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R Programming : S3 Class

Tutorial by:Maria Ghoste      Date: 2016-06-10 00:46:58

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S3 class is the most popular and prevalent class in R programming language. Most of the classes that come predefined in R are of this type. The fact that it is simple and easy to implement is the reason behind this.

Defining S3 Class and Creating S3 Objects

S3 class has no formal, predefined definition. Basically, a list with its class attribute set to some class name, is an S3 object. The components of the list become the member variables of the object. Following is a simple example of how an S3 object of class student can be created.

> # create a list with required components
> s <- list(name="John", age=21, GPA=3.5)

> # name the class appropriately
> class(s) <- "student"

> # That's it! we now have an object of class "student"
> s
$name
[1] "John"

$age
[1] 21

$GPA
[1] 3.5

attr(,"class")
[1] "student"

This might look awkward for programmers coming from C++, Java, Python etc. where there are formal class definitions and objects have properly defined attributes and methods. In R S3 system, it's pretty ad hoc. You can convert an object's class according to your will with objects of the same class looking completely different. It's all up to you.

Using Constructors to Create Objects

It is a good practice to use a function with the same name as class (not a necessity) to create objects. This will bring some uniformity in the creation of objects and make them look similar. We can also add some integrity check on the member attributes. Here is an example. Note that in this example we use the attr() function to set the class attribute of the object.

# a constructor function for the "student" class

student <- function(n,a,g) {
 # we can add our own integrity checks
 if(g>4 || g<0)  stop("GPA must be between 0 and 4")
 value <- list(name=n, age=a, GPA=g)
 
 # class can be set using class() or attr() function
 attr(value, "class") <- "student"
 value
}

Here is a sample run where we create objects using this constructor.

> s <- student("Paul", 26, 3.7)

> s
$name
[1] "Paul"

$age
[1] 26

$GPA
[1] 3.7

attr(,"class")
[1] "student"

> class(s)
[1] "student"

> s <- student("Paul", 26, 5)
Error in student("Paul", 26, 5) : GPA must be between 0 and 4

> # these integrity check only work while creating the object using constructor
> s <- student("Paul", 26, 2.5)

> # it's up to us to maintain it or not
> s$GPA <- 5

Methods and Generic Functions

In the above example, when we simply write the name of the object, its internals get printed. In interactive mode, writing the name alone will print it using the print() function.

> s
$name
[1] "Paul"

$age
[1] 26

$GPA
[1] 3.7

attr(,"class")
[1] "student"

Furthermore, we can use print() with vectors, matrix, data frames, factors etc. and they get printed differently according to the class they belong to. How does print() know how to print these variety of dissimilar looking object? The answer is, print() is a generic function. Actually, it has a collection of a number of methods. You can check all these methods with methods(print).


> methods(print)
 [1] print.acf*                                   
 [2] print.anova*
...
[181] print.xngettext*                             
[182] print.xtabs*                                 

  Non-visible functions are asterisked

We can see methods like print.data.frame and print.factor in the above list. So, when we call print() on a data frame, it is dispatched to print.data.frame(). If we had done the same with a factor, the call would dispatch to print.factor(). Here, we can observe that the method names are in the form generic_name.class_name(). This is how R is able to figure out which method to call depending on the class. Printing our object of class "student" looks for a method of the form print.student(), but there is no method of this form. So, which method did our object of class "student" call? It called print.default(). This is the fallback method which is called if no other match is found. Generic functions have a default method.

There are plenty of generic functions like print(). You can list them all with methods(class="default").

> methods(class="default")
 [1] add1.default*            aggregate.default*      
 [3] AIC.default*             all.equal.default
...

Writing Your Own Method

 

 
 

Now let us implement a method print.student() ourself.

print.student <- function(obj) {
 cat(obj$name, "\n")
 cat(obj$age, "years old\n")
 cat("GPA:", obj$GPA, "\n")
}

Now this method will be called whenever we print() an object of class "student". In S3 system, methods do not belong to object or class, they belong to generic functions. This will work as long as the class of the object is set.

> # our above implemented method is called
> s
Paul 
26 years old
GPA: 3.7 

> # removing the class attribute will restore as previous
> unclass(s)
$name
[1] "Paul"

$age
[1] 26

$GPA
[1] 3.7

Writing Your Own Generic Function

It is possible to make our own generic function like print() or plot(). Let us first look at how these functions are implemented.

> print
function (x, ...) 
UseMethod("print")
<bytecode: 0x0674e230>
<environment: namespace:base>

> plot
function (x, y, ...) 
UseMethod("plot")
<bytecode: 0x04fe6574>
<environment: namespace:graphics>

We can see that they have a single call to UseMethod() with the name of the generic function passed to it. This is the dispatcher function which will handle all the background details. It is this simple to implement a generic function. For the sake of example, we make a new generic function called grade.

grade <- function(obj) {
 UseMethod("grade")
}

A generic function is useless without any method. Let us implement the default method.

grade.default <- function(obj) {
 cat("This is a generic function\n")
}

Now let us make method for our class "student".

grade.student <- function(obj) {
 cat("Your grade is", obj$GPA, "\n")
}

A sample run.

> grade(s)
Your grade is 3.7

In this way, we implemented a generic function called grade and later a method for our class.

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R Programming

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