$ gem install prime_table

Notes

def sieve(upper) i = 0 list = (2..upper).to_a

(2..Math.sqrt(upper)).each do |mult|

if list[i] #proceed only when mult is prime
  init = mult + i

  (init..upper-1).step(mult) do |index|
    list[index] = nil
  end

end
i += 1

end list.compact end