The 2nd in series of blog posts in which I describe TDD katas & exercises that I’ve used for training purposes.
Multimap
A data structure that supports storing multiple values at a key. Think “English language dictionary,” which can contain multiple definitions for a single word.
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key => [values]
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You add to the multimap by specifying a key and a single value to be stored at that key: dictionary.put(‘a word’, ‘a definition’) // or dictionary[‘a word’] = ‘a definition’
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You retrieve from the multimap by specifying a key: var keys = dictionary[‘a word’] expect(keys).toEqual([‘1st definition’, ‘2nd definition’]
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Answers its size (the number of keys)
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Answers the total count of values stored across all keys (e.g. “count of all definitions”)
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Answers whether or not it is empty
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Throws / returns an error when attempting to store a null key
Duration: 45 minutes
Core themes:
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Test ordering
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Sticking with the R-G-R rhythm
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Incrementally moving from specific to more generalized implementations
I used the Multimap as an introductory TDD exercise for a while many years ago. In turn working backward, this exercise replaced my use of test-driving a stack as an exercise; I moved off of that due to the negative feedback it garnered because of its over-simplicity (which appeared to help reinforce resistances to TDD).
Test-driving a multimap works well as a first exercise (or second exercise if the first was done using “TDD paint by numbers,” in which case you’d want to discuss a starter test list with the group). It does provide a couple opportunities for the students to create mildly interesting defects. It is “real” enough in that I’ve found a few occasions to use it in production code.
Note that some languages and / or frameworks (e.g. Guava for Java and C++11 / later) already provide a Multimap implementation. In this case, test-driving a multimap can still work as an exercise, though it could also provoke thoughts of “why are we wasting our time building something already readily available?”
Some possibilities for extra credit / extending the exercise.
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Support adding multiple values at a key in addition to singular values
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Provide the ability to find a key based on a predicate against the values
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Support storing the values in a sorted order
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Support storing only unique values for a single key
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Support removing keys
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Support removing a specific value
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Support replacing a specific value
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Support interesting iteration
** For pairing TDD novices. Impacts to duration can include:
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Whether it’s their first TDD exercise or a subsequent one
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General level of programming proficiency
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Programming language used
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Exercise learning mode: pairing, mobbing, or solo?
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