Consider a program with 1000 lines. It is much easier to write a properly designed program consisting of 20 50-line functions than it is to write a program consisting of one 1000-line main program. Functions can be written, tested, and understood independently of one another. You can write a function, test it, and convince yourself that it is correct without worrying about the other functions in your program. It is much easier to get your mind around a 50-line thing than a 1000-line thing. Another benefit is that code can be reused between projects. If someone has previously written a...