S3 object system in R Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an S3 object of class "person".

R Programming

person <- list(name = "Alice", age = 30)
class(person) <- [1]

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

A"Person"

Blist

Cperson

D"person"

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to put the class name in quotes.

Using the wrong class name capitalization.

To create an S3 object, assign the class attribute as a character string with the class name, here "person".

2fill in blank

medium

Complete the code to define a print method for the "person" class.

R Programming

print.person <- function(x) {
  cat("Name:", x$name, "\nAge:", x$age, "\n")
}

p <- list(name = "Bob", age = 25)
class(p) <- "person"
print([1])

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

Ap

Bp$name

Cperson

Dprint.person

Attempts:

3 left

💡 Hint

Common Mistakes

Printing only a component like p$name instead of the whole object.

Trying to call the method name directly.

To call the print method for the S3 object, pass the object itself to print().

3fill in blank

hard

Fix the error in this S3 method definition for "summary.person".

R Programming

summary.person <- function(object, ...) {
  paste("Person:", object$name, "is", object$age, "years old")
}

p <- list(name = "Carol", age = 40)
class(p) <- "person"
summary([1])

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

Aobject

Bp

Cperson

Dsummary.person

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the class name instead of the object to summary().

Using the method name directly instead of summary().

The summary function should be called with the object variable, here p, to dispatch the S3 method.

4fill in blank

hard

Fill both blanks to create an S3 object and check its class.

R Programming

obj <- list(value = 100)
class(obj) <- [1]
result <- class([2])

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

A"myclass"

Bobj

C"obj"

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using the object name as a string when assigning class.

Checking class of a string instead of the object.

Assign the class name as a string to obj, then check the class of obj itself.

5fill in blank

hard

Fill all three blanks to define a generic function and an S3 method, then call it.

R Programming

myfunc <- function(x) {
  UseMethod([1])
}

myfunc.[2] <- function(x) {
  paste("Called method for class", class(x))
}

obj <- list()
class(obj) <- [3]
myfunc(obj)

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

A"myfunc"

Bmyclass

C"myclass"

Dmyfunc

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the generic function name as a string to UseMethod().

Not quoting the class name when assigning class.

Mismatch between class name and method suffix.

UseMethod() needs the generic function name without quotes; method name uses class name; class assigned as string.

Practice

1. What is the main purpose of the S3 object system in R?

easy

A. To speed up the execution of R code

B. To provide a graphical user interface for R

C. To allow functions to behave differently based on the class of their input

D. To manage memory allocation automatically

Solution

1. Step 1: Understand S3 system role

   S3 is designed to let functions act differently depending on the class of the object they receive.

2. Step 2: Compare options

   Only To allow functions to behave differently based on the class of their input describes this behavior. Other options describe unrelated features.

3. Final Answer:

   To allow functions to behave differently based on the class of their input -> Option C

4. Quick Check:

   S3 purpose = different behavior by class [OK]

Hint: Remember: S3 dispatches methods by object class [OK]

Common Mistakes:

• Confusing S3 with speed optimization
• Thinking S3 manages memory
• Assuming S3 provides a GUI

2. Which of the following is the correct way to define an S3 method for the generic function print for objects of class myclass?

easy

A. print_myclass <- function(x) { cat("My class object") }

B. print.myclass <- function(x) { cat("My class object") }

C. print(myclass) <- function(x) { cat("My class object") }

D. function print.myclass(x) { cat("My class object") }

Solution

1. Step 1: Recall S3 method naming convention

   S3 methods are named as generic.class, here print.myclass.

2. Step 2: Check syntax correctness

   print.myclass <- function(x) { cat("My class object") } uses correct assignment and function syntax. Others have invalid syntax or wrong naming.

3. Final Answer:

   print.myclass <- function(x) { cat("My class object") } -> Option B

4. Quick Check:

   S3 method = generic.class naming [OK]

Hint: Use generic.class naming to define S3 methods [OK]

Common Mistakes:

• Using underscores instead of dots in method names
• Incorrect function assignment syntax
• Trying to assign function to generic(class)

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

obj <- structure(list(a = 1), class = "myclass")
print.myclass <- function(x) { cat("Class is myclass\n") }
print(obj)

medium

A. Class is myclass

B. [1] 1

C. Error: could not find function "print.myclass"

D. NULL

Solution

1. Step 1: Understand object class and method

   The object has class "myclass" and a print.myclass method is defined.

2. Step 2: Method dispatch on print()

   Calling print(obj) triggers print.myclass, which prints "Class is myclass" with a newline.

3. Final Answer:

   Class is myclass -> Option A

4. Quick Check:

   print dispatches to print.myclass [OK]

Hint: print calls method matching object's class [OK]

Common Mistakes:

• Expecting default print output
• Thinking print.myclass is not found
• Ignoring class attribute

4. Consider this code snippet:

obj <- list(a = 10)
class(obj) <- "myclass"
print.myclass <- function(x) { cat("Value:", x$a, "\n") }
print(obj)

What is the error in this code if any?

medium

A. Error because print() does not dispatch to print.myclass

B. Error because print.myclass is not defined correctly

C. Error because class assignment is invalid

D. No error; it prints 'Value: 10'

Solution

1. Step 1: Check class assignment

   Assigning class "myclass" to obj is valid.

2. Step 2: Check method definition and dispatch

   print.myclass is correctly defined and print(obj) calls it, printing "Value: 10".

3. Final Answer:

   No error; it prints 'Value: 10' -> Option D

4. Quick Check:

   Class and method correctly set, print dispatches [OK]

Hint: Assign class before calling generic function [OK]

Common Mistakes:

• Defining method after calling print
• Not assigning class attribute
• Misnaming method function

5. You want to create an S3 generic function describe that behaves differently for numeric and character vectors. Which of the following code snippets correctly sets up the generic and methods?

hard

A. describe <- function(x) UseMethod("describe")
describe.numeric <- function(x) { paste("Numeric with length", length(x)) }
describe.character <- function(x) { paste("Character with length", length(x)) }

B. describe <- function(x) { if (is.numeric(x)) "numeric" else "character" }
describe.numeric <- function(x) {}
describe.character <- function(x) {}

C. describe.numeric <- function(x) { paste("Numeric", length(x)) }
describe.character <- function(x) { paste("Character", length(x)) }

D. describe <- function(x) { UseMethod(describe) }
describe.numeric <- function(x) { length(x) }
describe.character <- function(x) { length(x) }

Solution

1. Step 1: Define generic function correctly

   The generic must call UseMethod with the generic name as a string: UseMethod("describe").

2. Step 2: Define methods with correct naming and behavior

   Methods describe.numeric and describe.character are correctly named and return descriptive strings.

3. Final Answer:

   describe <- function(x) UseMethod("describe") describe.numeric <- function(x) { paste("Numeric with length", length(x)) } describe.character <- function(x) { paste("Character with length", length(x)) } -> Option A

4. Quick Check:

   Generic + methods correctly defined [OK]

Hint: Generic calls UseMethod with string name [OK]

Common Mistakes:

• Calling UseMethod without quotes
• Not defining generic before methods
• Empty method bodies