On 04/03/2012 22:26, Petr Savicky wrote:
On Sun, Mar 04, 2012 at 03:18:33PM -0600, Matyas Sustik wrote:
Hi All,
I am confused by a type conversion happening against my intent.
In an R script I allocate a matrix X, and I pass it to a C function
by using
tmp -< .C(..., as.double(X),...).
I use as.double() because I read that it makes sure that the
parameter passing is correct.
I return the matrix from the R script using:
return (list(..., X = tmp$X, ...))
The returned value is not a matrix but a flat vector.
I must not understand something fundamental that is happening
here.
Hi.
No, this is correct. See section 5.2 Interface functions .C and
.Fortran in R-exts.pdf for the list of C types. There is int * and
double *, which are vectors. Your R code should restore the dim
attribute after the return from C code, for example using matrix()
with appropriate nrow and ncol parameters.
'No, this is not correct'. If you want to use a double matrix in .C or
.Fortran, use
storage.mode(X) <- "double"
There are quite a few examples in R's own code, e.g. lm.fit.
If you want to pass really a matrix, use .Call, which is more
powerful.
But handling conversion in .Call is a lot trickier (especially if you
want to dispatch methods), so e.g. det() uses storage.mode at R level
before .Call.
Hope this helps.
Petr Savicky.
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