title: (Broad Question) How can you be certain a piece of code works correctly?
body: <p>An interviewer asked me how I can be sure that a piece of code works as intended to. I said to test the code through all the possible test cases. Are there any other ways you can be certain a piece of code would work? I was also thinking of writing a mathematical proof for the code but I'm not sure if that would apply to every type of code.</p>