Should I unit-test with data that should not be passed in a function (invalid input)?

Question

I am trying to use TDD for my coding practice. I would like to ask should I test with a data that should not happen in a function BUT this data may possibly break your program.

Here is one of a easy example to illustrate to what I ask :

a ROBOT function that has a one INT parameter. In this function I know that the valid range would only be 0-100. If -1, 101 is used, the function will be break.

function ROBOT (int num){
...
...
...
return result;
}

So I decided some automated test cases for this function...

1. function ROBOT with input argument 0
2. function ROBOT with input argument 1
3. function ROBOT with input argument 10
4. function ROBOT with input argument 100

But should I write test cases with input argument -1 or 101 for this ROBOT function IF I would guard that in my other function that call function ROBOT???

5. function ROBOT with input argument -1
6. function ROBOT with input argument 101

I don't know if it is necessary cause I think it is redundancy to test -1 and 101. And If it is really necessary to cover all the cases, I have to write more code to guard -1 and 101.

So in Common practice of TDD, will you write test case on -1 and 101 as well???

Answer 1

If the expected outcome is that an exception is thrown with invalid input values, then a test that the exceptions get properly thrown would be appropriate.

Edit:

As I noted in my comment below, if these cases will break your application, you should throw an exception. If it really is logically impossible for these cases to occur, then I would say no, you don't need to throw an exception, and you don't need test cases to cover it.

Note that if your system is well componentized, and this function is one component, the fact that it is logically impossible now doesn't mean it will always be logically impossible. It may be used differently down the road.