[R] Set without argument

[Martin Morgan]

Christophe Genolini <cgenolin at u-paris10.fr> writes:

> Hi the list,
>
> I am defining S4 objet. Is it possbile to define a method that change 
> the slot of an object without using <- ?
> My object contain a numeric and a matrix. At some point, I would like to 
> impute the missing value in the matrix. So I would like to use something 
> like :
>
> -----------------
> setClass("MyObj",representation(time="numeric",traj="matrix"))
> a <- new("MyObj",time=3,traj=matrix(c(1:6,NA,8:12),ncol=3))
> imputeMyObj(a)
> -----------------

Hi Christophe --

The 'usual' way to write the above code is

> a <- imputeMyObj(a)

with imputeMyObj designed to take a 'MyObj' as it's argument, and
return a modified 'MyObj' (that the user can assign to 'a', if they
like). Inside imputeMyObj, the developer would, in the end, write
something like

impuateMyObj <- function(obj) {
   # calculate imputed values 'imp'
   slot(obj, "traj") <- imp
   obj
}

A better design would have a 'setter' method, minimally

> setGeneric("traj<-",
+            function(object, ..., value) standardGeneric("traj<-"))
[1] "traj<-"
> setReplaceMethod("traj",
+                  signature=signature(
+                    object="MyObj",
+                    value="matrix"),
+                  function(object, ..., value) {
+                      slot(object, "traj") <- value
+                      object
+                  })
[1] "traj<-"

and then the impute code would have

impuateMyObj <- function(obj) {
   # calculate imputed values 'imp'
   traj(obj) <- imp
   obj
}

It's possible to design 'MyObj' so that it can be modified in-place
(e.g., storing data in a slot of class 'environment', which has
reference-like behavior) but this will probably surprise both you and
the user.

Martin

> I find 'setTime<-' to change le slot time, but it can not work in the 
> case of imputeMyObs since this mehod does not need a value...
>
> Any solution ?
>
> Thanks
>
> Christophe
>
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-- 
Martin Morgan
Computational Biology / Fred Hutchinson Cancer Research Center
1100 Fairview Ave. N.
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