R devel - Mar 2007 - Replacing slot of S4 class in method of S4 class?

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?

Best regards
Christian
_._._._._._._._._._._._._._._._
C.h.i.s.t.i.a.n S.t.r.a.t.o.w.a
V.i.e.n.n.a       A.u.s.t.r.i.a
_._._._._._._._._._._._._._._._

Please read the posting guide and use the appropriate mailing list (R- 
help) - your question has nothing to do with the development of R.

Cheers,
Simon

PS: look closely at your code - your mymethod is a noop.


On Mar 30, 2007, at 4:45 PM, 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?
>
> Best regards
> Christian
> _._._._._._._._._._._._._._._._
> C.h.i.s.t.i.a.n S.t.r.a.t.o.w.a
> V.i.e.n.n.a       A.u.s.t.r.i.a
> _._._._._._._._._._._._._._._._
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
>

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.