prime.rb

start_time = Time.now

max = 1000000
table = Array.new(max + 1, true)
prime_list = [2]


num = 3
if table[num] == true
  prime_list << num
  k = num * num
  while k <= max
    table[k] = false
    k += num * 2
  end
end

num = 5
while num <= max
  if table[num] == true
    prime_list << num
    k = num * num
    while k <= max
      table[k] = false
      k += num * 2
    end
  end
  num += 2

  if table[num] == true
    prime_list << num
    k = num * num
    while k <= max
      table[k] = false
      k += num * 2
    end
  end
  num += 4
end
p prime_list

puts Time.now - start_time