Tried simple Sieve of Eratosthenes in Ruby …

class MyPrime

  attr_reader :primes # there we can read then

  # compute prime numbers up to limit, default is 20
  def initialize(limit = 20)
    @primes = []
    limit = limit.to_i 
    return if limit.to_i < 2
    arr = Hash[*((2..limit).to_a*2).sort] # we can use .zip too
    (2..Math.sqrt(limit)).each do |i|
      next unless arr[i] # already removed
      (i*i..limit).step(i).each{|k| arr.delete(k)}
    end
    @primes = arr.keys.sort # return just keys
  end
end

we can add some tests

require 'test/unit'
require 'my_prime'

class TestMyPrime < Test::Unit::TestCase

  def test_20_primes
    primes = MyPrime.new(20)
    assert_equal [2, 3, 5, 7, 11, 13, 17, 19], primes.primes
  end

  def test_wo_param
    primes = MyPrime.new
    assert_equal [2, 3, 5, 7, 11, 13, 17, 19], primes.primes
  end

  def test_less_than_2
    primes = MyPrime.new(1)
    assert_equal [], primes.primes 
  end

  def test_limit_eq_2
    primes = MyPrime.new(2)
    assert_equal [2], primes.primes
  end

  def test_limit_is_string
    primes = MyPrime.new('20')
    assert_equal [2, 3, 5, 7, 11, 13, 17, 19], primes.primes
  end

  def test_limit_is_float
    primes = MyPrime.new(20.50)
    assert_equal [2, 3, 5, 7, 11, 13, 17, 19], primes.primes
  end

end