yields each element in this iterator together with its index.

- #with_object(obj)

  Returns an iterator that returns a `Tuple` of the element and a given object.

- #with_object(obj, &)

  Yields each element in this iterator together with obj.

- #zip(*others : Iterator) : Iterator

  Returns an iterator that returns the elements of this iterator and others traversed in tandem as `Tuple`s.

### Instance methods inherited from module `Enumerable(T)`

accumulate(initial : U) : Array(U) forall U
accumulate : Array(T)
accumulate(initial : U, &block : U, T -> U) : Array(U) forall U
accumulate(&block : T, T -> T) : Array(T) accumulate, all?(& : T -> ) : Bool
all?(pattern) : Bool
all? : Bool all?, any?(& : T -> ) : Bool
any?(pattern) : Bool
any? : Bool any?, chunks(&block : T -> U) forall U chunks, compact_map(& : T -> _) compact_map, count(& : T -> ) : Int32
count(item) : Int32 count, cycle(n, & : T -> ) : Nil
cycle(& : T -> ) : Nil cycle, each(& : T -> ) each, each_cons(count : Int, reuse = false, &) each_cons, each_cons_pair(& : T, T -> ) : Nil each_cons_pair, each_slice(count : Int, reuse = false, &) each_slice, each_step(n : Int, *, offset : Int = 0, & : T -> ) : Nil each_step, each_with_index(offset = 0, &) each_with_index, each_with_object(obj : U, & : T, U -> ) : U forall U each_with_object, empty? : Bool empty?, find(if_none = nil, & : T -> ) find, find!(& : T -> ) : T find!, find_value(if_none = nil, & : T -> ) find_value, first(&)
first(count : Int) : Array(T)
first : T first, first? : T | Nil first?, flat_map(& : T -> _) flat_map, group_by(& : T -> U) forall U group_by, in_groups_of(size : Int, filled_up_with : U = nil) forall U
in_groups_of(size : Int, filled_up_with : U = nil, reuse = false, &) forall U in_groups_of, in_slices_of(size : Int) : Array(Array(T)) in_slices_of, includes?(obj) : Bool includes?, index(& : T -> ) : Int32 | Nil
index(obj) : Int32 | Nil index, index!(& : T -> ) : Int32
index!(obj) : Int32 index!, index_by(& : T -> U) : Hash(U, T) forall U index_by, join(io : IO, separator = "") : Nil
join(separator, io : IO) : Nil
join(separator = "") : String
join(io : IO, separator = "", & : T, IO -> )
join(separator, io : IO, &)
join(separator = "", & : T -> ) join, map(& : T -> U) : Array(U) forall U map, map_with_index(offset = 0, & : T, Int32 -> U) : Array(U) forall U map_with_index, max(count : Int) : Array(T)
max : T max, max? : T | Nil max?, max_by(& : T -> U) : T forall U max_by, max_by?(& : T -> U) : T | Nil forall U max_by?, max_of(& : T -> U) : U forall U max_of, max_of?(& : T -> U) : U | Nil forall U max_of?, min(count : Int) : Array(T)
min : T min, min? : T | Nil min?, min_by(& : T -> U) : T forall U min_by, min_by?(& : T -> U) : T | Nil forall U min_by?, min_of(& : T -> U) : U forall U min_of, min_of?(& : T -> U) : U | Nil forall U min_of?, minmax : Tuple(T, T) minmax, minmax? : Tuple(T | Nil, T | Nil) minmax?, minmax_by(& : T -> U) : Tuple(T, T) forall U minmax_by, minmax_by?(& : T -> U) : Tuple(T, T) | Tuple(Nil, Nil) forall U minmax_by?, minmax_of(& : T -> U) : Tuple(U, U) forall U minmax_of, minmax_of?(& : T -> U) : Tuple(U, U) | Tuple(Nil, Nil) forall U minmax_of?, none?(& : T -> ) : Bool
none?(pattern) : Bool
none? : Bool none?, one?(& : T -> ) : Bool
one?(pattern) : Bool
one? : Bool one?, partition(& : T -> ) : Tuple(Array(T), Array(T))
partition(type : U.class) forall U partition, present? : Bool present?, product(initial : Number)
product
product(initial : Number, & : T -> )
product(& : T -> _) product, reduce(memo, &)
reduce(&) reduce, reduce?(&) reduce?, reject(& : T -> )
reject(type : U.class) forall U
reject(pattern) : Array(T) reject, sample(n : Int, random : Random = Random::DEFAULT) : Array(T)
sample(random : Random = Random::DEFAULT) : T sample, select(& : T -> )
select(type : U.class) : Array(U) forall U
select(pattern) : Array(T) select, size : Int32 size, skip(count : Int) skip, skip_while(& : T -> ) : Array(T) skip_while, sum(initial)
sum
sum(initial, & : T -> )
sum(& : T -> ) sum, take_while(& : T -> ) : Array(T) take_while, tally(hash)
tally : Hash(T, Int32) tally, tally_by(hash, &)
tally_by(&block : T -> U) : Hash(U, Int32) forall U tally_by, to_a : Array(T)
to_a(& : T -> U) : Array(U) forall U to_a, to_h
to_h(& : T -> Tuple(K, V)) forall K, V to_h, to_set : Set(T)
to_set(&block : T -> U) : Set(U) forall U to_set, zip(*others : Indexable | Iterable | Iterator, &)
zip(*others : Indexable | Iterable | Iterator) zip, zip?(*others : Indexable | Iterable | Iterator, &)
zip?(*others : Indexable | Iterable | Iterator) zip?

### Class methods inherited from module `Enumerable(T)`

element_type(x) element_type

##
Constructor Detail

def self.new(pull : JSON::PullParser) #

Creates a new iterator which iterates over a JSON array. See also `Iterator#from_json`.

WARNING The `JSON::PullParser` can't be used by anything else until the iterator is fully consumed.

##
Class Method Detail

def self.chain(iters : Iterator(Iter)) forall Iter #

The same as `#chain`, but have better performance when the quantity of iterators to chain is large (usually greater than 4) or undetermined.

    array_of_iters = [[1], [2, 3], [4, 5, 6]].each.map &.each
    iter = Iterator(Int32).chain array_of_iters
    iter.next # => 1
    iter.next # => 2
    iter.next # => 3
    iter.next # => 4

def self.chain(iters : Iterable(Iter)) forall Iter #

the same as `.chain(Iterator(Iter))`

def self.empty #

Returns an empty iterator.

def self.from_json(string_or_io) #

Reads the content of a JSON array into an iterator in a lazy way. With this method it should be possible to process a huge JSON array, without the requirement that the whole array fits into memory.

The following example produces a huge file, uses a lot of CPU but should not require much memory.

    File.open("/tmp/test.json", "w+") do |f|
      (0..1_000_000_000).each.to_json(f)
    end

    File.open("/tmp/test.json", "r") do |f|
      p Iterator(Int32).from_json(f).skip(1_000_000_000).to_a
    end

WARNING The `string_or_io` can't be used by anything else until the iterator is fully consumed.

def self.of(element : T) #
def self.of(&block : -> T) #
def self.stop #

Shortcut for `Iterator::Stop::INSTANCE`, to signal that there are no more elements in an iterator.

##
Instance Method Detail

def accumulate(initial : U) forall U #

Returns an iterator that returns initial and its prefix sums with the original iterator's elements.

Expects `U` to respond to the `#+` method.

    iter = (3..6).each.accumulate(7)
    iter.next # => 7
    iter.next # => 10
    iter.next # => 14
    iter.next # => 19
    iter.next # => 25
    iter.next # => Iterator::Stop::INSTANCE

def accumulate #

Returns an iterator that returns the prefix sums of the original iterator's elements.

Expects `T` to respond to the `#+` method.

    iter = (3..6).each.accumulate
    iter.next # => 3
    iter.next # => 7
    iter.next # => 12
    iter.next # => 18
    iter.next # => Iterator::Stop::INSTANCE

def accumulate(initial : U, &block : U, T -> U) forall U #

Returns an iterator that accumulates initial with the original iterator's elements by the given block.

Similar to `#accumulate(&block : T, T -> T)`, except the initial value is provided by an argument and needs not have the same type as the elements of the original iterator. This initial value is returned first.

    iter = [4, 3, 2].each.accumulate("X") { |x, y| x * y }
    iter.next # => "X"
    iter.next # => "XXXX"
    iter.next # => "XXXXXXXXXXXX"
    iter.next # => "XXXXXXXXXXXXXXXXXXXXXXXX"
    iter.next # => Iterator::Stop::INSTANCE

def accumulate(&block : T, T -> T) #

Returns an iterator that accumulates the original iterator's elements by the given block.

For each element of the original iterator the block is passed an accumulator value and the element. The result becomes the new value for the accumulator and is then returned. The initial value for the accumulator is the first element of the original iterator.

    iter = %w(the quick brown fox).each.accumulate { |x, y| "#{x}, #{y}" }
    iter.next # => "the"
    iter.next # => "the, quick"
    iter.next # => "the, quick, brown"
    iter.next # => "the, quick, brown, fox"
    iter.next # => Iterator::Stop::INSTANCE

def chain(other : Iterator(U)) forall U #

Returns an iterator that returns elements from the original iterator until it is exhausted and then returns the elements of the second iterator. Compared to `.chain(Iterator(Iter))`, it has better performance when the quantity of iterators to chain is small (usually less than 4). This method also cannot chain iterators in a loop, for that see `.chain(Iterator(Iter))`.

    iter = (1..2).each.chain(('a'..'b').each)
    iter.next # => 1
    iter.next # => 2
    iter.next # => 'a'
    iter.next # => 'b'
    iter.next # => Iterator::Stop::INSTANCE

def chunk(reuse = false, &block : T -> U) forall T, U #

Returns an Iterator that enumerates over the items, chunking them together based on the return value of the block.

Consecutive elements which return the same block value are chunked together.

For example, consecutive even numbers and odd numbers can be chunked as follows.

    [3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5].chunk(&.even?).each do |even, ary|
      p [even, ary]
    end

    # => [false, [3, 1]]
    #    [true, [4]]
    #    [false, [1, 5, 9]]
    #    [true, [2, 6]]
    #    [false, [5, 3, 5]]

The following key values have special meaning:

- `Enumerable::Chunk::Drop` specifies that the elements should be dropped
- `Enumerable::Chunk::Alone` specifies that the element should be chunked by itself

By default, a new array is created and yielded for each chunk when invoking `#next`.

- If reuse is given, the array can be reused
- If reuse is an `Array`, this array will be reused
- If reuse is truthy, the method will create a new array and reuse it.

This can be used to prevent many memory allocations when each slice of interest is to be used in a read-only fashion.

See also: `Enumerable#chunks`.

def chunk_while(reuse : Bool | Array(T) = false, &block : T, T -> B) forall B #

Returns an iterator for each chunked elements where elements are kept in a given chunk as long as the block's value over a pair of elements is truthy.

For example, one-by-one increasing subsequences can be chunked as follows:

    ary = [1, 2, 4, 9, 10, 11, 12, 15, 16, 19, 20, 21]
    iter = ary.chunk_while { |i, j| i + 1 == j }
    iter.next # => [1, 2]
    iter.next # => [4]
    iter.next # => [9, 10, 11, 12]
    iter.next # => [15, 16]
    iter.next # => [19, 20, 21]
    iter.next # => Iterator::Stop::INSTANCE

By default, a new array is created and yielded for each slice when invoking `#next`.

- If reuse is `false`, the method will create a new array for each chunk
- If reuse is `true`, the method will create a new array and reuse it.
- If reuse is an `Array`, that array will be reused

This can be used to prevent many memory allocations when each slice of interest is to be used in a read-only fashion.

See also `#slice_when`, which works similarly but the block's condition is inverted.

def compact_map(&func : T -> _) #

Returns an iterator that applies the given function to the element and then returns it unless it is `nil`. If the returned value would be `nil` it instead returns the next non `nil` value.

    iter = [1, nil, 2, nil].each.compact_map { |e| e.try &.*(2) }
    iter.next # => 2
    iter.next # => 4
    iter.next # => Iterator::Stop::INSTANCE

def cons(n : Int, reuse = false) #

Returns an iterator that returns consecutive chunks of the size n.

    iter = (1..5).each.cons(3)
    iter.next # => [1, 2, 3]
    iter.next # => [2, 3, 4]
    iter.next # => [3, 4, 5]
    iter.next # => Iterator::Stop::INSTANCE

By default, a new array is created and returned for each consecutive call of `#next`.

- If reuse is given, the array can be reused
- If reuse is `true`, the method will create a new array and reuse it.
- If reuse is an instance of `Array`, `Deque` or a similar collection type (implementing `#<<`, `#shift` and `#size`) it will be used.
- If reuse is falsey, the array will not be reused.

This can be used to prevent many memory allocations when each slice of interest is to be used in a read-only fashion.

Chunks of two items can be iterated using `#cons_pair`, an optimized implementation for the special case of `n == 2` which avoids heap allocations.

def cons_pair : Iterator(Tuple(T, T)) #

Returns an iterator that returns consecutive pairs of adjacent items.

    iter = (1..5).each.cons_pair
    iter.next # => {1, 2}
    iter.next # => {2, 3}
    iter.next # => {3, 4}
    iter.next # => {4, 5}
    iter.next # => Iterator::Stop::INSTANCE

Chunks of more than two items can be iterated using `#cons`. This method is just an optimized implementation for the special case of `n == 2` to avoid heap allocations.

def cycle(n : Int) #

Returns an iterator that repeatedly returns the elements of the original iterator starting back at the beginning when the end was reached, but only n times.

    iter = ["a", "b", "c"].each.cycle(2)
    iter.next # => "a"
    iter.next # => "b"
    iter.next # => "c"
    iter.next # => "a"
    iter.next # => "b"
    iter.next # => "c"
    iter.next # => Iterator::Stop::INSTANCE

def cycle #

Returns an iterator that repeatedly returns the elements of the original iterator forever starting back at the beginning when the end was reached.

    iter = ["a", "b", "c"].each.cycle
    iter.next # => "a"
    iter.next # => "b"
    iter.next # => "c"
    iter.next # => "a"
    iter.next # => "b"
    iter.next # => "c"
    iter.next # => "a"
    # and so an and so on

def each #
def each(& : T -> ) : Nil #

Calls the given block once for each element, passing that element as a parameter.

    iter = ["a", "b", "c"].each
    iter.each { |x| print x, " " } # Prints "a b c"

def each_slice(n, reuse = false) #

Returns an iterator that then returns slices of n elements of the initial iterator.

    iter = (1..9).each.each_slice(3)
    iter.next # => [1, 2, 3]
    iter.next # => [4, 5, 6]
    iter.next # => [7, 8, 9]
    iter.next # => Iterator::Stop::INSTANCE

By default, a new array is created and yielded for each consecutive when invoking `#next`.

- If reuse is given, the array can be reused
- If reuse is an `Array`, this array will be reused
- If reuse is truthy, the method will create a new array and reuse it.

This can be used to prevent many memory allocations when each slice of interest is to be used in a read-only fashion.

def first(n : Int) #

Returns an iterator that only returns the first n elements of the initial iterator.

    iter = ["a", "b", "c"].each.first 2
    iter.next # => "a"
    iter.next # => "b"
    iter.next # => Iterator::Stop::INSTANCE

def flat_map(&func : T -> _) #

Returns a new iterator with the concatenated results of running the block once for every element in the collection. Only `Array` and `Iterator` results are concatenated; every other value is returned once in the new iterator.

    iter = [1, 2, 3].each.flat_map { |x| [x, x] }

    iter.next # => 1
    iter.next # => 1
    iter.next # => 2

    iter = [1, 2, 3].each.flat_map { |x| [x, x].each }

    iter.to_a # => [1, 1, 2, 2, 3, 3]

def flatten #

Returns an iterator that flattens nested iterators and arrays into a single iterator whose type is the union of the simple types of all of the nested iterators and arrays (and their nested iterators and arrays, and so on).

    iter = [(1..2).each, ('a'..'b').each].each.flatten
    iter.next # => 1
    iter.next # => 2
    iter.next # => 'a'
    iter.next # => 'b'
    iter.next # => Iterator::Stop::INSTANCE

def in_groups_of(size : Int, filled_up_with = nil, reuse = false) #

Returns an iterator that chunks the iterator's elements in arrays of size filling up the remaining elements if no element remains with `nil` or a given optional parameter.

    iter = (1..3).each.in_groups_of(2)
    iter.next # => [1, 2]
    iter.next # => [3, nil]
    iter.next # => Iterator::Stop::INSTANCE

    iter = (1..3).each.in_groups_of(2, 'z')
    iter.next # => [1, 2]
    iter.next # => [3, 'z']
    iter.next # => Iterator::Stop::INSTANCE

By default, a new array is created and yielded for each group.

- If reuse is given, the array can be reused
- If reuse is an `Array`, this array will be reused
- If reuse is truthy, the method will create a new array and reuse it.

This can be used to prevent many memory allocations when each slice of interest is to be used in a read-only fashion.

def map(&func : T -> U) forall U #

Returns an iterator that applies the given block to the next element and returns the result.

    iter = [1, 2, 3].each.map &.*(2)
    iter.next # => 2
    iter.next # => 4
    iter.next # => 6
    iter.next # => Iterator::Stop::INSTANCE

abstract def next #

Returns the next element in this iterator, or `Iterator::Stop::INSTANCE` if there are no more elements.

def reject(&func : T -> U) forall U #

Returns an iterator that only returns elements for which the passed in block returns a falsey value.

    iter = [1, 2, 3].each.reject &.odd?
    iter.next # => 2
    iter.next # => Iterator::Stop::INSTANCE

def reject(type : U.class) forall U #

Returns an iterator that only returns elements that are not of the given type.

    iter = [1, false, 3, true].each.reject(Bool)
    iter.next # => 1
    iter.next # => 3
    iter.next # => Iterator::Stop::INSTANCE

def reject(pattern) #

Returns an iterator that only returns elements where `pattern === element` does not hold.

    iter = [2, 3, 1, 5, 4, 6].each.reject(3..5)
    iter.next # => 2
    iter.next # => 1
    iter.next # => 6
    iter.next # => Iterator::Stop::INSTANCE

def select(&func : T -> U) forall U #

Returns an iterator that only returns elements for which the passed in block returns a truthy value.

    iter = [1, 2, 3].each.select &.odd?
    iter.next # => 1
    iter.next # => 3
    iter.next # => Iterator::Stop::INSTANCE

def select(type : U.class) forall U #

Returns an iterator that only returns elements of the given type.

    iter = [1, false, 3, nil].each.select(Int32)
    iter.next # => 1
    iter.next # => 3
    iter.next # => Iterator::Stop::INSTANCE

def select(pattern) #

Returns an iterator that only returns elements where `pattern === element`.

    iter = [1, 3, 2, 5, 4, 6].each.select(3..5)
    iter.next # => 3
    iter.next # => 5
    iter.next # => 4
    iter.next # => Iterator::Stop::INSTANCE

def skip(n : Int) #

Returns an iterator that skips the first n elements and only returns the elements after that.

    iter = (1..3).each.skip(2)
    iter.next # -> 3
    iter.next # -> Iterator::Stop::INSTANCE

def skip_while(&func : T -> U) forall U #

Returns an iterator that only starts to return elements once the given block has returned falsey value for one element.

    iter = [1, 2, 3, 4, 0].each.skip_while { |i| i < 3 }
    iter.next # => 3
    iter.next # => 4
    iter.next # => 0
    iter.next # => Iterator::Stop::INSTANCE

def slice(n : Int, reuse = false) #

Alias of `#each_slice`.

def slice_after(reuse : Bool | Array(T) = false, &block : T -> B) forall B #

Returns an iterator over chunks of elements, where each chunk ends right after the given block's value is truthy.

For example, to get chunks that end at each uppercase letter:

    ary = ['a', 'b', 'C', 'd', 'E', 'F', 'g', 'h']
    #                   ^         ^    ^
    iter = ary.slice_after(&.uppercase?)
    iter.next # => ['a', 'b', 'C']
    iter.next # => ['d', 'E']
    iter.next # => ['F']
    iter.next # => ['g', 'h']
    iter.next # => Iterator::Stop::INSTANCE

By default, a new array is created and yielded for each slice when invoking `#next`.

- If reuse is `false`, the method will create a new array for each chunk
- If reuse is `true`, the method will create a new array and reuse it.
- If reuse is an `Array`, that array will be reused

This can be used to prevent many memory allocations when each slice of interest is to be used in a read-only fashion.

def slice_after(pattern, reuse : Bool | Array(T) = false) #

Returns an iterator over chunks of elements, where each chunk ends right after the given pattern is matched with `pattern === element`.

For example, to get chunks that end at each ASCII uppercase letter:

    ary = ['a', 'b', 'C', 'd', 'E', 'F', 'g', 'h']
    #                   ^         ^    ^
    iter = ary.slice_after('A'..'Z')
    iter.next # => ['a', 'b', 'C']
    iter.next # => ['d', 'E']
    iter.next # => ['F']
    iter.next # => ['g', 'h']
    iter.next # => Iterator::Stop::INSTANCE

By default, a new array is created and yielded for each slice when invoking `#next`.

- If reuse is `false`, the method will create a new array for each chunk
- If reuse is `true`, the method will create a new array and reuse it.
- If reuse is an `Array`, that array will be reused

This can be used to prevent many memory allocations when each slice of interest is to be used in a read-only fashion.

def slice_before(reuse : Bool | Array(T) = false, &block : T -> B) forall B #

Returns an iterator over chunks of elements, where each chunk ends right before the given block's value is truthy.

For example, to get chunks that end just before each uppercase letter:

    ary = ['a', 'b', 'C', 'd', 'E', 'F', 'g', 'h']
    #              ^         ^    ^
    iter = ary.slice_before(&.uppercase?)
    iter.next # => ['a', 'b']
    iter.next # => ['C', 'd']
    iter.next # => ['E']
    iter.next # => ['F', 'g', 'h']
    iter.next # => Iterator::Stop::INSTANCE

By default, a new array is created and yielded for each slice when invoking `#next`.

- If reuse is `false`, the method will create a new array for each chunk
- If reuse is `true`, the method will create a new array and reuse it.
- If reuse is an `Array`, that array will be reused

This can be used to prevent many memory allocations when each slice of interest is to be used in a read-only fashion.

def slice_before(pattern, reuse : Bool | Array(T) = false) #

Returns an iterator over chunks of elements, where each chunk ends right before the given pattern is matched with `pattern === element`.

For example, to get chunks that end just before each ASCII uppercase letter:

    ary = ['a', 'b', 'C', 'd', 'E', 'F', 'g', 'h']
    #              ^         ^    ^
    iter = ary.slice_before('A'..'Z')
    iter.next # => ['a', 'b']
    iter.next # => ['C', 'd']
    iter.next # => ['E']
    iter.next # => ['F', 'g', 'h']
    iter.next # => Iterator::Stop::INSTANCE

By default, a new array is created and yielded for each slice when invoking `#next`.

- If reuse is `false`, the method will create a new array for each chunk
- If reuse is `true`, the method will create a new array and reuse it.
- If reuse is an `Array`, that array will be reused

This can be used to prevent many memory allocations when each slice of interest is to be used in a read-only fashion.

def slice_when(reuse : Bool | Array(T) = false, &block : T, T -> B) forall B #

Returns an iterator for each chunked elements where the ends of chunks are defined by the block, when the block's value over a pair of elements is truthy.

For example, one-by-one increasing subsequences can be chunked as follows:

    ary = [1, 2, 4, 9, 10, 11, 12, 15, 16, 19, 20, 21]
    iter = ary.slice_when { |i, j| i + 1 != j }
    iter.next # => [1, 2]
    iter.next # => [4]
    iter.next # => [9, 10, 11, 12]
    iter.next # => [15, 16]
    iter.next # => [19, 20, 21]
    iter.next # => Iterator::Stop::INSTANCE

By default, a new array is created and yielded for each slice when invoking `#next`.

- If reuse is `false`, the method will create a new array for each chunk
- If reuse is `true`, the method will create a new array and reuse it.
- If reuse is an `Array`, that array will be reused

This can be used to prevent many memory allocations when each slice of interest is to be used in a read-only fashion.

See also `#chunk_while`, which works similarly but the block's condition is inverted.

def step(n : Int) #

Returns an iterator that only returns every nth element, starting with the first.

    iter = (1..6).each.step(2)
    iter.next # => 1
    iter.next # => 3
    iter.next # => 5
    iter.next # => Iterator::Stop::INSTANCE

def stop #

Shortcut for `Iterator::Stop::INSTANCE`, to signal that there are no more elements in an iterator.

def take_while(&func : T -> U) forall U #

Returns an iterator that returns elements while the given block returns a truthy value.

    iter = (1..5).each.take_while { |i| i < 3 }
    iter.next # => 1
    iter.next # => 2
    iter.next # => Iterator::Stop::INSTANCE

def tap(&block : T -> ) #

Returns an iterator that calls the given block with the next element of the iterator when calling `#next`, still returning the original element.

    a = 0
    iter = (1..3).each.tap { |x| a += x }
    iter.next # => 1
    a         # => 1
    iter.next # => 2
    a         # => 3
    iter.next # => 3
    a         # => 6
    iter.next # => Iterator::Stop::INSTANCE

def to_json(json : JSON::Builder) #

Converts the content of an iterator into a JSON array in lazy way. See `Iterator#from_json` for an example.

def to_yaml(yaml : YAML::Nodes::Builder) #

Converts the content of an iterator to YAML. The conversion is done in a lazy way. In contrast to `Iterator#to_json` this operation requires memory for the for the complete YAML document

def uniq #

Returns an iterator that only returns unique values of the original iterator.

    iter = [1, 2, 1].each.uniq
    iter.next # => 1
    iter.next # => 2
    iter.next # => Iterator::Stop::INSTANCE

def uniq(&func : T -> U) forall U #

Returns an iterator that only returns unique values of the original iterator. The provided block is applied to the elements to determine the value to be checked for uniqueness.

    iter = [["a", "a"], ["b", "a"], ["a", "c"]].each.uniq &.first
    iter.next # => ["a", "a"]
    iter.next # => ["b", "a"]
    iter.next # => Iterator::Stop::INSTANCE

def with_index(offset : Int = 0) #

Returns an iterator that returns a `Tuple` of the element and its index.

    iter = (1..3).each.with_index
    iter.next # => {1, 0}
    iter.next # => {2, 1}
    iter.next # => {3, 2}
    iter.next # => Iterator::Stop::INSTANCE

def with_index(offset : Int = 0, &) #

Yields each element in this iterator together with its index.

def with_object(obj) #

Returns an iterator that returns a `Tuple` of the element and a given object.

    iter = (1..3).each.with_object("a")
    iter.next # => {1, "a"}
    iter.next # => {2, "a"}
    iter.next # => {3, "a"}
    iter.next # => Iterator::Stop::INSTANCE

def with_object(obj, &) #

Yields each element in this iterator together with obj. Returns that object.

def zip(*others : Iterator) : Iterator #

Returns an iterator that returns the elements of this iterator and others traversed in tandem as `Tuple`s.

Iteration stops when any of the iterators runs out of elements.

    iter1 = [4, 5, 6].each
    iter2 = [7, 8, 9].each
    iter3 = ['a', 'b', 'c', 'd'].each
    iter = iter1.zip(iter2, iter3)
    iter.next # => {4, 7, 'a'}
    iter.next # => {5, 8, 'b'}
    iter.next # => {6, 9, 'c'}
    iter.next # => Iterator::Stop::INSTANCE