Unit Testing for mapping int to string

How about using enums(mapper int to string) and you don't need to unit test anything.

enum Days {Sat=1, Sun, Mon, Tue, Wed, Thu, Fri};

Now let's assume your mapper is used by some method that returns different results based on the mapper. Here is how I would test it.

1. Programatically get all the values in the mapper.
2. Iterate through each and call your method with that value.
3. Assert after each response.

The example above would do well if you are loading something from filesystem that corresponds to your enum values or something similar. Your situation could vary.

If the function does several different things (and the description "...a function which (among other things) maps an integer to a string" indicates that yours does), you should probably divide it into several functions and test each of them separately. Then, as Daniel T. suggested, make sure to test every branch through the function(s).

In addition to the answer above, I'd also recommend testing out edge cases - so e.g. -1, 0, and then a large number. You could also test that passing something other than an integer makes the method behave the way it is supposed to behave.

I suggest you have one unit test for each linearly independent path through the function. In practice, this means one test, plus an additional test for each branch.

To learn more, I suggest you read about Cyclomatic complexity (https://en.wikipedia.org/wiki/Cyclomatic_complexity)