Hands-On Programming with R

Chapter 8. S3

You may have noticed that your slot machine results do not look the way I promised they would. I suggested that the slot machine would display its results like this:

play()
## 0 0 DD
## $0

But the current machine displays its results in a less pretty format:

play()
## "0"  "0" "DD"
## 0

Moreover, the slot machine uses a hack to display symbols (we call print from within play). As a result, the symbols do not follow your prize output if you save it:

one_play <- play()
## "B" "0" "B"

one_play
## 0

You can fix both of these problems with R’s S3 system.

The S3 System

S3 refers to a class system built into R. The system governs how R handles objects of different classes. Certain R functions will look up an object’s S3 class, and then behave differently in response.

The print function is like this. When you print a numeric vector, print will display a number:

num <- 1000000000
print(num)
## 1000000000

But if you give that number the S3 class POSIXct followed by POSIXt, print will display a time:

class(num) <- c("POSIXct", "POSIXt")
print(num)
## "2001-09-08 19:46:40 CST"

If you use objects with classes—and you do—you will run into R’s S3 system. S3 behavior can seem odd at first, but is easy to predict once you are familiar with it.

R’s S3 system is built around three components: attributes (especially the class attribute), generic functions, and methods.

Attributes

In “Attributes”, you learned that many R objects come with attributes, pieces of extra information that ...