How do I generate the first n prime numbers?

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.

Check out Sieve of Eratosthenes. This is not Ruby specific but it is an algorithm to generate prime numbers. The idea behind this algorithm is that you have a list/array of numbers say

2..1000

You grab the first number, 2. Go through the list and eliminate everything that is divisible by 2. You will be left with everything that is not divisible by 2 other than 2 itself (e.g. [2,3,5,7,9,11...999]

Go to the next number, 3. And again, eliminate everything that you can divide by 3. Keep going until you reach the last number and you will get an array of prime numbers. Hope that helps.

http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes

require 'prime'

Prime.first(10) # => [2, 3, 5, 7, 11, 13, 17, 19, 23, 29]

If you'd like to do it yourself, then something like this could work:

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 

    def next_prime
        n = self+1
        n = n + 1 until n.is_prime?
        n   
    end 
end

Now to get the first 10 primes:

e = Enumerator.new do |y|
    n = 2
    loop do
        y << n
        n = n.next_prime
    end
end

primes = e.take 10