R S4 Class - r - learn r - r programming



  • In this article, you'll learn everything about S4 classes in R; how to define them, create them, access their slots, and use them efficiently in your program.
 r s4 class

  • Unlike S3 classes and objects which lacks formal definition, we look at S4 class which is stricter in the sense that it has a formal definition and a uniform way to create objects.
  • This adds safety to our code and prevents us from accidentally making naive mistakes.

How to define S4 Class?

  • S4 class is defined using the setClass() function.
  • In R terminology, member variables are called slots.
  • r programming class object

    Sample class object image in C

  • While defining a class, we need to set the name and the slots (along with class of the slot) it is going to have.

Example 1: Definition of S4 class

setClass("student", slots=list(name="character", age="numeric", GPA="numeric"))
  • In the above example, we defined a new class called student along with three slots it's going to have name, age and GPA.
  • There are other optional arguments of setClass() which you can explore in the help section with ?setClass.

How to create S4 objects?

  • S4 objects are created using the new() function.

Example 2: Creation of S4 object

># create an object using new()
># provide the class name and value for slots
> s <- new("student",name="John", age=21, GPA=3.5)

> s
Anobject of class"student"
Slot"name":
[1] "John"

Slot"age":
[1] 21

Slot"GPA":
[1] 3.5
  • We can check if an object is an S4 object through the function isS4().
> isS4(s)
[1] TRUE
  • The function setClass() returns a generator function.
  • This generator function (usually having same name as the class) can be used to create new objects.
  • It acts as a constructor.
> student <- setClass("student", slots=list(name="character", age="numeric", GPA="numeric"))

> student
class generator functionforclass “student” frompackage‘.GlobalEnv’
function (...) 
new("student", ...)
  • Now we can use this constructor function to create new objects.
  • Note above that our constructor in turn uses the new() function to create objects. It is just a wrap around.

Example 3: Creation of S4 objects using generator function

>student(name="John", age=21, GPA=3.5)
Anobject of class"student"
Slot"name":
[1] "John"

Slot"age":
[1] 21

Slot"GPA":
[1] 3.5

How to access and modify slot?

  • Just as components of a list are accessed using $, slot of an object are accessed using @.

Accessing slot

>[email protected]
[1] "John"

>[email protected]
[1] 3.5

>[email protected]
[1] 21

Modifying slot directly

  • A slot can be modified through reassignment.
># modify GPA
>[email protected]<- 3.7

> s
Anobject of class"student"
Slot"name":
[1] "John"

Slot"age":
[1] 21

Slot"GPA":
[1] 3.7

Modifying slots using slot() function

  • Similarly, slots can be access or modified using the slot() function.
> slot(s,"name")
[1] "John"

> slot(s,"name") <- "Paul"

> s
Anobject of class"student"
Slot"name":
[1] "Paul"

Slot"age":
[1] 21

Slot"GPA":
[1] 3.7

Methods and Generic Functions

  • As in the case of S3 class, methods for S4 class also belong to generic functions rather than the class itself.
  • Working with S4 generics is pretty much similar to S3 generics.
  • You can list all the S4 generic functions and methods available, using the function showMethods().

Example 4: List all generic functions

>showMethods()
Function: - (packagebase)

Function: != (packagebase)
...
Function: trigamma (packagebase)

Function: trunc (packagebase)
  • Writing the name of the object in interactive mode prints it.
  • This is done using the S4 generic function show().
  • You can see this function in the above list.
  • This function is the S4 analogy of the S3 print()function.

Example 5: Check if a function is a generic function

> isS4(print)
[1] FALSE

> isS4(show)
[1] TRUE
  • We can list all the methods of show generic function using showMethods(show).

Example 6: List all methods of a generic function

>showMethods(show)
Function: show (package methods)
object="ANY"
object="classGeneratorFunction"
...
object="standardGeneric"
(inherited from: object="genericFunction")
object="traceable"

How to write your own method?

  • We can write our own method using setMethod() helper function.
  • For example, we can implement our class method for the show() generic as follows.
setMethod("show",
"student",
function(object) {
cat([email protected], "\n")
cat([email protected], "years old\n")
cat("GPA:", [email protected], "\n")
         }
)
  • Now, if we write out the name of the object in interactive mode as before, the above code is executed.
> s <- new("student",name="John", age=21, GPA=3.5)

> s    # this is same as show(s)
John
21years old
GPA: 3.5
  • In this way we can write our own S4 class methods for generic functions.

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