Module # 7 R Object: S3 vs. S4 assignment

For this assignment, I used the built-in R dataset mtcars, which comes from the R datasets package.

data("mtcars")

head(mtcars, 6)

The mtcars dataset contains information about different car models, including:

• mpg (miles per gallon)
• cyl (number of cylinders)
• hp (horsepower)
• wt (weight)
• am (transmission type)

To check its structure, I got this below:

Output shows:

• Class: "data.frame"

• Base type: "list"

A good question to ask is if a generic function can be assigned to this 

A generic function in R is a function that behaves differently depending on the class of the object passed to it.

For example:

Both print() and summary() are generic functions.

And to check if a function is generic: methods(summary)

This shows different methods like:

• summary.data.frame
• summary.lm
• Since mtcars is a data.frame, R automatically dispatches:
• summary.data.frame(mtcars)

A Generic Function Be Assigned

Because mtcars is an S3 object, data.frame is an S3 class, generic functions like: print() summary() and plot()

automatically dispatch methods for it.

If a dataset does not have a defined class or method, R will use the default method, such as:

summary.default()

If no method exists at all, R can just give an error.

S3 Assignment

Assign a new S3 class to mtcars:

mydata <- mtcars
class(mydata) <- "car_data"

summary.car_data <- function(object) {
  cat("Custom Summary for Car Data\n")
  cat("Number of Cars:", nrow(object), "\n")
  cat("Average MPG:", mean(object$mpg), "\n")
}

Then:

summary(mydata)

This demonstrates S3 method dispatch.

---

S4 Assignment

setClass("CarData",

         slots = list(
           data = "data.frame"
         ))

car_obj <- new("CarData", data = mtcars)

To define a method:

setGeneric("carSummary", function(object) standardGeneric("carSummary"))

setMethod("carSummary", "CarData", function(object) {
  cat("S4 Car Summary\n")
  cat("Number of Cars:", nrow(object@data), "\n")
})

Then call:

carSummary(car_obj)

Both S3 and S4 can be assigned to this dataset.

1. How do you tell what OO system (S3 vs. S4) an object is associated with?

For S3:

class(object)

S3 objects have a class attribute but no formal slot definition.

For S4:

isS4(object)

If TRUE it is an S4 object.

Also:

slotNames(object)

Only works for S4 objects.

2. How do you determine the base type of an object?

Use:

typeof(object) or mode(object)

For example:

typeof(mtcars)

Returns:

[1] "list"

So even though mtcars is a data.frame, its base type is a list.

3. What is a Generic Function?

A generic function is a function that performs method dispatch based on the object’s class.

Example:

• summary()
• print()
• plot()

It calls class-specific methods like:

• summary.data.frame
• summary.lm
• summary.default

Generic functions can enable polymorphism.

4. Main Differences Between S3 and S4

S3 has no formal class definition while S4 does. S4 also has multiple dispatch while S3 has a single dispatch. S3 is simpler while S4 is more complex.