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R S4 Show Method

A show method controls how an S4 object prints. print() runs the show method in place of the plain slot dump.

A show method takes over print

The first print shows the plain slot dump. After the show method is written, the same print shows one short line.

setClass("Wallet", slots = c(balance = "numeric", owner = "character"), prototype = list(balance = 0, owner = "Ana"))
w <- new("Wallet", balance = 10, owner = "Ana")
print(w)
setMethod("show", "Wallet", function(object) cat(object@owner, "wallet:", object@balance, "coins\n"))
print(w)

getSlots lists the slot classes

getSlots() takes the class name. The result pairs each slot name with the class of that slot.

setClass("Wallet", slots = c(balance = "numeric", owner = "character"), prototype = list(balance = 0, owner = "Ana"))
print(getSlots("Wallet"))

The show method reads each object

Two Wallet objects hold different values. Each print shows the values stored in that object.

setClass("Wallet", slots = c(balance = "numeric", owner = "character"), prototype = list(balance = 0, owner = "Ana"))
setMethod("show", "Wallet", function(object) cat(object@owner, "wallet:", object@balance, "coins\n"))
w <- new("Wallet", balance = 25, owner = "Bo")
print(w)
w2 <- new("Wallet", balance = 8, owner = "Cy")
print(w2)

getSlots includes inherited slots

Savings builds on Wallet, so its slot list has three entries. The rate slot comes first, ahead of the two inherited slots.

setClass("Wallet", slots = c(balance = "numeric", owner = "character"), prototype = list(balance = 0, owner = "Ana"))
setClass("Savings", contains = "Wallet", slots = c(rate = "numeric"), prototype = list(rate = 2))
print(getSlots("Savings"))

Example

setClass("Wallet", slots = c(balance = "numeric", owner = "character"), prototype = list(balance = 0, owner = "Ana"))
setMethod("show", "Wallet", function(object) cat(object@owner, "wallet:", object@balance, "coins\n"))
w <- new("Wallet", balance = 42, owner = "Dee")
print(w)
print(getSlots("Wallet"))

Key points

  • setMethod() with the name "show" writes the show method for a class.
  • The show method takes the object as its argument.
  • The method reads the slots with @ and prints them with cat().
  • print() runs the show method without any extra call.
  • getSlots() returns a named character vector that pairs each slot name with its class.
  • For a subclass, getSlots() lists the inherited slots as well as its own slots.

In this example, the show method prints Dee with a balance of 42 in one short line. getSlots() then lists balance as numeric and owner as character.

Try it Yourself

Edit the code, then press Run.

Running is simulated · rdrr.io R Snippets

Exercise

What does getSlots("Wallet") return?