Sieve of Eratosthenes in Ruby

sieve_of_eratosthenes.rb

#!/usr/bin/env ruby

def gen_primes(n)
  sieve = Array.new(n) { |i| i > 1 }

  (0...Math.sqrt(n)).each do |i|
    next unless sieve[i]
    (i * i...n).step(i) { |j| sieve[j] = false }
  end

  sieve.map.with_index { |prime, index| index if prime }.compact

  # for count
  # primes.count { |p| p }
end

print gen_primes(ARGV.first.to_i)