Enumerator (opens new window) is an object that implements iteration in a controlled fashion.
Instead of looping until some condition is satisfied, the object enumerates values as needed. Execution of the loop is paused until the next value is requested by the owner of the object.
Enumerators make infinite streams of values possible.
# Custom enumerators
fibonacci = Enumerator.new do |yielder| a = b = 1 loop do yielder << a a, b = b, a + b end end
We can now use any
Enumerable (opens new window) method with
fibonacci.take 10 # => [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]
# Existing methods
If an iteration method such as
each is called without a block, an
Enumerator (opens new window) should be returned.
This can be done using the
enum_for (opens new window) method:
def each return enum_for :each unless block_given? yield :x yield :y yield :z end
This enables the programmer to compose
Enumerable (opens new window) operations:
each.drop(2).map(&:upcase).first # => :Z
rewind (opens new window) to restart the enumerator.
ℕ = Enumerator.new do |yielder| x = 0 loop do yielder << x x += 1 end end ℕ.next # => 0 ℕ.next # => 1 ℕ.next # => 2 ℕ.rewind ℕ.next # => 0
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