Finding the largest prime factor in a given number

LargestPrimeFactor.rb

class LargestPrimeFactor
  def largest_prime_of(number)
    return nil if number == nil
    arr = []
    (2..number).each do |n|
      if prime?(n)
        arr << n
      end
    end
    arr.reject!{|n| number % n != 0}
    arr[-1]
  end
  def prime?(number)
    (2..number-1).each do |num|
      if number%num == 0
        return false
      end
    end
    true
  end
end