[Rd] Replacing slot of S4 class in method of S4 class?

[Ross Boylan]

On Fri, Mar 30, 2007 at 10:45:38PM +0200, cstrato wrote:
> Dear all,
> 
> Assume that I have an S4 class "MyClass" with a slot "myname", which
> is initialized to:  myname="" in method("initialize"):
>    myclass <- new("MyClass", myname="")
> 
> Assume that class "MyClass" has a method "mymethod":
>    "mymethod.MyClass" <-
>    function(object, myname=character(0), ...) {
>        object at myname <- myname;
>    #or:    myName(object) <- myname
>    }
>    setMethod("mymethod", "MyClass", mymethod.MyClass);
> 
> Furthermore, I have a replacement method:
> setReplaceMethod("myName", signature(object="MyClass", value="character"),
>    function(object, value) {
>       object at myname <- value;
>       return(object);
>    }
> )
> 
> I know that it is possible to call:
>    myName(myclass) <- "newname"
> 
> However, I want to replace the value of slot "myname" for object "myclass"
> in method "mymethod":
>    mymethod(myclass,  myname="newname")
> 
> Sorrowly, the above code in method "mymethod" does not work.
> 
> Is there a possibility to change the value of a slot in the method of a 
> class?

Yes, but to make the effect persistent (visible might be a more
accurate description) that method must return the
object being updated, and you must use the return value.  R uses call
by value semantics, so in the definition of mymethod.MyClass when you
change object you only change a local copy.  It needs to be
 "mymethod.MyClass" <-
    function(object, myname=character(0), ...) {
        object at myname <- myname;
	object
  }

Further, if you invoke it with
mymethod(myclass, "new name")
you will discover myclass is unchanged.  You need
myclass <- mymethod(myclass, "new name")

You might consider using the R.oo package, which probably has
semantics closer to what you're expecting.  Alternately, you could
study more about R and functional programming.

Ross Boylan

P.S. Regarding the follow up saying that this is the wrong list, the
guide to mailing lists says of R-devel
"This list is intended for questions and discussion about code
development in R. Questions likely to prompt discussion unintelligible
to non-programmers or topics that are too technical for R-help's
audience should go to R-devel,"
The question seems to fall under this description to me, though I am
not authoritative.  It is true that further study would have disclosed
what is going on.  Since the same thing tripped me up too, I thought
I'd share the answer.