How to get the nth element of an Enumerable in Ruby

Question

For example, to return the 10,000th prime number I could write:

require \'prime\'
Prime.first(10000).last #=> 104729

But creating a huge

Answer 1

module Enumerable
  def at(n)
    enum = is_a?(Enumerator) ? self : each
    (n-1).times { enum.next }
    enum.next
  end
end

(0..4).at(3)
  #=> 2 
{ a:1, b:2, c:3, d:4, e:5 }.at(3)
  #=> [:c, 3] 
'0ab_1ab_2ab_3ab_4ab'.gsub(/.(?=ab)/).at(3)
  #=> "2" 
require 'prime'; Prime.at(10000)
  #=> 104729 
e = 1.step; e.at(10)
  #=> 10 
[0,1,2,3,4].at(6)
  #=> nil 
'0ab_1ab_2ab_3ab_4ab'.gsub(/.(?=ab)/).at(6)
  #=> StopIteration (iteration reached an end)

Note that '0ab_1ab_2ab_3ab_4ab'.gsub(/.(?=ab)/).class #=> Enumerator.

Some refinements would be needed, such as checking that n is an integer greater than zero and improving the exception handling, such as dealing with the case when self is not an enumerator but has no method each.