zlacker

Generally, the purposes of the exercises in a class is to reinforce the material that has been taught up to that point and to demonstrate that the student can use it.

For example, if early in an elementary number theory class the student is asked to prove that there are in infinite number of primes of the form 4n+3, a solution that just invokes Dirichlet's theorem on primes in arithmetic progressions would probably not be acceptable. That approach does work to show that there are an infinite number of 4n+3 primes, but completely fails to show that that the student understood the material actually taught in class.

It's the exact same thing with the digit counting problem. Solving it by just invoking the built in string length function does little to demonstrate that the student understands the material taught so far.