R help - Mar 2012 - type conversion

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.

Should I remove the as.double() from the .C() call?

Here is what I am trying to do: allocate a matrix (with doubles)
that is modified by the .C() call and return it from the script.

I am also not sure how to make sure that the allocated matrix
is made up of double values.  If I use diag(n) to initialize
it will that do the trick?

Thanks!
-Matyas

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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.

If you want to pass really a matrix, use .Call, which is more
powerful.

Hope this helps.

Petr Savicky.

-------- Original message --------
Subject: type conversion
Date: Sun Mar 04 15:18:33 CST 2012
From: Matyas Sustik <msustik@gmail.com>
To: r-help@r-project.org

I got it.  Just documenting it on the list.
I need do: 
dim(tmp$X) <- c(n, k)
return (list(..., X = tmp$X, ...))

-Matyas

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