Do I need to explicitly handle negative numbers or zero when summing squared digits?

Question

I recently had a test in my class. One of the problems was the following:

Given a number n, write a function in C/C++ that returns the su

Answer 1

As others have pointed out, the special treatment for n==0 is nonsense, since for every serious C programmer it is obvious that "while(n)" does the job.

The behaviour for n<0 is not that obvious, that's why I would prefer to see those 2 lines of code:

if (n < 0) 
    n = -n;

or at least a comment:

// don't worry, works for n < 0 as well

Honestly, at what time did you start considering that n might be negative? When writing the code or when reading your teacher's remarks?