Not quite sure how to ask this question without going into a great deal of detail about my specific programming problem. So, this question is slightly related, but much simpler. I'm trying to understand the procedure R applies when assigning to a slot of an S4 class. I have custom access and assignment functions, "$" and "$<-".
I note two things:
Following is a toy example for illustration. Comment on point 1 and an answer to point 2 are much appreciated.
setClass("Person"
, representation(FirstName = "character"
, LastName = "character"
, Birthday = "Date")
)
setMethod("$", signature(x = "Person"), function(x, name) {
print("Just called $ accessor")
slot(x, name)
})
setMethod("$<-", signature(x = "Person"), function(x, name, value) {
print("Just called $ assignment")
slot(x, name) = value
x
})
objPeople = new("Person"
, FirstName=c("Ambrose", "Victor", "Jules")
, LastName=c("Bierce", "Hugo", "Verne")
, Birthday=seq(as.Date("2001/01/01"), as.Date("2003/12/31"), by="1 year"))
# This assignment will work. When assigning, there will be a call to the "$" accessor function. Why?
objPeople$FirstName[2] = "Joe"
# This assignment will not make a call to the "$" accessor function. Why?
objPeople$FirstName = "Ambroze"
Remember that [ is just another function, as is [<-. So in order to do
objPeople$FirstName[2] = "Joe"
The $ is going to run first and return something that the [<- can operate on. Something like
'$<-'(objPeople, "FirstName", '[<-'( '$'(objPeople, "FirstName"), 2, "Joe"))
So in order to subset, it has to extract the first name. But with
objPeople$FirstName = "Ambroze"
That's just a
'$<-'( objPeople, "FirstName", "Ambroze")
so you don't need to call the accessor. You are just directly calling the assignment function.
If you wanted to have a custom subsetter on your class, it would be at the Person[] level. If you wanted a custom subsetter on the FirstName slot, you would have to make the FirstName slot a class of it's own where you could re-define the [ method.
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