Example
The S3 object system is a very simple OO system in R.
Every object has an S3 class. It can be get (got?) with the function class.
> class(3)
[1] "numeric"
It can also be set with the function class:
> bicycle <- 2
> class(bicycle) <- 'vehicle'
> class(bicycle)
[1] "vehicle"
It can also be set with the function attr:
> velocipede <- 2
> attr(velocipede, 'class') <- 'vehicle'
> class(velocipede)
[1] "vehicle"
An object can have many classes:
> class(x = bicycle) <- c('human-powered vehicle', class(x = bicycle))
> class(x = bicycle)
[1] "human-powered vehicle" "vehicle"
When using a generic function, R uses the first element of the class that has an available generic.
For example:
> summary.vehicle <- function(object, ...) {
+ message('this is a vehicle')
+ }
> summary(object = my_bike)
this is a vehicle
But if we now define a summary.bicycle:
> summary.bicycle <- function(object, ...) {
+ message('this is a bicycle')
+ }
> summary(object = my_bike)
this is a bicycle
