$ 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