[R] Method for $

[Duncan Temple Lang]

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Ulrike Grömping wrote:
> Uwe,
> 
> thank thank you very much for your suggestion. This does in fact work, if I 
> define
> 
> "$.myclass"<-function(x,slotname){slot(x,slotname)}
> 
> which solves my immediate need.
> 
> However, the setMethod version
> setMethod("$", signature(x="myclass"), function(x,slotname){ 
>   slot(x,slotname) 
>   } 
>  ) 
> still produces the same error message as before (cf. below). If anyone has a 
> solution for this, I would appreciate it.
> 

For the record, the problem you are experiencing is that
you are not using the same argument names in your
method as the function "$" uses. Hence there is a mismatch.
Use
 setMethod("$", signature(x="myclass"), function(x, name){
   slot(x, name)
 } )

i.e. rather than slotname as the second argument, use name
as that is what $ uses.

But, as Martin pointed out, this is not an advisable thing
to do.
And if you want to use an S4 class to house sub-elements,
either explicitly specify their types with
  setClass('myclass', representation( a = "integer",
                                      b = "matrix", ....)

or if you want them to be arbitary objects, then think
about why you are using S4.

> Uwe, my best wishes also to Dortmund and Lena, and by the way, it's your book 
> that got me started with understanding (part of) the programming aspects of R 
> relatively fast.
> 
> Regards,
> Ulrike Grömping
> 
> ---------- Original Message ----------- 
> From: Uwe Ligges <ligges at statistik.uni-dortmund.de> 
> To: Ulrike Grömping <groemp at tfh-berlin.de> 
> Cc: r-help at stat.math.ethz.ch 
> Sent: Fri, 18 Nov 2005 17:45:40 +0100 
> Subject: Re: [R] Method for $
> 
> 
>>Ulrike Grömping wrote: 
>>
>>
>>>Dear R experts, 
>>>
>>>I have defined a class "myclass" and would like the slots to be 
> 
> extractable 
> 
>>>not only by "@" but also by "$". I now try to write a method for "$" that 
>>>simply executes the request object at slotname, whenever someone calls 
>>>object$slotname for any object of class "myclass". 
>>>I don't manage to find out how I can provide this function 
> 
> with "slotname", 
> 
>>>so that one and the same function works for any arbitrary slotname a user 
>>>might choose. 
>>>
>>>I have tried 
>>>
>>>setMethod("$", signature(x="myclass"), function(x,slotname){ 
>>> x at slotname 
>>> } 
>>>) 
>>
>>Ulrike, 
>>
>>what about (untested!): 
>>slot(x, slotname) 
>>
>>Best wishes from Dortmund and Lena, 
>>Uwe Ligges 
>>
>>
>>>This produced the error message: 
>>>In method for function "$": expanding the signature to 
>>>include omitted arguments in definition: name = "missing" 
>>>Error in rematchDefinition(definition, fdef, mnames, fnames, signature) : 
>>>        methods can add arguments to the generic only if '...' is an 
> 
> argument 
> 
>>>to the generic 
>>>
>>>My searches for a solution of this problem have not been successful. Can 
>>>someone help? 
>>>
>>>Thanks and regards, 
>>>Ulrike Grömping, Berlin 
>>>
>>>______________________________________________ 
>>>R-help at stat.math.ethz.ch mailing list 
>>>https://stat.ethz.ch/mailman/listinfo/r-help 
>>>PLEASE do read the posting guide! http://www.R-project.org/posting-
> 
> guide.html 
> ------- End of Original Message -------
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html

- --
Duncan Temple Lang                duncan at wald.ucdavis.edu
Department of Statistics          work:  (530) 752-4782
371 Kerr Hall                     fax:   (530) 752-7099
One Shields Ave.
University of California at Davis
Davis, CA 95616, USA
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