First, what is Test Driven Development (TDD)? Well, I’m not an expert, so don’t quote me, but in practice it means that you develop your code to fulfill tests that you define prior to beginning your work. You do not define all your tests first, though. You define a single test, and produce code to pass it. Then you define another code, and produce code to pass both. And so forth until you complete your specification.

This is good because:

The steps of TDD can be described as from Wikipedia

1. Add tests: These should be useful, and should fail.
2. Run tests: Verify that the test fails. If not, go back to 1.
3. Write code: Write enough code to pass the test.
4. Run tests: Verify that all tests pass. If some of the tests fail, go back to 3.
5. Refactor: Now that everything passes, make the code looks nicer. This is harder for non-seasoned programmers, becauses it’s vague. Essentially, it means removing duplicate code, magic numbers, clarifying names, etc.
6. Run tests: Again. Should be done during refactoring, to guarantee you’re not breaking anything. But just to be very clear: your tests should pass at the end of refactoring.
7. Repeat.

This is one way of describing TDD, but there are other. Many others, by the way. In fact, there are many images describing it, so you can print one and staple it around.

## Julia

First, we are gonna follow the package layout in Julia. This post mentions it at the end. Basically, we need

• Folder PackageName.jl
• Folder src
• PackageName.jl
• Folder test
• runtests.jl

In our example, we’re gonna write a program to convert Roman numbers to decimal, and vice-versa. This was inspired by this site.

Important: You should use git, but I’ll skip it here

Let’s begin writing the outline of the project

This defines the building blocks. Note that test_digits.jl does not exist. We’re gonna create it to test the individuals digits.

Our testing environment will consist of having a terminal open at all times at the root of this project. Our testing command will be

There are different ways to issue the same command, but this is locally good.

## Tests

Julia comes with a Base.Test package, which is the least you should use. For all basic things it is enough. It provides the @test macro, which you can use as

We’re gonna implement the function roman_to_dec which receives a string with roman numerals and returns the decimal equivalent of the number. With FactCheck, our first test will be

When we run our test, we’ll get

Look, roman_to_dec not defined. Well, let’s define it.

Running again, we get an even better message

Expected 1, nothing ocurred. Well, that’s easy.

Done. We’re successful. Rejoice. Back to work.

We’ve written a test, we’ve tested it, we’ve written code to fix it, we tested it. Not much to refactor, this is a silly example.

Repeat. Let’s improve the tests.

Running, we obtain

Now, that’s better. Improving the code.

This too will pass. Notice that this example is very silly. It is instructional, of course. On a real application, you could start with all digits at once, for instance.

More tests and solutions:

Now we can refactor, because it’s getting very ugly.

We can also refactor the test.

Test. Now we can add more tests for digits, and it will be much easier (because it’s refactored) to both create the test and to solve it.

Understanding the logic now, you can add all the rest of the digits at once. Remember to test before start fixing, even though is very easy now. This could be a breaking moment on your code. If, when trying to fix it, you realize it’s not as simple as you expected. Remove the test, and add a smaller one. At this time it will be very useful to have been using git.

## Next test

We’ve completed a test. Let’s do the next.

Double digits are more complex that single digits (by at least at factor of 2? :) ). Let’s break it down using context.

Testing this will fail (as it should), with a KeyError: II not found, because we’re using the dictionary, and “II” is not in it.

Before reading the solution, try to fix it yourself. There are many ways to do it.

This fixes it. Now to refactor. You may have noticed that s[i] does not work inside digits. That is because julia differentiates characters and single digits strings (like C, unlike Python). One refactor option is to change the dictionary to use chars. Another option is to use a better variable instead of s, since it start to become a nuisance to read. Yet another, is to use another way to make the sum.

Since this post explains the usage of TDD, it ends here. You can continue with this problem until you can make a complete conversor of roman to decimal.