Re: [R] Testing for S4 objects (redux)

Spencer Graves Sat, 04 Dec 2004 17:00:48 -0800

Which version of R?

I just replicated it in R 2.0.0patched. I know that 2.0.1 is now available, but I haven't found time to upgrade yet.

By the way, I can replicate the error message with only one line:

> getClass("by")
Error in getClass("by") : "by" is not a defined class

     hope this helps.  spencer graves

John Fox wrote:

Dear John,
I've encountered the following problem:
x <- rnorm(100) y <- sample(2, 100, replace=TRUE) res <- by(x, y, mean) res

INDICES: 1 [1] 0.1129494 ------------------------------------------------------------ INDICES: 2 [1] -0.2066684

isS4object <- function(object)(length(attr(object, "class"))==1 &&

+ !is.null(getClass(class(object))))

isS4object(res)

Error in getClass(class(object)) : "by" is not a defined class
For a simple question, this has gotten rather complicated. Any suggestions
would be appreciated.
Regards,
John
-------------------------------- John Fox Department of Sociology McMaster University Hamilton, Ontario Canada L8S 4M4 905-525-9140x23604 http://socserv.mcmaster.ca/jfox --------------------------------
-----Original Message----- From: John Chambers [mailto:[EMAIL PROTECTED] Sent: Tuesday, November 30, 2004 9:40 AM To: John Fox Cc: Martin Maechler; [EMAIL PROTECTED] Subject: Re: [R] Testing for S4 objects

Let me suggest a different test, because slotNames was written to work differently when given a string or a class definition. With your definition,
R> x <- "classRepresentation"
R> isS4object(x)
[1] TRUE
which I assume is not what you wanted.  (Given a single string,
slotNames() tries to look up the class definition of that name.)
How about the following? The logic is that an S4 object must have an actual class attribute of length 1 (that rules out basic data types, where class(x) is a string but there is no actual attribute, and also rules out some S3 objects). Then if that's true, try to look up the class definition. If it is non-null, seems like an S4 object.
R> isS4object <- function(object)(length(attr(object, "class"))==1 &&
+     !is.null(getClass(class(object))))
R> isS4object(x)
[1] FALSE
R> isS4object(getClass(class(x)))
[1] TRUE
This definition seems to work, at least on the examples I could think of right away. Notice though, that some classes, such as "ts", that have been around for a long while are nevertheless legitimate S4 classes, so:
R> t1 = ts(1:12)
R> isS4object(t1)
[1] TRUE
(this applies to either version of isS4object).
There are a couple of details, more appropriate for the r-devel list. Seems a good candidate for a function to add to R.

On Sat, 27 Nov 2004 17:48:30 -0500, John Fox <[EMAIL PROTECTED]> wrote:
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:[EMAIL PROTECTED]
Sent: Friday, November 26, 2004 3:18 AM
To: John Fox
Cc: [EMAIL PROTECTED]
Subject: Re: [R] Testing for S4 objects
"JohnF" == John Fox <[EMAIL PROTECTED]> 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
______________________________________________ [EMAIL PROTECTED] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
______________________________________________ [EMAIL PROTECTED] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html


--
Spencer Graves, PhD, Senior Development Engineer
O:  (408)938-4420;  mobile:  (408)655-4567

______________________________________________
[EMAIL PROTECTED] mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html

Re: [R] Testing for S4 objects (redux)

Reply via email to