Oo Cash Register


  1. Build a class with instance methods.
  2. Call instance methods inside of other instance methods.
  3. Use instance methods to track information pertinent to an instance of a class.


We're going to create an Object Oriented Cash Register that can:

  • Add items of varying quantities and prices
  • Calculate discounts
  • Keep track of what's been added to it
  • Void the last transaction


This is a test-driven lab! You will need to read the spec file and the test output very carefully to solve this one.

Note that a discount is calculated as a percentage off of the total cash register price (e.g. a discount of 20 means the customer receives 20% off of their total price).

Hint #1: Keep in mind that to call an instance method inside another instance method, we use the self keyword to refer to the instance on which we are operating. For example:

class Person

  attr_accessor :age

  def initialize(age = 0)
    @age = age 

  def birthday
    self.age += 1

Follow along with the specs in spec/cash_register_spec.rb. Reading along with what the tests are looking for can be really helpful!

Take it one step at a time!

Hint #2: The apply_discount requires some knowledge about working with an Integer versus a Float in Ruby. Note that 100.class returns Integer while 100.0.class returns Float. Ruby provides methods for changing an object of type Integer to a Float and vice versa.

Hint #3: The void_last_transaction method will remove the last transaction from the total. You'll need to make an additional attribute accessor and keep track of that last transaction amount somehow. In what method of the class are you working with an individual item?

View Object Oriented Cash Register on Learn.co and start learning to code for free.

Unlock your future in tech
Learn to code.

Learn about Flatiron School's Mission

With a new take on education that falls somewhere between self-taught prodigy and four-year computer science degree, the Flatiron School promises to turn students with little programming experience into developers.

In the six months since the Manhattan coding school was acquired by WeWork, it has spawned locations in Washington, D.C., Brooklyn, and London. Now, WeWork is opening a fourth Flatiron School location, this time in Houston.

Adam Enbar, Flatiron School's cofounder, believes now is the time to grow. "How the world is changing has impacted working and learning in very similar ways. We think education fundamentally is about one thing: enabling people to pursue a better life."

Learn. Love. Code.
Students come to Flatiron School to change their lives. Join our driven community of career-changers and master the skills you need to become a software engineer or a data scientist.
Find Us