Ruby enumerators

Enumerators are objects that yield things to a code block.

  e = Enumerator.new do |y|
    [1, 2, 3].each do |x|
      y << x
    end
  end

  e.map { |x| x**2 }

#+RESULTS:
# | 1 | 4 | 9 |

The y is a “yielder,” an instance of Enumerator::Yielder. The yielder is populated in the code block passed to the enumerator, and the enumerator looks to the yielder to determine what to return at any particular iteration of an each call.

a = [1, 2, 3]
e = a.each
e.class

#+RESULTS:
# : Enumerator

You can create an enumerator from most iterator methods by withholding a code block.

a = (1..10)
e = a.each
e.select { |x| x > 6 }

#+RESULTS:
# | 7 | 8 | 9 | 10 |

a = (1..10)
e = a.map
return e, e.each { |x| x**2 }

#+RESULTS:
# : '(#<Enumerator: 1..10:map>  (1  4  9  16  25  36  49  64  81  100))

The enumerator retains whatever method it’s created from — here e.each is created from a.map and functions like a.map.

def is_prime?(n)
  return 1 if n == 1
  (2..n/2).each do |x|
    return n if n % x == 0
  end
  return 'Prime'
end

def primes(n)
  (1..Float::INFINITY).lazy.map { |x| is_prime?(x) }.first(n)
end

primes(5)

#+RESULTS:
# | 1 | Prime | Prime | 4 | Prime |

You can even use an enumerator to do work on infinite sets by making it a “lazy” enumerator. Here I wrote a function to find primes in the first n positive natural numbers.