why-s of method dispatching

Thu, Mar 18, 2004 11:58 AM

I see. Thank you very much!

This brings another question. Does R-Core have any plan to promote
data.frame to an S4 class? In general, is there any "road-map" (formal
or informal) to phasing out S3 classes?

Thanks,
Vadim

-----Original Message-----
From: Prof Brian Ripley [mailto:ripley at stats.ox.ac.uk]
Sent: Thursday, March 18, 2004 12:24 AM
To: Vadim Ogranovich
Cc: R Help List
Subject: Re: [R] why-s of method dispatching


On Wed, 17 Mar 2004, Vadim Ogranovich wrote:

I am having a problem to understand why as.data.frame

method doesn't

dispatch properly on my class:

setClass("Foo", "character")

[1] "Foo"

as.data.frame(list(foo=new("Foo", .Data="a")))

Error in as.data.frame.default(x[[i]], optional = TRUE) :  can't 
coerce Foo into a data.frame

I was expecting that this would call as.data.frame.character.

You have set an S4 class and as.data.frame is an S3 generic.

list(foo=new("Foo", .Data="a"))

$foo
An object of class "Foo"
[1] "a"

and what as.data.frame sees is

attributes(list(foo=new("Foo", .Data="a"))$foo)

$class
[1] "Foo"
attr(,"package")
[1] ".GlobalEnv"

so thinks this is an S3 class it knows nothing about.

Another puzzle. If I explicitly call as.data.frame.character() it
would fail but for a different reason:

as.data.frame.character(list(foo=new("Foo", .Data="a")))

Error in unique.default(x) : unique() applies only to vectors

I was under an impression that an instance of "Foo" would

be welcome

anywhere a "character" was, but it seems to be more subtle.

What am I

missing?

The difference between S3 and S4 classes.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595