rpn_calculator

rpn_calculator.rb

class RPNCalculator
  attr_accessor :calculator
  
  def initialize
    @calculator = []
  end
  
  def push(x)
    @calculator << x
  end
  
    def value
    @calculator.last
  end

  def plus
    if @calculator.size >= 2
    sum = @calculator.pop + @calculator.pop
    @calculator << sum
    else
      raise "calculator is empty"
    end
  end
  
  def minus
    if @calculator.size >= 2
      second = @calculator.pop
      first = @calculator.pop
      minus = first - second
      @calculator << minus
    else
      raise "calculator is empty"
    end
  end
  
   def divide
    if @calculator.size >= 2
      second = @calculator.pop.to_f
      first = @calculator.pop.to_f
      division = first / second
      @calculator << division
    else
      raise "calculator is empty"
    end
  end
  
    def times
    if @calculator.size >= 2
      second = @calculator.pop.to_f
      first = @calculator.pop.to_f
      multiply = first * second
      @calculator << multiply
    else
      raise "calculator is empty"
    end
  end
  
  def tokens(array)
    new = array.split(" ")
    new.collect do |x|
      if ( x == "+" || x == "-" || x == "*" || x == "/")
        x.to_sym
      else
        x.to_i
      end
    end
end

  
  end

rpn_calculator_spec.rb

# # Topics
# * arrays
# * arithmetic
# * strings
#
# # RPN Calculator
#
# "RPN" stands for "Reverse Polish Notation". (See [the wikipedia entry](http://en.wikipedia.org/wiki/Reverse_Polish_notation) for more information on this colorful term.) Briefly, in an RPN world, instead of using normal "infix" notation, e.g.
#
# 2 + 2
#
# you use "postfix" notation, e.g.
#
# 2 2 +
#
# While this may seem bizarre, there are some advantages to doing things this way. For one, you never need to use parentheses, since there is never any ambiguity as to what order to perform operations in. The rule is, you always go from the back, or the left side.
#
# 1 + 2 * 3 =>
# (1 + 2) * 3 or
# 1 + (2 * 3)
#
# 1 2 + 3 * => (1 + 2) * 3
# 1 2 3 * + => 1 + (2 * 3)
#
# Another advantage is that you can represent any mathematical formula using a simple and elegant data structure, called a [stack](http://en.wikipedia.org/wiki/Stack_(data_structure)).
#
# # Hints
#
# Ruby doesn't have a built-in stack, but the standard Array has all the methods you need to emulate one (namely, `push` and `pop`, and optionally `size`).
#

# See
# * <http://en.wikipedia.org/wiki/Reverse_Polish_notation>
# * <http://www.calculator.org/rpn.aspx>
#
require "rpn_calculator"

describe RPNCalculator do

  attr_accessor :calculator

  before do
    @calculator = RPNCalculator.new
  end

  it "adds two numbers" do
    calculator.push(2)
    calculator.push(3)
    calculator.plus
    calculator.value.should == 5
  end

  it "adds three numbers" do
    calculator.push(2)
    calculator.push(3)
    calculator.push(4)
    calculator.plus
    calculator.value.should == 7
    calculator.plus
    calculator.value.should == 9
  end

  it "subtracts the second number from the first number" do
    calculator.push(2)
    calculator.push(3)
    calculator.minus
    calculator.value.should == -1
  end

  it "adds and subtracts" do
    calculator.push(2)
    calculator.push(3)
    calculator.push(4)
    calculator.minus
    calculator.value.should == -1
    calculator.plus
    calculator.value.should == 1
  end

  it "multiplies and divides" do
    calculator.push(2)
    calculator.push(3)
    calculator.push(4)
    calculator.divide
    calculator.value.should == (3.0 / 4.0)
    calculator.times
    calculator.value.should == 2.0 * (3.0 / 4.0)
  end

  it "resolves operator precedence unambiguously" do
    # 1 2 + 3 * => (1 + 2) * 3
    calculator.push(1)
    calculator.push(2)
    calculator.plus
    calculator.push(3)
    calculator.times
    calculator.value.should == (1+2)*3

    # 1 2 3 * + => 1 + (2 * 3)
    calculator.push(1)
    calculator.push(2)
    calculator.push(3)
    calculator.times
    calculator.plus
    calculator.value.should == 1+(2*3)
  end

  it "fails informatively when there's not enough values stacked away" do
    expect {
      calculator.plus
    }.to raise_error("calculator is empty")

    expect {
      calculator.minus
    }.to raise_error("calculator is empty")

    expect {
      calculator.times
    }.to raise_error("calculator is empty")

    expect {
      calculator.divide
    }.to raise_error("calculator is empty")
  end

  # extra credit
  it "tokenizes a string" do
    calculator.tokens("1 2 3 * + 4 5 - /").should ==
      [1, 2, 3, :*, :+, 4, 5, :-, :/]
  end

  # extra credit
  xit "evaluates a string" do
    calculator.evaluate("1 2 3 * +").should ==
      ((2 * 3) + 1)

    calculator.evaluate("4 5 -").should ==
      (4 - 5)

    calculator.evaluate("2 3 /").should ==
      (2.0 / 3.0)

    calculator.evaluate("1 2 3 * + 4 5 - /").should ==
      (1.0 + (2 * 3)) / (4 - 5)
  end

end