What is modular code?

To understand the importance of modular code, consider a car. A car is not created of a single piece of material. Instead, a car is the amalgamation of individual components that, when combined, form a fully working vehicle. For example, a car has an engine, a transmission, brakes, and so on. By creating a car in components, the designers can swap out broken parts, upgrade individual components, use certain parts in other cars, and so on without redesigning the whole car.

A program should follow the same logic. Code needs to be placed into logical containers that organize code. The most basic of these containers is called a function. A function is analogous to a patch and a program is analogous to a quilt. A well-written program is a program that is composed of functions that are stitched together to form a fully working program.

With that in mind, what is the underlying purpose of a function, or module? The answer to that question is simple but will usually...