R – Inheritance

Inheritance is one of the concept in object oriented programming by which new classes can derived from existing or base classes helping in re-usability of code. Derived classes can be the same as a base class or can have extended features which creates a hierarchical structure of classes in the programming environment. In this article, we’ll discuss how inheritance is followed out with three different types of classes in R programming.

Inheritance in S3 Class

S3 class in R programming language has no formal and fixed definition. In an S3 object, a list with its class attribute is set to a class name. S3 class objects inherit only methods from its base class.

Example:

# Create a function to create an object of class
student <- function(n, a, r){
  value <- list(name = n, age = a, rno = r)
  attr(value, "class") <- student
  value
}

# Method for generic function print()
print.student <- function(obj){
  cat(obj$name, "\n")
  cat(obj$age, "\n")
  cat(obj$rno, "\n")
}

# Create an object which inherits class student
s <- list(name = "Utkarsh", age = 21, rno = 96,
          country = "India")

# Derive from class student
class(s) <- c("InternationalStudent", "student")

cat("The method print.student() is inherited:\n")
print(s)

# Overwriting the print method
print.InternationalStudent <- function(obj){
cat(obj$name, "is from", obj$country, "\n")
}

cat("After overwriting method print.student():\n")
print(s)

# Check imheritance
cat("Does object 's' is inherited by class 'student' ?\n")
inherits(s, "student")

Output:

The method print.student() is inherited:
Utkarsh 
21 
96 
After overwriting method print.student():
Utkarsh is from India
Does object 's' is inherited by class 'student' ?
[1] TRUE

Inheritance in S4 Class

S4 class in R programming have proper definition and derived classes will be able to inherit both attributes and methods from its base class.

Example:

# Define S4 class
setClass("student",
         slots = list(name = "character",
                      age = "numeric", rno = "numeric")
)

# Defining a function to display object details
setMethod("show", "student",
          function(obj){
            cat(obj@name, "\n")
            cat(obj@age, "\n")
            cat(obj@rno, "\n")
          }
)

# Inherit from student
setClass("InternationalStudent",
slots = list(country = "character"),
contains = "student"
)

# Rest of the attributes will be inherited from student
s <- new("InternationalStudent", name = "Utkarsh",
         age = 21, rno = 96, country="India")
show(s)

Output:

Utkarsh 
21 
96 

Inheritance in Reference Class

Inheritance in reference class is almost similar to the S4 class and uses setRefClass() function to perform inheritance.

Example:

# Define class
student <- setRefClass("student",
   fields = list(name = "character",
                 age = "numeric", rno = "numeric"),
   methods = list(
     inc_age <- function(x) {
       age <<- age + x
     },
     dec_age <- function(x) {
       age <<- age - x
     }
   )
)

# Inheriting from Reference class
InternStudent <- setRefClass("InternStudent",
   fields = list(country = "character"),
   contains = "student",
   methods = list(
   dec_age <- function(x) {
     if((age - x) < 0)  stop("Age cannot be negative")
     age <<- age - x
   }
   )
)

# Create object
s <- InternStudent(name = "Utkarsh",
                   age = 21, rno = 96, country = "India")

cat("Decrease age by 5\n")
s$dec_age(5)
s$age

cat("Decrease age by 20\n")
s$dec_age(20)
s$age

Output:

[1] 16 
Error in s$dec_age(20) : Age cannot be negative
[1] 16