Type:
Class

A class which allows both internal and external iteration.

An Enumerator can be created by the following methods.

  • Kernel#to_enum

  • Kernel#enum_for

  • ::new

Most methods have two forms: a block form where the contents are evaluated for each item in the enumeration, and a non-block form which returns a new Enumerator wrapping the iteration.

enumerator = %w(one two three).each
puts enumerator.class # => Enumerator

enumerator.each_with_object("foo") do |item, obj|
  puts "#{obj}: #{item}"
end

# foo: one
# foo: two
# foo: three

enum_with_obj = enumerator.each_with_object("foo")
puts enum_with_obj.class # => Enumerator

enum_with_obj.each do |item, obj|
  puts "#{obj}: #{item}"
end

# foo: one
# foo: two
# foo: three

This allows you to chain Enumerators together. For example, you can map a list's elements to strings containing the index and the element as a string via:

puts %w[foo bar baz].map.with_index { |w, i| "#{i}:#{w}" }
# => ["0:foo", "1:bar", "2:baz"]

An Enumerator can also be used as an external iterator. For example, #next returns the next value of the iterator or raises StopIteration if the Enumerator is at the end.

e = [1,2,3].each   # returns an enumerator object.
puts e.next   # => 1
puts e.next   # => 2
puts e.next   # => 3
puts e.next   # raises StopIteration

You can use this to implement an internal iterator as follows:

def ext_each(e)
  while true
    begin
      vs = e.next_values
    rescue StopIteration
      return $!.result
    end
    y = yield(*vs)
    e.feed y
  end
end

o = Object.new

def o.each
  puts yield
  puts yield(1)
  puts yield(1, 2)
  3
end

# use o.each as an internal iterator directly.
puts o.each {|*x| puts x; [:b, *x] }
# => [], [:b], [1], [:b, 1], [1, 2], [:b, 1, 2], 3

# convert o.each to an external iterator for
# implementing an internal iterator.
puts ext_each(o.to_enum) {|*x| puts x; [:b, *x] }
# => [], [:b], [1], [:b, 1], [1, 2], [:b, 1, 2], 3
next

e.next â object Instance Public methods Returns the next object in the enumerator

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inspect

e.inspect â string Instance Public methods Creates a printable version of

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rewind

e.rewind â e Instance Public methods Rewinds the enumeration sequence to

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peek

e.peek â object Instance Public methods Returns the next object in the enumerator

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size

e.size â int, Float::INFINITY or nil Instance Public methods Returns

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peek_values

e.peek_values â array Instance Public methods Returns the next object as

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new

Enumerator.new(size = nil) { |yielder| ... }Enumerator.new(obj, method = :each, *args) Class Public methods

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feed

e.feed obj â nil Instance Public methods Sets the value to be returned by

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with_index

e.with_index(offset = 0) {|(*args), idx| ... }e.with_index(offset = 0) Instance Public methods

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next_values

e.next_values â array Instance Public methods Returns the next object as

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