- 1_8_6_287
- 1_8_7_72
- 1_8_7_330
- 1_9_1_378 (0)
- 1_9_2_180 (38)
- 1_9_3_125 (15)
- 1_9_3_392 (0)
- 2_1_10 (0)
- 2_2_9 (0)
- 2_4_6 (0)
- 2_5_5 (0)
- 2_6_3 (0)
- What's this?
A class which allows both internal and external iteration.
An Enumerator can be created by the following methods.
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Kernel#to_enum
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Kernel#enum_for
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, Enumerator#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
Getting the return value from the underlying method of an Enumerator
This is documented in the example code, but easy to miss.
When you get an Enumerator using #to_enum(:method_name, …), you can get all of the yielded values using #next, but not the value that is finally returned.
That value can be retrieved via the #result attribute of the StopIteration exception object that is raised when calling #next after the underlying method has returned.