Determine if a contiguous set of positive integers in an array add up to n

array_contains_sum.rb

class Array
  def contains_sum?(n)
    return false if n.nil?
    low = 0
    high = 0
    sum = first

    while sum != n && low < size
      if sum < n || low == high
        return false if high == size - 1
        high += 1
        sum += self[high]
      elsif sum > n
        sum -= self[low]
        low += 1
      end
    end

    sum == n
  end
end

Testing:

irb(main):022:0> a = [5, 3, 8, 10, 1]
=> [5, 3, 8, 10, 1]
irb(main):023:0> a.contains_sum?(11)
=> true
irb(main):024:0> a.contains_sum?(1)
=> true
irb(main):025:0> a.contains_sum?(2)
=> false
irb(main):026:0> a.contains_sum?(21)
=> true
irb(main):027:0> a.contains_sum?(5)
=> true
irb(main):028:0> [].contains_sum?(10)
=> false