We defined the idea of control flow structures in Chapter 7, Basic Programming, and introduced the idea of an S3 class in Chapter 8 ,S3 Classes. We will now introduce the idea of S4 classes, which is a more formal way to implement classes in R. One of the odd quirks of S4 classes is that you first define the class along with its data, and then you define the methods separately.

As a result of this separation in the way a class is defined, we will first discuss the general idea of how to define a class and its data. We will then discuss how to add a method to an existing class. Next, we will discuss how inheritance is implemented. Finally, we will provide a few notes about other options that do not fit nicely in the categories mentioned earlier.

In the previous chapter, we took an example and then modified it. The approach associated with an S4 class is less flexible and requires a bit more forethought in terms of how a class is defined. We will take a different approach...