How do I generate the first n prime numbers?

Question

I am learning Ruby and doing some math stuff. One of the things I want to do is generate prime numbers.

I want to generate the first ten prime numbers and the first ten only. I have no problem testing a number to see if it is a prime number or not, but was wondering what the best way is to do generate these numbers?

I am using the following method to determine if the number is prime:

class Integer < Numeric
  def is_prime?
    return false if self <= 1
    2.upto(Math.sqrt(self).to_i) do |x|
      return false if self%x == 0
    end
    true
  end
end

Answer 1

In Ruby 1.9 there is a Prime class you can use to generate prime numbers, or to test if a number is prime:

require 'prime'

Prime.take(10) #=> [2, 3, 5, 7, 11, 13, 17, 19, 23, 29]
Prime.take_while {|p| p < 10 } #=> [2, 3, 5, 7]
Prime.prime?(19) #=> true

Prime implements the each method and includes the Enumerable module, so you can do all sorts of fun stuff like filtering, mapping, and so on.