This is due to the following message Problems passing arguments by calling NextMethod () in R
I am writing accessors for the two classes S4, "foo" and "bar". "bar" is inherited from foo and applies only to a few slots. Instead of writing a full accessor function for objects of class "bar", I want to pass arguments to the call of NextMethod () when accessing a slot that is inherited by "foo". My code is as follows:
foo <- setClass("foo", representation(x = "numeric", y = "numeric")) bar <- setClass("bar", representation(distance = "numeric"), contains = "foo") setMethod("[", "bar", function(x, i, j, drop) { if (i == "distance") { return( x@distance ) } else { callNextMethod() } } ) setMethod("[", "foo", function(x, i, j, drop) { if (i == "x") { return( x@x ) } else { if (i == "y") { return( x@y ) } } } )
Now try this:
f <- new("foo", x = 1, y = 2) b <- new("bar", x = 3, y = 4, distance = 5) f["x"] f["y"]
The accessor for the object 'f' of class 'foo' correctly returns:
> f["x"] [1] 1 > f["y"] [1] 2
If I try to access the slot "distance" of the object "b" of the class "bar", then the accessor will also correctly return:
> b["distance"] [1] 5
The problem occurs when I try to access any of the slots of object "b" of class "bar" that are inherited from "foo". If I try:
b["x"] b["y"]
The following error message appears:
Error in callNextMethod() : bad object found as method (class "function")
I read the recommendations from this post Problems with passing arguments by calling NextMethod () to R , but I can’t define a common one for '[' and trying to pass arguments explicitly to call NextMethod () also failed.
Of course I'm doing something wrong! I have read several resources by inheritance and cannot determine the problem, and I hope that you people can guide me in the right direction.
thanks
Max