[Rd] method using several (and different) arguments in turn

james.foadi at diamond.ac.uk james.foadi at diamond.ac.uk
Tue Feb 14 18:57:14 CET 2012

Dear Martin,
I could not entirely follow your suggestion.
I can see how you define two classes inheriting from TypeOfSample, and how these two
classes have two associated methods (incidentally, I'm unfamiliar with the:

           function(type,x=numeric(),...)to do.


expression. What is type?

But then I'm lost. I wouldn't know how to carry on from here. 

I don't know if I have explained the key point clear enough before. For the person using
"newsample" it makes sense which name (x or y or z, etc) is used. newsample(x=12.4) would
give something different from newsample(y=12.4); and yet another result would be obtained if
using a combination, like newsample(x=12.4,y=12.4).

I wanted to use a simple function with default values at first, but I'm in the middle of developing
a package using S4 formalism. I'm not sure this would be a wise thing 
Sent: 14 February 2012 17:00
To: Foadi, James (Imperial Coll.,RAL,DIA)
Cc: r-devel at r-project.org
Subject: Re: [Rd] method using several (and different) arguments in turn

On 02/14/2012 08:43 AM, james.foadi at diamond.ac.uk wrote:
> Dear R-developers community, I have the following generic:
> setGeneric(
>                     name="newsample",
>                     def=function(x,y,z,a,b,c,...){standardGeneric("newsample")}
> And I can build several methods for this generic. One useful thing is to use "newsample"
> with only one of the 6 arguments listed. At the moment this is what I do:
> setMethod(
>                     f="newsample",
>                     signature=c("missing","missing","numeric","missing","missing","missing"),
>                     function(x,y,z,a,b,c,...)
>                     {
>                         ..............................
>                         ..............................
>                      }
>                      )
> This would be used when the single argument is z:
> newsample(z=12.5)
> To use newsample with another argument (say x) I should implement the same as before,
> but with signature c("numeric","missing","missing","missing","missing","missing").
> Is there another shorter and easier way to do this?

Hi James --

A matter of opinion, but multiple dispatch like this can be very
complicated, e.g., figuring out the 'next' method when dispatching on
two or more arguments; I'd really discourage it.

A different approach, assuming that x, y, z, ... are all numeric() but
that the sample to be drawn differs, is to define a small class
hierarchy to be used for dispatch.


   setClass("XSample", contains="TypeOfSample")
   XSample <- new("XSample")  ## a 'singleton', used for dispatch

   setClass("YSample", contains="TypeOfSample")
   YSample <- new("YSample")

and then

       function(type, x=numeric(), ...) standardGeneric("newsample"),

   setMethod("newsample", "XSample", function(type, x=numeric(), ...) {

   setMethod("newsample", "YSample", function(type, x=numeric(), ...) {

One could implement a default method on "TypeOfSample", and use
callNextMethod() after initial transformation, if that were the pattern.

To use:

   newsample(XSample, x=1:100)


> J

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