Finding a good first exercise for TDD learners can be challenging. Some of the things I look for:
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Not so simple that it’s trivial, e.g. Stack, lest students dismiss it as pointless. There should be opportunities for students to make a few simple mistakes, but…
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…not so complex that students mire in the algorithm itself instead of focusing on technique.
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Something students can relate to–not so obscure that students struggle with understanding the domain or problem.
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Games and fluff (bowling, fizz-buzz, e.g.) can create a first impression–and first impressions can be very hard to break–that TDD is for trivialities only, despite what you say. These things can be fine as later exercises.
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Not something that already exists in the library. (“Why am building a multi-map when C++ already has one?”)
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Can be test-driven in 10 to 15 minutes by an experienced TDDer, meaning that it’s probably a 30-to-45-minute exercise for newbies pairing up.
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Provides a good demonstration of the ability to incrementally grow a solution, including opportunities to spot and remove duplication.
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Provides an opportunity to show how to deal with at least one exceptional case.
I’ve been demoing a stock portfolio tracker for some time–a simple collection class that allows purchases of stock symbols. With Java students, I follow up with a multi-map, a class that would be useful in most shops (though a similar jakarta implementation exists). Both have worked well.
The message an exercise sends can be damaging. As a second or third exercise, lately I’ve been using the Roman numeral converter kata. I think it’s a cool exercise that can show how you can build an elegant algorithm by following a simple, incremental approach. It’s had 99% effectiveness: Out of the past ~100 students, one guy–an algorithm specialist–took a completely negative view of TDD after it. His stance was that he could have derived the algorithm much more quickly using a top-down, non-test-driven approach. From that he dismissed TDD completely. During subsequent pairings, I think he saw some of its benefits (we talk a lot about algorithms and TDD), but it’s an uphill battle and it might have been too late.
Currently I’m experimenting with an exercise to implement the soundex algorithm. More on that in an upcoming blog post.
Dave Thomas’s PragProg CodeKata site is a great place to get ideas. You might also check out the TDD Problems site (to which I contributed a handful of problems).
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