Nth_Prime

nth_prime.rb

class Prime
  def self.nth(nth_prime)
    raise ArgumentError if nth_prime == 0
    return 2 if nth_prime == 1
    primes = [2,3]

    5.step(by:2) do |potential_prime|
      break if primes.size == nth_prime
      primes << potential_prime if prime? potential_prime
    end

    primes.last
  end

  def self.prime?(potential_prime)
    sqrt = Math.sqrt(potential_prime).round # I don't need to round here, but for some reason doing so makes it almost twice as fast.

    result = 2.upto(sqrt) do |number|
      return false if potential_prime % number == 0
    end

    result == false ? false : true
  end
end

# I could have defined #prime? as follows below, but then it would return 2 if the potential_prime was prime.
# Since 2 is truthy the method below will also work the way it needs to work in the #nth method, 
# but it wouldn't make sense to call Prime.prime?(5) and have the number 2 returned so I would make 
#prime? a private_class_method, but it seems like a waste to not be able to use #prime? on it's own so 
# I added some extra lines to #prime? above so it will return true or false instead of 2 or false.
# I imagine it's probably better ruby to have a method ending with ? explictly return true or false
# instead of a truthy or falsey value as well. 

def self.prime?(potential_prime)
  sqrt = Math.sqrt(potential_prime).round

  2.upto(sqrt) do |number|
    return false if potential_prime % number == 0
  end
end