>>>>> "BDR" == Prof Brian Ripley <[EMAIL PROTECTED]>
>>>>> on Mon, 6 Feb 2006 19:44:50 +0000 (GMT) writes:
BDR> An S4 object is just a list with attributes, so a
BDR> vector type. match() works with all vector types
BDR> including lists, as you found out (or could have read).
yes, the internal representation of S4 objects is such -- seen
from a non-S4 perspective.
BDR> If in the future those proposing it do re-implement an
BDR> S4 object as an new SEXP then this will change, but for
BDR> now the cost of detecting objects which might have an
BDR> S4 class defined somewhere is just too high (and would
BDR> fall on those who do not use S4 classes).
Just for further explanation, put into other words and a
slightly changed point of view:
Yes, many R functions get confused by S4 objects,
most notably, c() (!)
- because they only look at the "internal representation"
- and because it's expensive to always ``look twice'';
particularly from the internal C code.
There's a relatively simple check from R code which we've
using for str() :
>> if(has.class <- !is.null(cl <- attr(object, "class"))) { # S3 or S4 class
>> ## FIXME: a kludge
>> S4 <- !is.null(attr(cl, "package")) || cl == "classRepresentation"
>> ## better, but needs 'methods': length(methods::getSlots(cl)) > 0
>> }
which --- when only testing for S4-presence --- you could collapse to
if(!is.null(cl <- attr(object, "class")) &&
(!is.null(attr(cl, "package")) ||
cl == "classRepresentation")) {
...have.S4.object...
}
but note the comment >>>> ## FIXME: a kludge <<<
The solution has been agreed to be changing the internal
representation of S4 objects making them a new SEXP (basic R
"type"); and as Brian alludes to, the problem is that those in
R-core that want to and are able to do this didn't have the time
for that till now.
Martin Maechler, ETH Zurich
BDR> On Mon, 6 Feb 2006, Seth Falcon wrote:
>> If one accidentally calls match(x, obj), where obj is any S4 instance,
>> the result is NA.
>>
>> I was expecting an error because, in general, if a match method is not
>> defined for a particular S4 class, I don't know what a reasonable
>> default could be. Specifically, here's what I see
>>
>> setClass("FOO", representation(a="numeric"))
>> foo <- new("FOO", a=10)
>> match("a", foo)
>> [1] NA
>>
>> And my thinking is that this should be an error, along the lines of
>> match("a", function(x) x)
>>
>> Unless, of course, a specific method for match, table="FOO" has been
>> defined.
BDR> --
BDR> Brian D. Ripley, [EMAIL PROTECTED]
BDR> Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
BDR> University of Oxford, Tel: +44 1865 272861 (self)
BDR> 1 South Parks Road, +44 1865 272866 (PA)
BDR> Oxford OX1 3TG, UK Fax: +44 1865 272595
BDR> ______________________________________________
BDR> R-devel@r-project.org mailing list
BDR> https://stat.ethz.ch/mailman/listinfo/r-devel
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