Working with the S3 object system

The S3 object system in R is a simple, loose, object-oriented system. Every basic object type has an S3 class name. For example, integer, numeric, character, logical, list,  data.frame, and so on are all S3 classes.

For example, the type of vec1 class is double, which means the internal type or storage mode of vec1 is double floating numbers. However, its S3 class is numeric:

vec1 <- c(1, 2, 3) 
typeof(vec1) 
## [1] "double" 
class(vec1) 
## [1] "numeric" 

The type of data1 class is list, which means the internal type or storage mode of data1 is a list, but its S3 class is data.frame:

data1 <- data.frame(x = 1:3, y = rnorm(3)) 
typeof(data1) 
## [1] "list" 
class(data1) 
## [1] "data.frame" 

In the following sections, we'll explain the difference between the internal type of an object and its S3 class.

Understanding generic functions and method dispatch

As we mentioned earlier in this chapter, a class can...