Test Driven Development

Written by Gregory McIntyre, @gregmcintyre

This exercise is intended to teach you what we’re talking about when we say Test Driven Development (TDD).

Background information

Roman Numerals

If you are not already familiar with Roman numerals, please read up on how Roman numerals work before continuing.

In summary, here are some examples of how Roman people wrote numbers:

Hindu-Arabic Roman
1 I
4 IIII (or IV)
5 V
6 VI
9 VIIII (or IX)
10 X
50 L
100 C
500 D
1000 M

We are going to write a program that takes an integer value in the left column and calculates the equivalent string value in the right column. If we finish that, we will then make it work with the subtractive digits like IV.

Guide for working in a group

We encourage doing this exercise in a group of 2-4 people. The rules that govern how this works are very similar to how programmers do pair programming and this exercise is also intended to give you some exposure to that practice also.

Help from the coach

Explain how pair programming can be useful.

1. Initial code

Copy this code into a file called roman.rb:

def roman(n)
  return "?"

require "minitest/spec"
require "minitest/autorun"

describe "roman" do
  it "converts the number 1 to the string I" do
    roman(1).must_equal "I"

Run your tests

Run the command following into the terminal:

ruby roman.rb


You should see the following output from the tests:

roman#test_0001_converts the number 1 to the string I [tdd1.rb:11]:
Expected: "I"
  Actual: "?"

1 tests, 1 assertions, 1 failures, 0 errors, 0 skips

Take a moment to read this output carefully. It is quite a mouthful.

Your tests are now red. i.e. One or more of the tests are failing. You can tell you have a failing test by checking the summary at the end: 1 tests, 1 assertions, 1 failures, 0 errors, 0 skips.

Stand up and give the hot seat to the next person.

Help from the coach

Explain how TDD can be useful.

2. Make the tests pass

It is time to make the test pass. Do this however you see fit. It’s fine if the change is just an extra if statement or one extra character. In fact, that is encouraged: you generally shouldn’t write unnecessary code. If you’re stuck, you can ask the people around you for their opinions.

Here is a way that you could make the first test pass, just to get you into the swing of things:

def roman(n)
  return "I"

If this seems facetious, you’re right but it is a valid solution because it makes all the tests pass. When your tests all pass, we call them green.

3. Refactor your code

Look over the code and decide if it’s a good idea to refactor it (clean up the code and make it easier to read). If you decide to not to refactor, skip this step.

Hint: It’s a good time to refactor when you notice repetition. If you like, you can also refactor the tests.

Run your tests after refactoring. If they fail, you accidentally broke something.

Help from the coach

Explain how focusing on something small enough to test can be useful.

4. Write a new failing test

If you all agree that the code should work in general, and you can’t think of any more cases to test and everything passes, you can stop here. You win!

Otherwise, your last job in the hot seat is to write a new test. We currently have a test that checks that the number one is turned into an "I", but we need more tests to verify that all other numbers convert as expected. When you add a new test for another number, be sure to run the tests to find your test fail. If you’re stuck, there are some suggestions at the bottom of this page.

You can copy and paste the previous test and alter it. You can change it to be anything you like. Your tests should probably test the next trickiest situation, but if you feel like going back and adding a simpler case, that’s fine too as long as it fails.

The other members of the group can chime in and ask questions or spot problems for you.

Here is an example of an expanded test suite:

describe "roman" do
  it "converts the number 1 to the string I" do
    roman(1).must_equal "I"

  it "converts the number 2 to the string II" do
    roman(2).must_equal "II"

Your tests are now red again; at least one is failing.

Stand up and offer the hot seat to the next person in your group.


Keep repeating steps 2 through 4, making sure to continue switching at the end of step 4. You are done when your team feels like they are done.

Don’t worry about finishing all cases. The goal is to practice the steps and learn to work together in this way. Get used to writing tests as well as getting them to pass. Practice. Good luck!


If you are stuck for ideas, here is a list of Roman numerals to write tests for, in this order. Note the way that the build up incrementally in complexity.

Input Output
1 "I"
5 "V"
4 "IIII"
6 "VI"
7 "VII"
10 "X"

If you get this far, you earn partial credit. Romans used to use IIII for 4. That’s why 4 on an analog watch is written as IIII. Later on, they added subtractive digits. These are harder to program. Once you feel confident that your program works with all the numbers above, try dealing with subtractive digits.

Input Output
4 "IV"
14 "XIV"

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