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).
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:
|4||IIII (or IV)|
|9||VIIII (or IX)|
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.
- Each group has one hot seat with laptop and Sublime Text ready.
- Everybody else should close their laptops and sit around the hot seat chair.
- You will all regularly stand up and rotates chairs so the next person is in the hot seat. The steps below explain when to do that.
- Pick somebody to start in the hot seat. That person should follow all the steps until swapping seats is mentioned.
Explain how pair programming can be useful.
1. Initial code
Copy this code into a file called
Run your tests
Run the command following into the terminal:
You should see the following output from the tests:
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.
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:
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.
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:
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.
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
Want to learn more? View more guides!