The Art of R Programming

Theplot()function is another generic function. You can useplot()on

just about any R object. R will find an appropriate plotting function based

on the object’s class.

Classes are used to organize objects. Together with generic functions,

they allow flexible code to be developed for handling a variety of different

but related tasks. Chapter 9 covers classes in depth.

1.5 Extended Example: Regression Analysis of Exam Grades.......................

For our next example, we’ll walk through a brief statistical regression analysis.

There isn’t much actual programming in this example, but it illustrates how

some of the data types we just discussed are used, including R’s S3 objects.

Also, it will serve as the basis for several of our programming examples in

subsequent chapters.

I have a file,ExamsQuiz.txt, containing grades from a class I taught. Here

are its first few lines:

2 3.3 4

3.3 2 3.7

4 4.3 4

2.3 0 3.3

The numbers correspond to letter grades on a four-point scale; 3.3 is a

B+, for instance. Each line contains the data for one student, consisting of

the midterm examination grade, final examination grade, and average quiz

grade. It might be interesting to see how well the midterm and quiz grades

predict the student’s grade on the final examination.

Let’s first read in the data file:

> examsquiz <- read.table("ExamsQuiz.txt",header=FALSE)

Our file does not include a header line naming the variables in each

student record, so we specifiedheader=FALSEin the function call. This is an

example of a default argument, which we talked about earlier. Actually, the

default value of theheaderargument isFALSEalready (which you can check by

consulting R’s online help forread.table()), so we didn’t need to specify this

setting, but it’s clearer if we do.

Our data is now inexamsquiz, which is an R object of classdata.frame.

> class(examsquiz)

[1] "data.frame"

Just to check that the file was read in correctly, let’s take a look at the

first few rows:

> head(examsquiz)

V1 V2 V3

16 Chapter 1