Separation of concerns

Separation of concerns means each part of a program handles one kind of job. The most useful split for your projects has three parts. Input and output covers reading files, asking the user and printing.

Core logic covers calculations and rules. Data covers the structures that hold your records. When the logic never prints or reads input itself, you can test it with plain values.

You can also reuse it from a web page later, and change the output format without touching the rules. The milestone asks for exactly this split. The warning sign is a function that asks for input, does the maths and prints the answer all at once. The only way to test it is to type into it.