[R] The object argument of NextMethod.

[Gabor Grothendieck]

In section 5.5 of the language manual it says:

"It is important to realize that the choice of the next method depends on the
current values of .Generic and .Class and not on the object. So changing
the object in a call to NextMethod affects the arguments received by
the next method but does not affect the choice of the next method."

I think this needs to be put into ?NextMethod too since the current
wording in ?NextMethod appears to contract the language manual:

  "object: an object whose class will determine the method to be
          dispatched.  Defaults to the first argument of the enclosing
          function."

At any rate, try using .Class to direct it to the appropriate method like this:

> foo <- function(x) UseMethod("foo")
> foo.cls1 <- function(x) { x <- x+1; .Class <- class(x) <- "ncls"; NextMethod() }
> foo.ncls <- function(x) cat("ncls\n")
> foo.cls2=function(x) { cat("cls2\n"); print(x) }
> a <- 1; class(a) <- c("cls1","cls2")
>
> foo(a)
ncls

On 4/14/06, ronggui <ronggui.huang at gmail.com> wrote:
> My question is when the object argument of NexthMethod be used?
>
> In the following example, weather object argument is used will not
> affects the result.
>
> ###
> foo=function(x)  {UseMethod("foo")}
>
> foo.cls1=function(x)
> {
>  x=x+1;class(x)<-"ncls"
>  NextMethod()
> }
>
> foo.ncls=function(x)
> {
> cat("ncls\n")
> }
>
> foo.cls2=function(x)
> {
> cat("cls2\n");print(x)
> }
>
> a=1;class(a)=c("cls1","cls2")
>
> > foo(a)
> cls2
> [1] 2
> attr(,"class")
> [1] "ncls"
>
> ###
> > foo=function(x)  {UseMethod("foo")}
> >
> > foo.cls1=function(x)
> + {
> +   x=x+1;class(x)<-"ncls"
> +   NextMethod(,x)
> + }
> >
> > foo.ncls=function(x)
> + {
> + cat("ncls\n")
> + }
> >
> > foo.cls2=function(x)
> + {
> + cat("cls2\n");print(x)
> + }
> >
> > a=1;class(a)=c("cls1","cls2")
> >
> > foo(a)
> cls2
> [1] 2
> attr(,"class")
> [1] "ncls"
>
> Thank you very much.
>
> --
> »ÆÈÙ¹ó
> Deparment of Sociology
> Fudan University
>
>
>
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