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 take 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

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)

Defined in:

collection/unique.cr

Instance Method Summary

• #unique
• #unique(&func : T -> U) forall U

Instance Method Detail