[R] How to change the value of a class slot

[Henrik Bengtsson]

Berton Gunter wrote:
>>Second, in my experiments I couldn't get setReplacementMethod to work:
>>
>>"bumpIndex<-" <- function(pm, value) {
>>  pm at i <- pm at i+as.integer(value)
>>  pm
>>}
>>
>># I get an error without the next function definition
>>bumpIndex <- function(pm) pm at i
>>
>>setReplaceMethod("bumpIndex",
>>                 signature=signature(pm="CompletePathMaker",
>>                   value="numeric"), bumpIndex) 
>>
> 
> 
> 
> 'bumpIndex' is apparently not a generic function (created by setGeneric)
> signature is not an argument to setReplaceMethod 
> The ... argument to setReplaceMethod must include a function to do the
> replacement, which is why you need the bumpIndex function (I agree that this
> is not cleanly documented, but it's fairly easy to see since
> setReplaceMethod calls setMethod and that requires the function argument).
> 
> so I don't see why you expect this to work at all. At the very least,
> shouldn't you do what the documentation tells you to? I think that you are
> expecting R to work according to your paradigm rather than trying to
> understand how it actually works. If you feel that R's paradigm (and/or
> documentation) is too awful to bother with, you might consider the R.oo
> package, which does OOP an entirely different way.

I have not followed the thread before, but here is an how it could look 
like with R.oo:

setConstructorS3("CompletePathMaker", function(index=0) {
   extend(Object(), "CompletePathMaker",
     index = as.integer(index)
   )
})

setMethodS3("increase", "CompletePathMaker", function(this, dindex=+1) {
   this$index <- this$index + as.integer(dindex);
   # You do not have to return anything here....
})

cpm <- CompletePathMaker()
cpm$index <- 3
increase(cpm, 2)
print(cpm$index)

To add some control of what type of values you can assign to 'index', 
you can define a virtual field function set<Field>(), e.g.

setMethodS3("setIndex", "CompletePathMaker", function(this, index) {
   if (!is.numeric(index) || length(index) != 1)
     stop("Argument 'index' must be a single numeric: ", mode(index));
   if (index < 0)
     stop("Argument 'index' must be non-negative: ", index);
   this$index <- as.integer(index);
   # ... or here.
})

cpm$index <- 4
cpm$index <- -3  # Gives an error as wanted!

(Again...) note that setConstructorS3() and setMethodS3() are 
conveniency wrappers to define the CompletePathMaker(), 
increaseBumpIndex.CompletePathMaker() and 
setBumpIndex.CompletePathMaker(). The implementation of references is 
taken care of by the special class Object, which also implements support 
for virtual fields.

/Henrik

> -- Bert Gunter
> 
> 
> 
> 
>>When I try to load this, I get
>>arguments in definition changed from (spec) to (object)
>>arguments in definition changed from (self) to (object)
>>arguments in definition changed from (self) to (object)
>>Creating a new generic function for 'bumpIndex<-' in '.GlobalEnv'
>>Error in conformMethod(signature, mnames, fnames, f) : 
>>	In method for function "bumpIndex<-": formal arguments 
>>omitted in the
>>method definition cannot be in the signature (value = "numeric")
>>
>>All the errors are triggered by setReplaceMethod.  Can anyone help me
>>interpret them?  Or, maybe better, tell me how to debug the
>>"compilation"?
>>
>>
>>
>>>I leave it to language "experts" to say whether S4 formal 
>>
>>classes and
>>
>>>methods are wise or not in comparison to others. From my 
>>
>>fairly ignorant
>>
>>>perspective, that always seems to be a matter of taste.
>>
>>There are actually two related issues on that score: first, 
>>whether the
>>complex of expectation set up by talking about "objects" and "classes"
>>are met by what R/S does, and second the wisdom of what R/S 
>>does in its
>>own right.
>>
>>
>>>Cheers,
>>>Bert
>>>
>>>
>>
>>-- 
>>Ross Boylan                                      wk:  (415) 502-4031
>>530 Parnassus Avenue (Library) rm 115-4          ross at biostat.ucsf.edu
>>Dept of Epidemiology and Biostatistics           fax: (415) 476-9856
>>University of California, San Francisco
>>San Francisco, CA 94143-0840                     hm:  (415) 550-1062
>>
>>
> 
> 
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