Sieve of Eratosthene

eratosthenes.rb

def era(n)
    $primes = Array.new(n, true)
    $primes[0], $primes[1] = false, false
    (2..Math.sqrt(n)).each do |i|
        if $primes[i]
            (i * i).step(n, i).each do |j|
                $primes[j] = false
            end
        end
    end
end

max = 1000000
era(max)

loop do
    n = gets.to_i
    if n == 0
        break
    end
    number_of_primes = $primes.slice(0..n).count(true)
    puts "There are #{number_of_primes} prime numbers."
    puts (0..n).select{|i| $primes[i]}
end