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Something of a tried, tested, and testing classic, sorting routines offer an illustrative example. Implementing a sorting algorithm is not necessarily an everyday task for a programmer, but sorting is such a familiar idea that most people believe they know what to expect from it. This casual familiarity, however, can make it harder to see past certain assumptions.
When programmers are asked “What would you test for?” by far and away the most common response is “The result of sorting is a sorted sequence of elements.” While this is true, it is not the whole truth. When prompted for a more precise condition, many programmers add that the resulting sequence should be the same length as the original. Although correct, this is still not enough. For example, given the following sequence:
The following sequence satisfies a postcondition of being sorted in non-descending order and having the same length as the original sequence:
The full postcondition is that the result is sorted and that it holds a permutation of the original values. This appropriately constrains the required behavior. That the result length is the same as the input length comes out in the wash and doesn’t need restating.
Even stating the postcondition in the way described is not enough to give you a good test. A good test should be readable. It should be comprehensible and simple enough that you can see readily that it is correct (or not). Unless you already have code lying around for checking that a sequence is sorted and that one sequence contains a permutation of values in another, it is quite likely that the test code will be more complex than the code under test. As Tony Hoare observed:
The result of sorting is the following:
No other answer will do. Accept no substitutes.
Concrete examples helps to illustrate general behavior in an accessible and unambiguous way. The result of adding an item to an empty collection is not simply that it is not empty: It is that the collection now has a single item. And that the single item held is the item added. Two or more items would qualify as not empty. And would also be wrong. A single item of a different value would also be wrong. The result of adding a row to a table is not simply that the table is one row bigger. It also entails that the row’s key can be used to recover the row added. And so on.
