Module # 7 R Object: S3 vs. S4 assignment

 data("mtcars")

head(mtcars, 6)

str(mtcars)

class(mtcars)

typeof(mtcars)

What I found:

• class(mtcars) → "data.frame"

• typeof(mtcars) → "list" (because data frames are lists under the hood)

• str(mtcars) shows it’s a list of columns, each column is numeric.

• Yes. Generic functions work great with mtcars.

• A generic function is a function that chooses which version of the function to run based on the class of the object you pass in.

  Example: print() is a generic.

  print(mtcars)     # uses print.data.frame behind the scenes
  summary(mtcars)   # uses summary.data.frame
  

  To prove it’s generic:

  isS4(mtcars)      # FALSE
  methods("print")  # shows lots of print methods
  methods("summary")
  

  If a generic function didn’t work, it would usually be because the object’s class has no method implemented for that generic (so it falls back to a default method or errors).

• Step 3 — Can S3 and S4 be assigned to this dataset?

  ✅ S3 can be assigned easily by setting a class attribute.
  ✅ S4 can be assigned by defining an S4 class and using new().

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

  Use these:

  class(mtcars)   # shows S3 class name(s)
  isS4(mtcars)    # tells if it’s an S4 object
  

  • If isS4(x) is TRUE → it’s S4

  • If it has a class() but isS4() is FALSE → it’s almost always S3

  Also helpful:

  methods(class = "data.frame")
  

  ---

  2) How do you determine the base type (like integer or list) of an object?

  Use:

  typeof(mtcars)
  mode(mtcars)
  storage.mode(mtcars)
  

  For data frames:

  • typeof(mtcars) is "list" because each column is stored as an element of a list.

  ---

  3) What is a generic function?

  A generic function is a function that dispatches (chooses) a method based on an object’s class.

  Examples:

  • print(x) calls print.classname(x) if it exists

  • summary(x) calls summary.classname(x) if it exists

  To check if something is generic:

  is.primitive(summary)  # not always helpful
  methods("summary")
  

  ---

  4) What are the main differences between S3 and S4?

  S3

  • Informal, simple

  • Classes are just a class attribute

  • Methods named like generic.class

  • No strict definition of fields/structure

  • Fast to create and flexible

  S4

  • Formal and strict

  • You define a class with setClass()

  • Objects have slots

  • Stronger checks/validation

  • Methods created with setGeneric() and setMethod()