eratosthenes sample

under_prime.rb

def eratosthenes( search_list, prime_list=[] )
  prime_list << search_list.first

  shake!( search_list, prime_list )

  if prime_list.last**2 > search_list.last
    return prime_list + search_list
  end

  eratosthenes( search_list, prime_list )
end

def shake!( target_list, filter_list )
  filter_list.each do |i|
    target_list.delete_if{ |j| j % i == 0 }
  end
end

def under_prime max_num
  eratosthenes Array( 2..max_num )
end

p under_prime(100_000)