>>>>> "Roger" == Roger Bivand <[EMAIL PROTECTED]>
>>>>> on Wed, 15 Nov 2006 19:27:11 +0100 (CET) writes:
Roger> On Wed, 15 Nov 2006, "BaRow" == Barry Rowlingson wrote:
> BillD> It does not remove names in Splus either, just all
> BillD> other attributes. I see c() used in several Splus
> BillD> functions as a way to convert a matrix into a vector
> BillD> (by removing the .Dims and .Dimnames attributes).
>>
BaRow> I dont see the logic in certain attribute names
BaRow> (?attr lists 'class', 'comment', 'dim', 'dimnames',
BaRow> 'names', 'row.names' and 'tsp') being 'special'. Why
BaRow> are matrices vectors with some 'magic' attribute
BaRow> 'dim', and possibly some non-magic attributes like
BaRow> 'umbongo' and 'fnord'? I'd have the dim (and other
BaRow> essential properties of data structures) more tightly
BaRow> bound to the data. attr(M,'dim')=NULL seems a very
BaRow> odd way of turning a matrix into a vector.
BaRow>
BaRow> If I was the R bdfl[1] I'd deprecate using 'attr' to
BaRow> access these aspects of data objects and insist on
BaRow> using "dim(M)" or "row.names(M)=foo".
Roger> I have a less than vague feeling that you as bdfl are
Roger> describing new-style classes:
indeed, thank you, Roger!
Roger> library(Matrix)
Roger> x <- Matrix(1:6, nrow=2, ncol=3)
Roger> getSlots(class(x))
Roger> slot(x, "Dim")
Roger> slot(x, "Dim") <- NULL
which gives
Error in checkSlotAssignment(object, name, value) :
assignment of an object of class "NULL" is not valid for slot "Dim" in
an object of class "dgeMatrix"; is(value, "integer") is not TRUE
Roger> but perhaps unfortunately:
Roger> slot(x, "Dim") <- as.integer(c(0,0,0))
Roger> works! Maybe not quite what was intended?
Indeed it does not signal an error, surprisingly to you and others.
Note however that subsequently
> validObject(x)
Error in validObject(x) : invalid class "dgeMatrix" object: Dim slot must be
of length 2
. . . . .
This seems to have become a QFAQ (quite frequently asked question):
Q: Why does R not signal an error when I assign invalid contents
to a slot of an S4 object?
A: Calling validObject() on every slot assignment would
- inhibit building S4 objects incrementally ``until they are valid''
- potentially lead to too slowly executing code.
Consequence:
Only ``low-level'' programming should use
slot(A, "sn") <- .... or the equivalent
A @ sn <- ....
and then the programmeR is solely responsible to construct a
valid object -- or needs to expliclictly call validObject() at
the end of the construction process.
Normal construction of S4 objects should use
- either new("..", .....)
which calls validObject()
[unless for new("...") with no extra args]
- or higher level constructors
such as you did in the example by calling Matrix().
John Chambers has much more to say on this, but I think most of
it has already been written in the Green Book.
--
Martin Maechler, ETH Zurich
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