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

[Uwe Ligges]

Ross Boylan wrote:

> On Sat, Jun 04, 2005 at 07:08:56PM +0200, Martin Maechler wrote:
> 
>>    Ross> nextPath <- function(pm){ #pm is a CompletePathMaker
>>    Ross>    pm at i <- pm at i+as.integer(1)
>>    Ross> [etc]
>>
>>If your nextPath   function has  'pm' as its last statement it
>>will return the updated object, and if you call it
>>as
>>	mypm <- nextPath(mypm)
>>
>>you are
>>    1) updating  mypm
>>    2) in a proper S way (i.e. no cheating).
>>
>>Regards,
>>Martin
> 
> 
> Wow.  This is almost the exact inverse of the usual object behavior,
> in which only the class itself can update the slots (aka instance
> variables).  None of the methods of the class can update instances of
> the class persistently without the help of outsiders, and only
> outsiders can change the slot values.
> 
> (Yes, I realize that using the idiom you suggest of returning a new
> object one can have only class methods actually fiddling with the
> slots.)
> 
> The inability of a class method to change a class object without
> outside help seems unfortunate.
> 
> It looks as if instances of class objects are best thought of as
> immutable once created.

Obviously, there are many definition of "object oriented" programming, 
and yours seems to be different from the S4 definition.

I was going to answer your first question at first, but you have not 
given enough details - in particular it was not clear to me why your 
approach did not work. I assumed that you are assigning the new object 
again, which is the S way. You have to think about scoping rules and it 
will be clear that the approach you are expecting is not a clean one in S.

Uwe Ligges


> Ross
> 
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