# [Rd] R and C pointers

Simon Urbanek simon.urbanek at r-project.org
Tue Jun 19 18:38:43 CEST 2012

```On Jun 19, 2012, at 11:40 AM, Adrian Duşa wrote:

> Dear R devel,
>
> Apologies for these (most probably trivial) questions, doing my first
> attempt to call C from R (and actually learning C in the process).
>
> I need to pass a matrix to C, and after reading R-exts.pdf (many
> times), I was unable to find how to handle matrices at C-level...
> except for, what probably is the answer, that matrices are in fact
> vectors with dimensions.
>
> This is a sample code I am using at C level:
> ±±±±±±±±±±±±±±±±±±±±±±±±
> # include <R.h>
> # include <Rinternals.h>
>
> SEXP foo(SEXP x) {
>    SEXP dimx;
>    double *px, *pdimx;
>
>    PROTECT(dimx = getAttrib(x, R_DimSymbol));
>    px = REAL(x);
>    UNPROTECT(1);
>    return(dimx);
> }
> ±±±±±±±±±±±±±±±±±±±±±±±±
>
> The question is: how to create pointers to dimx (in order to extract
> individual values)?
>
> I tried:
> pdimx = REAL(dimx);
>
> This is where R complains that:
> REAL() can only be applied to a 'numeric', not a 'integer'
>

And it is right - dimensions are integers, not reals, you want

int *pdimx = INTEGER(dimx);

You can use REAL() only on real vectors, not on anything else.

> Much in the same line, what would be the procedure to create pointers
> to a logical vector y?
> I tried:
> PROTECT(y = coerceVector(y, LGLSXP));
> py = REAL(y);
>
> where R throws a similar error.
>

Obviously, you want

Rboolean *py = LOGICAL(y);

Cheers,
Simon

> Thanks very much in advance,
>
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```