Fundamental rule for testing functions

Question

"Every function should be tested in a program in which every other function in that program has already been fully tested and debugged"
what does that mean? please help..

Answer 1

well it means you should use a proper, working program, as test case.

to debug any function you created, implement it in your test case, so that you won't have to deal with unnecessary problems and thus making it easier for you to debug the function.