R6 classes in R Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new R6 class named 'Person'.

R Programming

Person <- R6Class([1] = "Person")

Drag options to blanks, or click blank then click option'

Aclassname

Bid

Cname

Dclass

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'class' or 'name' instead of 'classname' causes errors.

The argument to R6Class() to name the class is classname.

2fill in blank

medium

Complete the code to add a public method 'greet' that prints 'Hello!'.

R Programming

Person <- R6Class(classname = "Person",
  public = list(
    greet = function() {
      print([1])
    }
  )
)

Drag options to blanks, or click blank then click option'

A"Hello!"

B'Hi!'

CHello!

D'Hello'

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes or using single quotes without exclamation mark.

The print function requires a string in quotes. Double quotes around 'Hello!' are correct.

3fill in blank

hard

Fix the error in the code to correctly initialize a field 'name' with a default value 'John'.

R Programming

Person <- R6Class(classname = "Person",
  public = list(
    name = [1],
    initialize = function(name = "John") {
      self$name <- name
    }
  )
)

Drag options to blanks, or click blank then click option'

ANA

B"John"

CNULL

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Setting the field directly to a string causes errors before initialize runs.

Fields should be initialized as NULL or a default value. Setting to NULL avoids errors before initialization.

4fill in blank

hard

Fill both blanks to add a private field 'age' and a public method 'get_age' that returns it.

R Programming

Person <- R6Class(classname = "Person",
  private = list(
    [1] = NULL
  ),
  public = list(
    get_age = function() {
      return(private$[2])
    }
  )
)

Drag options to blanks, or click blank then click option'

Aage

Cname

Dage_value

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the private field and in the method causes errors.

The private field is named 'age', and the public method accesses it as 'private$age'.

5fill in blank

hard

Fill all three blanks to create a method 'set_name' that updates the 'name' field and prints confirmation.

R Programming

Person <- R6Class(classname = "Person",
  public = list(
    name = NULL,
    set_name = function(new_name) {
      self$[1] <- [2]
      print(paste("Name set to", [3]))
    }
  )
)

Drag options to blanks, or click blank then click option'

Aname

Bnew_name

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up variable names or forgetting 'self$' causes errors.

The method sets self$name to new_name and prints it.

Practice

1. What is the main purpose of using R6Class() in R?

easy

A. To create plots and visualizations

B. To perform statistical tests on data

C. To import data from external files

D. To create a class that groups data and functions together

Solution

1. Step 1: Understand the role of R6Class()

   R6Class() is used to define a class that can hold both data (fields) and functions (methods) together.

2. Step 2: Compare with other options

   Creating plots and visualizations, performing statistical tests on data, and importing data from external files are unrelated tasks, not the purpose of R6Class().

3. Final Answer:

   To create a class that groups data and functions together -> Option D

4. Quick Check:

   R6Class() = class with data and methods [OK]

Hint: R6Class groups data and actions in one place [OK]

Common Mistakes:

• Thinking R6Class is for data analysis only
• Confusing R6Class with plotting functions
• Assuming R6Class imports data

2. Which of the following is the correct way to define a method inside an R6 class?

easy

A. methods = { my_method <- function() { print(self$field) } }

B. methods = list(my_method = function() { print(self$field) })

C. method <- function() { print(self$field) }

D. methods = function(my_method) { print(self$field) }

Solution

1. Step 1: Recall R6 method syntax

   Methods are defined inside a list with names as method names and values as functions, e.g., methods = list(name = function() {...}).

2. Step 2: Check each option

   methods = list(my_method = function() { print(self$field) }) correctly uses a list with a named function. The other options do not follow the required list structure.

3. Final Answer:

   methods = list(my_method = function() { print(self$field) }) -> Option B

4. Quick Check:

   Methods = list of named functions [OK]

Hint: Methods must be in a named list inside R6Class [OK]

Common Mistakes:

• Defining methods as a single function instead of list
• Using assignment inside methods block incorrectly
• Omitting the list() wrapper

3. What will be the output of this R code?

Person <- R6Class("Person",
  public = list(
    name = NULL,
    initialize = function(name) {
      self$name <- name
    },
    greet = function() {
      paste0("Hello, ", self$name, "!")
    }
  )
)
p <- Person$new("Anna")
p$greet()

medium

A. NULL

B. Error: object 'self' not found

C. "Hello, Anna!"

D. "Hello, name!"

Solution

1. Step 1: Understand object creation and initialization

   The class Person has a field 'name' set during initialization with self$name <- name. The object p is created with name "Anna".

2. Step 2: Evaluate the greet method

   The greet method returns paste0("Hello, ", self$name, "!"), which becomes "Hello, Anna!" for p.

3. Final Answer:

   "Hello, Anna!" -> Option C

4. Quick Check:

   p$greet() = "Hello, Anna!" [OK]

Hint: self$name holds the name; greet returns greeting string [OK]

Common Mistakes:

• Expecting NULL because name is NULL initially
• Confusing self with private or global variables
• Thinking greet prints instead of returns

4. Identify the error in this R6 class definition:

Car <- R6Class("Car",
  public = list(
    speed = 0,
    accelerate = function() {
      speed <- speed + 10
    }
  )
)
c <- Car$new()
c$accelerate()
c$speed

medium

A. The method should use self$speed, not speed alone

B. The class name must be lowercase

C. The accelerate method must return speed explicitly

D. The speed field cannot be initialized to 0

Solution

1. Step 1: Check variable usage inside method

   Inside accelerate(), speed <- speed + 10 updates a local variable speed, not the field. It should be self$speed to update the object's field.

2. Step 2: Verify other options

   Class names can be uppercase, methods don't need explicit return, and initializing speed to 0 is valid.

3. Final Answer:

   The method should use self$speed, not speed alone -> Option A

4. Quick Check:

   Use self$ to access fields inside methods [OK]

Hint: Use self$ to access or change fields inside methods [OK]

Common Mistakes:

• Forgetting self$ prefix for fields
• Assuming local variable changes affect fields
• Thinking class names must be lowercase

5. You want to create an R6 class BankAccount with a field balance starting at 0 and methods deposit(amount) and withdraw(amount). The withdraw method should not allow the balance to go below zero. Which code snippet correctly implements this?

hard

A. BankAccount <- R6Class("BankAccount",
public = list(
balance = 0,
deposit = function(amount) {
self$balance <- self$balance + amount
},
withdraw = function(amount) {
if (self$balance >= amount) {
self$balance <- self$balance - amount
}
}
)
)

B. BankAccount <- R6Class("BankAccount",
public = list(
balance = 0,
deposit = function(amount) {
balance <- balance + amount
},
withdraw = function(amount) {
balance <- balance - amount
}
)
)

C. BankAccount <- R6Class("BankAccount",
public = list(
balance = 0,
deposit = function(amount) {
self$balance <- amount
},
withdraw = function(amount) {
self$balance <- self$balance - amount
}
)
)

D. BankAccount <- R6Class("BankAccount",
public = list(
balance = 0,
deposit = function(amount) {
self$balance <- self$balance + amount
},
withdraw = function(amount) {
self$balance <- self$balance - amount
}
)
)

Solution

1. Step 1: Check deposit method correctness

   Deposit should add amount to self$balance. Some options do this correctly; others do not use self$ or overwrite balance incorrectly.

2. Step 2: Check withdraw method logic

   Withdraw must check if balance >= amount before subtracting to avoid negative balance. Only BankAccount <- R6Class("BankAccount", public = list( balance = 0, deposit = function(amount) { self$balance <- self$balance + amount }, withdraw = function(amount) { if (self$balance >= amount) { self$balance <- self$balance - amount } } ) ) has this check.

3. Final Answer:

   The code snippet with self$balance updates and balance check before withdrawal -> Option A

4. Quick Check:

   Use self$ and check balance before withdrawal [OK]

Hint: Withdraw must check balance before subtracting [OK]

Common Mistakes:

• Not using self$ to update balance
• Overwriting balance instead of adding
• Allowing negative balance without check