Sieve.rb

def sieve(num)
  first_arr = Array.new(num) { |i| i }
  2.times { first_arr.delete_at(0) }
  primes = []
  while true
    break if first_arr[0]**2 >= num
    first_arr.delete_if { |n| n % first_arr[0] == 0 && first_arr[0] != n } 
    primes << first_arr.slice!(0)
  end
  primes += first_arr
  primes.count
end
 
 
st = Time.new
puts sieve(97531)
en = Time.new - st
puts en