S3 object system in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this S3 method call?

Consider the following R code using S3 classes and methods. What will be printed when print(obj) is called?

R Programming

obj <- structure(list(name = "Alice", age = 30), class = "person")
print.person <- function(x) {
  cat("Person:", x$name, "is", x$age, "years old\n")
}
print(obj)

A Person: Alice is 30 years old

B [1] "Person: Alice is 30 years old"

C list(name = "Alice", age = 30)

D Error in print.person(obj) : could not find function "print.person"

💡 Hint

Remember that S3 methods are functions named like generic.class and are called automatically by the generic.

The print.person function is the S3 method for class 'person'. When print(obj) is called, R dispatches to print.person, which uses cat to print the formatted string without quotes or index.

🧠 Conceptual

intermediate

How does S3 method dispatch work in R?

Which statement best describes how R finds the correct S3 method to call when a generic function is used?

A R always calls the generic function without checking the object's class.

B R uses formal class definitions and inheritance trees to find the method in a class hierarchy.

C R looks for a function named <code>generic.class</code> where <code>class</code> is the class of the object, and calls it if found.

D R requires the user to manually call the method function for each class.

💡 Hint

Think about the naming pattern of S3 methods.

S3 dispatch is based on naming conventions. When calling a generic like print(), R looks for a function named print.classname matching the object's class and calls it if it exists.

🔧 Debug

advanced

Why does this S3 method not get called?

Given the code below, why does summary(obj) not call summary.myclass as expected?

R Programming

obj <- structure(list(x = 1:5), class = "myclass")
summary.myclass <- function(object) {
  # Custom summary
}
summary(obj)

A The class attribute is not set correctly on <code>obj</code>.

B The method <code>summary.myclass</code> does not return a value explicitly, so it returns NULL and appears not to work.

C The generic <code>summary</code> does not support S3 dispatch.

D The method name should be <code>summary_myclass</code> with underscore.

💡 Hint

Check what the method function returns.

The summary.myclass function contains only a comment and produces no output. R prints the result of summary(obj), which is NULL because the function lacks an explicit return or print statement.

📝 Syntax

advanced

Which option correctly defines an S3 method for class 'data.frame' for the generic 'head'?

Choose the correct syntax to define an S3 method head.data.frame that returns the first 3 rows of a dataframe.

A

head.data.frame <- function(x) {
  x[1:3, ]
}

B

head.data.frame <- function(x, n) {
  x[1:n]
}

C

head.data.frame <- function(x) {
  return(x[1:3])
}

D

head.data.frame <- function(x, n = 3) {
  x[1:n, ]
}

💡 Hint

The original head generic accepts an argument n with default 6.

The correct method matches the generic signature including the argument n with default value. It returns the first n rows of the dataframe. Option D lacks the n argument, C hard-codes 3 rows without the n argument, and D lacks default for n.

🚀 Application

expert

How many items are in the list returned by this S3 generic with multiple classes?

Consider this R code where an object has multiple classes and a generic returns a list. How many elements does the list returned by foo(obj) contain?

R Programming

foo <- function(x) UseMethod("foo")
foo.class1 <- function(x) list(a = 1, b = 2)
foo.class2 <- function(x) list(c = 3)
obj <- structure(list(), class = c("class1", "class2"))
result <- foo(obj)
length(result)

A 2

B 3

C 1

D 0

💡 Hint

S3 dispatch calls the method for the first class in the class vector.

The object has classes c("class1", "class2"). S3 dispatch calls foo.class1 because it is the first class. That method returns a list with two elements named 'a' and 'b'. So length(result) is 2.