Message-ID: <20041127224829.WDSK25979.tomts25-srv.bellnexxia.net@JohnDesktop8300>
Date: 2004-11-27T22:48:30Z
From: John Fox
Subject: Testing for S4 objects
In-Reply-To: <16806.59063.246234.858186@gargle.gargle.HOWL>
Dear Martin,
As it turns out, the test that I proposed (i.e., testing for NULL slotNames)
sometimes fails. For example:
> library(car)
> data(Prestige)
> sum <- summary(lm(prestige ~ income + education, data=Prestige))
> slotNames(sum)
character(0)
The following, however, seems to work (at least as far as I've been able to
ascertain):
isS4object <- function(object) length(slotNames(object)) != 0
I hope that this is a more robust test.
John
--------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario
Canada L8S 4M4
905-525-9140x23604
http://socserv.mcmaster.ca/jfox
--------------------------------
> -----Original Message-----
> From: Martin Maechler [mailto:maechler at stat.math.ethz.ch]
> Sent: Friday, November 26, 2004 3:18 AM
> To: John Fox
> Cc: r-help at stat.math.ethz.ch
> Subject: Re: [R] Testing for S4 objects
>
> >>>>> "JohnF" == John Fox <jfox at mcmaster.ca>
> >>>>> on Thu, 25 Nov 2004 22:28:50 -0500 writes:
>
> JohnF> Dear r-help list members, Is there a way to test
> JohnF> whether an object is an S4 object? The best that I've
> JohnF> been able to come up with is
>
> JohnF> isS4object <- function(object)
> !(is.null(slotNames(object)))
>
> you can drop one pair of "(..)" to give
>
> isS4object <- function(object) !is.null(slotNames(object))
>
>
> JohnF> which assumes that an S4 object has at least one
> JohnF> slot. I think this is safe, but perhaps I'm missing
> JohnF> something.
>
> The question is a very good one -- that I have posed to
> R-core a while ago myself.
>
> Inside utils:::str.default {which doesn't show the many
> commments in the *source* of str.default()}, I have wanted a
> way that even works when the 'methods' package is not
> attached and use the more obscure
>
> #NOT yet:if(has.class <- !is.null(cl <- class(object)))
> if(has.class <- !is.null(cl <- attr(object, "class")))#
> S3 or S4 class
> S4 <- !is.null(attr(cl, "package"))## <<<'kludge' FIXME!
> ##or length(methods::getSlots(cl)) > 0
>
> For the time being, I'd keep your function, but I don't think
> we'd guarantee that it will remain the appropriate test in
> all future. But till then many things will have happened (if
> not all of them ;-).
>
> Martin Maechler, ETH Zurich
>