Sieve of Eratosthenes

prime_number.rb

STDIN.read.split("\n").map {|i| i.chomp.to_i}.each do |n|
  x = n
  # lst = (3..x).to_a.select(&:odd?).unshift(2)
  lst = (2..x).to_a
  result = []
  sqrt = Math.sqrt(x)

  loop do
    elem = lst.shift
    result << elem
    break if elem >= sqrt
    lst.delete_if { |i| i % elem == 0 }
  end

  puts (result + lst).count
end