Using multiple constructors for R classes and subclasses - r

Question

I would like to use multiple constructors in my R S4 class.
I have an object that has three slots. To make that object, sometimes I want to just give the values for the three slots outright. But sometimes I'd like to provide a matrix, and I have a function that can take a matrix and return what those three slots should be.
At first, it seems like I could write a function as a constructor. So I could write objectFromMatrix(matrix) --> object with three slots. The problem is that I also have sub-classes that inherit from that main class, and I want to be able to use that constructor with them as well.
So I could just write functions as extra constructors for each of the subclasses, but that would be a bit tedious and not super OO-like.
To make my problem a little more tangible, I'll try to write a minimal example below. I'll write it in Java, but I'm a bit rusty so let me know if it doesn't make sense.
Desired structure, in Java:
// An abode is a place where you live and it has a size
class Abode {
int size = 1;
// Main constructor that just assigns args to fields
Abode(int size) {
this.size = size;
}
// Alternative constructor that takes in a different datatype
// and computes args to assign to fields
Abode(string description) {
if(description eq "Large") {
this.size = 5;
}
if(description eq "Small") {
this.size = 1;
}
}
// To keep it simple, a house is just an abode with a name
class House extends Abode {
String name;
House(int size, String name) {
super(size);
this.name = name;
}
House(string size, String name) {
super(size);
this.name = name;
}
}
This implementation works nicely because I can call Abode("big") or House("big", "Casa de me"), and both of those get passed to the extra constructor I built in the Abode class.
Keeping up with the house analogy, this is the best I've been able to do in R:
# An abode is a place you live and it has a size
setClass("Abode",
slots =
list(size = "numeric")
)
# Alternative constructor that takes in a different datatype
# and computes args to assign to fields
abode_constructor_2 <- function(sizeString) {
if (sizeString == "big") {return new("Abode", size = 5)}
if (sizeString == "small") {return new("Abode", size = 1)}
}
# A house is an abode with a name
setClass("House",
slots =
list(name = "string"),
contains = "Abode"
)
# I already defined this constructor but I have to do it again
house_constructor_2 <- function(sizeString, name) {
if (sizeString == "big") {return new("House", size = 5, name = name)}
if (sizeString == "small") {return new("House", size = 1, name = name)}
}
In case it helps, here is a minimal example of the real context where this problem is coming up. I define an extra constructor for the Sensor class, sensor_constructor_2, as a function. But then, when I have a class that inherits from Sensor, I have to make that constructor over again.
# A sensor has three parameters
setClass("Sensor",
slots =
list(Rmin = "numeric", Rmax = "numeric", delta = "numeric")
)
# I also like to make sensors from a matrix
sensor_constructor_2 <- function(matrix) {
params <- matrix_to_params(matrix)
return (new("Sensor", Rmin = params[1], Rmax = params[2], delta = params[3]))
}
# A redoxSensor is just a sensor with an extra field
setClass("redoxSensor",
slots =
list(e0 = "numeric"),
contains = "Sensor"
)
# Goal: make this extra constructor unnecessary by making sensor_constructor_2 a property of the sensor class
extraConstructor_redox <- function(matrix, e0) {
params <- matrix_to_params(matrix)
return (new("redoxSensor", Rmin = params[1], Rmax = params[2], delta = params[3]), e0 = e0)
}

Answer 1

There is no reason why you can't do this with one S4 constructor by using default arguments and a little extra logic, along the lines of
setClass("Abode",
slots = list(size = "numeric")
) -> Abode
setClass("House",
slots = list(name = "character"),
contains = "Abode"
) -> House
createDwelling <- function(size=0,name,sizeString){
if(!missing(sizeString)){
if(sizeString == "Large") size <- 5
else if(sizeString == "Small") size <- 1
else stop("invalid sizeString")
}
if(missing(name)) return(Abode(size=size))
else return(House(size=size,name=name))
}
example usage:
> createDwelling(size=3)
An object of class "Abode"
Slot "size":
[1] 3
> createDwelling(sizeString="Small")
An object of class "Abode"
Slot "size":
[1] 1
> createDwelling(sizeString="Small",name="my house")
An object of class "House"
Slot "name":
[1] "my house"
Slot "size":
[1] 1