Takes an english math expression (e.g. What is 4 plus 2?), and returns the result. Assumes order of operations are those that the user inputs.

wordy_calculator.rb

class Calculator
  def phrase_cleaner(phrase)
    operands = phrase.match(/([0-9,-]+.*[\w])/)[0]
  end

  def number_filter(phrase)
    numbers = phrase.scan(/[0-9,-]+/)
    numbers.map { |number| number.to_i }
  end

  def operator_filter(phrase)
    operators = phrase.scan(/[0-9,-]+[\s](\S+)/)
    operators.map { |operator| operator.pop }
  end

  def ask(question)

    operators_gloss = {
      "plus" => :plus,
      "add" => :plus, 
      "minus" => :minus, 
      "take" => :minus, 
      "times" => :multiply, 
      "divided" => :divide,
      "divide" => :divide, 
      "multiplied" => :multiply, 
      "squared" => :square, 
      "cubed" => :cube,
      "to" => :power,
    }

    operands = phrase_cleaner(question)
    operators = operator_filter(operands)
    numbers = number_filter(operands)

    total ||= numbers.shift

    operators.each do |operator|
      operator = operators_gloss[operator]
      case operator
        when :plus
          total += numbers.shift
        when :minus
          total -= numbers.shift
        when :multiply
          total *= numbers.shift
        when :divide
          total /= numbers.shift
        when :power
          total = total ** numbers.shift
        when :square
          total = total ** 2
        when :cube
          total = total ** 3
      end
    end
    total
  end
end

wordy_calculator_spec.rb

require 'rspec'
require 'calculator'

describe 'calculator' do
  describe 'ask' do
    it 'calculates addition equations in English' do
      calculator = Calculator.new
      calculator.ask('What is 1 plus 1?').should eq 2
    end

    it 'calculates subtraction equations in English' do
      calculator = Calculator.new
      calculator.ask('What is the value of 4 minus 5?').should eq -1
      calculator.ask('What is 5 take away 4?').should eq 1
    end

    it 'calculates multiplication equations in English' do
      calculator = Calculator.new
      calculator.ask('What is the value of 4 times 6?').should eq 24
    end

    it 'calculates division equations in English' do
      calculator = Calculator.new
      calculator.ask('What is 5 divided by 5?').should eq 1
    end

    it 'calculates expressions with multiple operators (e.g. 5 plus 6 plus 7)' do
      calculator = Calculator.new
      calculator.ask('What is 5 plus 6 plus 7').should eq 18
    end

    it 'calculates with negative numbers' do
      calculator = Calculator.new
      calculator.ask('Tell me 5 minus -4!').should eq 9
    end

    it 'calculates powers' do
      calculator = Calculator.new
      calculator.ask('What is 5 plus 6 to the power of 2?').should eq 121
    end

    it 'calculates squares' do
      calculator = Calculator.new
      calculator.ask('What is 5 squared?').should eq 25
    end

    it 'calculates cubes' do
      calculator = Calculator.new
      calculator.ask('What is 2 cubed?').should eq 8
    end
  end

  describe 'phrase_cleaner' do
    it 'strips off ".", "!", "?", from the expression' do
      calculator = Calculator.new
      calculator.phrase_cleaner('7 times 8?!.').should eq '7 times 8'
    end

    it 'strips off superfulous words' do
      calculator = Calculator.new
      calculator.phrase_cleaner('What is 7 times 8').should eq '7 times 8'
    end
  end

  describe 'number filter' do
    it 'returns an array of all of the numbers in a string' do
      calculator = Calculator.new
      calculator.number_filter('1 to the power of 35 divided by -6').should eq [1, 35, -6]
    end
  end

  describe 'operator filter' do
    it 'returns an array of all of the operators in a string' do
      calculator = Calculator.new
      calculator.operator_filter('1 to the power of 35 divided by -6').should eq ['to', 'divided']
    end
  end
end