calculator.rb

class Calculator

  def multiply(first, second)
    first * second
  end

  def two_raised_by(exponent)
    if exponent == 0
      return 0
    end

    result = 1
    (1 .. exponent).each do
      result = multiply(2, result)
    end
    result
  end
end

calculator_spec.rb

require 'spec_helper'

describe Calculator do

  let(:subject) {Calculator.new}

  describe '#multiply' do
    it 'returns 4 if you multiply 2 and 2' do
      subject.multiply(2, 2).should eq(4)
    end

    it 'returns 9 if you multiply 3 and 3' do
      subject.multiply(3, 3).should eq(9)
    end
  end

  describe '#two_raised_by' do
    it 'returns 0 if your exponent is 0' do
      subject.two_raised_by(0).should eq(0)
    end

    it 'returns 8 if your exponent is 3' do
      subject.two_raised_by(3).should eq(8)
    end
  end
end