S4 object system in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this S4 class slot access?

Consider the following R code using S4 classes. What will be printed when accessing the slot @name?

R Programming

setClass("Person", slots = list(name = "character", age = "numeric"))
p <- new("Person", name = "Alice", age = 30)
print(p@name)

A [1] "Alice"

B [1] 30

C Error: no slot of name "name" for this object of class "Person"

D [1] "p@name"

💡 Hint

Remember that slots in S4 objects are accessed with the @ operator.

The slot 'name' contains the character 'Alice'. Accessing p@name returns this value.

🧠 Conceptual

intermediate

Which statement about S4 class inheritance is true?

In R's S4 system, which of the following is true about class inheritance?

A An S4 class can inherit from multiple classes simultaneously.

B An S4 class can only inherit from one class at a time.

C Inheritance is automatically applied to all slots without declaration.

D Inheritance is not supported in S4 classes.

💡 Hint

Think about how S4 supports complex class hierarchies.

S4 classes support multiple inheritance by specifying multiple classes in the 'contains' argument.

🔧 Debug

advanced

Identify the error in this S4 method definition

What error will this code produce when defining an S4 method?

R Programming

setGeneric("greet", function(object) standardGeneric("greet"))
setMethod("greet", "Person", function(obj) {
  paste("Hello", obj@name)
})

A Error: no slot of name "name" for this object of class "Person"

B Error: formal argument "object" matched by multiple actual arguments

C Error: unused argument (obj)

D No error; method defined successfully

💡 Hint

Check the argument names in generic and method functions.

The generic expects an argument named 'object', but the method uses 'obj', causing argument matching issues.

📝 Syntax

advanced

Which option correctly defines an S4 class with a numeric slot 'score' and a character slot 'grade'?

Choose the correct syntax to define an S4 class named 'Result' with slots 'score' (numeric) and 'grade' (character).

A setClass("Result", slots = list(score = numeric, grade = character))

B setClass("Result", slot = list(score = numeric, grade = character))

C setClass("Result", slots = c(score = numeric, grade = character))

D setClass("Result", slots = list(score = "numeric", grade = "character"))

💡 Hint

Slots must be declared as a named list with class names as strings.

Slots argument requires a named list with slot names and their class names as strings.

🚀 Application

expert

How many slots does this S4 object have after inheritance?

Given these class definitions, how many slots does an object of class 'Student' have?

R Programming

setClass("Person", slots = list(name = "character", age = "numeric"))
setClass("Employee", slots = list(employeeID = "character"))
setClass("Student", contains = c("Person", "Employee"), slots = list(grade = "character"))
obj <- new("Student", name = "Bob", age = 22, employeeID = "E123", grade = "A")

A 5

B 2

C 4

D 3

💡 Hint

Count all slots from inherited classes plus new slots.

Student inherits slots 'name' and 'age' from Person, 'employeeID' from Employee, and adds 'grade'. Total slots = 4.