How to decide test cases for unit tests?

Question

I'm just getting into unit testing, and have written some short tests to check if function called isPrime() works correctly.

I've got a test that checks that the function works, and have some test data in the form of some numbers and the expected return value.

How many should I test? How do I decide on which to test? What's the best-practices here?

One approach would be to generate 1000 primes, then loop through them all, another would be to just select 4 or 5 and test them. What's the correct thing to do?

Answer 1

I've also been informed that every time a bug is found, you should write a test to verify that it is fixed. It seems reasonable to me, anyway.

Answer 2

You'd want to check edge cases. How big a prime number is your method supposed to be able to handle? This will depend on what representation (type) you used. If you're only interested in small (really relative term when used in number theory) primes, you're probably using int or long. Test a handful of the biggest primes you can in the representation you've chosen. Make sure you check some non-prime numbers too. (These are much easier to verify independently.)

Naturally, you'll also want to test a few small numbers (primes and non-primes) and a few in the middle of the range. A handful of each should be plenty. Also make sure you throw an exception (or return an error code, whichever is your preference) for numbers that are out of range of your valid inputs.