eratosthenes.rb

MAX = 100
target = Array(2..MAX)
prime_numbers = []

LIMIT = Math.sqrt(MAX)
until target.first > LIMIT
  sieve = target.shift
  prime_numbers << sieve
  target.delete_if {|n| (n % sieve).zero? }
end

p prime_numbers + target #=> [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97]