R6 classes in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of method call in R6 class

What is the output of the following R code?

R Programming

library(R6)
Person <- R6Class("Person",
  public = list(
    name = NULL,
    initialize = function(name) {
      self$name <- name
    },
    greet = function() {
      paste("Hello, my name is", self$name)
    }
  )
)
p <- Person$new("Alice")
p$greet()

A Error: object 'name' not found

B "Hello, my name is NULL"

C "Hello, my name is Alice"

D NULL

💡 Hint

Check how the initialize method sets the name and how greet accesses it.

The initialize method sets self$name to 'Alice'. The greet method returns a string combining 'Hello, my name is' with self$name, which is 'Alice'.

🧠 Conceptual

intermediate

Understanding private fields in R6

Which statement about private fields in R6 classes is TRUE?

A Private fields are global variables shared by all instances of the class.

B Private fields can be accessed directly from outside the object using $ operator.

C Private fields are automatically copied to public fields when the object is created.

D Private fields are defined inside the private list and cannot be accessed directly from outside the object.

💡 Hint

Think about encapsulation and how private fields protect data.

Private fields are defined inside the private list and are only accessible within the class methods, not from outside using $.

🔧 Debug

advanced

Identify the error in R6 method definition

What error will this code produce when run?

R Programming

library(R6)
Counter <- R6Class("Counter",
  public = list(
    count = 0,
    increment = function() {
      count <- count + 1
    }
  )
)
c <- Counter$new()
c$increment()
c$count

A 0

B 1

C Error: object 'count' not found

D Error: attempt to modify a locked binding

💡 Hint

Check how the increment method modifies the count variable.

Inside increment, count <- count + 1 creates a local variable count instead of modifying self$count. So self$count remains 0.

📝 Syntax

advanced

Correct syntax for defining an active binding in R6

Which option correctly defines an active binding named 'double' that returns twice the value of private$x?

A

active = list(
  double = function(value) {
    if (missing(value)) private$x * 2 else private$x <- value / 2
  }
)

B

active = list(
  double = function() {
    private$x * 2
  }
)

C

active = list(
  double = function(value) {
    private$x * 2
  }
)

D

active = list(
  double = function(value) {
    if (!missing(value)) private$x * 2 else private$x <- value / 2
  }
)

💡 Hint

Active bindings use a function with an optional argument to get or set values.

Active bindings in R6 use a function with an optional argument 'value'. If value is missing, it returns the computed value; otherwise, it sets the underlying variable.

🚀 Application

expert

Predict the number of items in a list after method calls

Consider this R6 class that stores unique names in a list. After running the code below, how many items are in the list stored in the object?

R Programming

library(R6)
UniqueNames <- R6Class("UniqueNames",
  private = list(
    names = list()
  ),
  public = list(
    add_name = function(name) {
      if (!(name %in% private$names)) {
        private$names <- c(private$names, name)
      }
    },
    get_names = function() {
      private$names
    }
  )
)
obj <- UniqueNames$new()
obj$add_name("Anna")
obj$add_name("Bob")
obj$add_name("Anna")
obj$add_name("Cara")
length(obj$get_names())

A Error: object of type 'closure' is not subsettable

B 3

C 4

D 2

💡 Hint

Check how the add_name method checks for duplicates before adding.

The add_name method adds a name only if it is not already in private$names. 'Anna' is added once, so total unique names are 3.