[R] Call and memory

[Bob Wheeler]

I use a large real matrix, X, in C code that is passed from R and 
transposed in place in the C code. I would like to conserve memory and, 
if possible, allocate space for only one copy of X -- hence I would like 
to pass a pointer to the data in the X object to the C code.

The Writing R Extensions manual says that neither .Call nor .External 
copy their arguments. They also say that these arguments should be 
treated as read only.

Fine, but in testing I seem to be able to transpose very large X's in 
place, in C code without an error. This leads me to assume that the 
manual was just giving good advice about treating arguments as read 
only. However, I find that I have done nothing to the X in R. It seems 
that a copy has been made after all. I may as well call the code with .C 
  and avoid the use of macros.

Could someone please point out the error in my thinking or suggest a way 
to accomplish my goal?

My code follows:

"mListTest" <-
function(X,N,k) {

	.Call("mList",as.double(X),as.integer(N),as.integer(k));
}


SEXP mList(
	SEXP Xi,
	SEXP Ni,
	SEXP ki
)
{
	double *pX=NUMERIC_POINTER(Xi);
	int	N=INTEGER_POINTER(Ni)[0];
	int k=INTEGER_POINTER(ki)[0];
	SEXP alist;
	SEXP avector;
	SEXP nvector;
	SEXP rvector;
	SEXP kvector;
	int n;
	int i;

	transposeMatrix(pX,N,k);

	n=4;
	PROTECT(alist=NEW_LIST(n));
	PROTECT(avector=NEW_NUMERIC(200));

	for (i=0;i<200;i++) {
		NUMERIC_POINTER(avector)[i]=pX[i];
	}
	SET_ELEMENT(alist,0,avector);
	UNPROTECT(1);
	PROTECT(nvector=NEW_INTEGER(1));
	INTEGER_POINTER(nvector)[0]=N;
	SET_ELEMENT(alist,1,nvector);
	UNPROTECT(1);
	PROTECT(rvector=NEW_NUMERIC(1));
	NUMERIC_POINTER(rvector)[0]=0.5;
	SET_ELEMENT(alist,2,rvector);
	UNPROTECT(1);
	PROTECT(kvector=NEW_INTEGER(1));
	INTEGER_POINTER(kvector)[0]=k;
	SET_ELEMENT(alist,3,kvector);
	UNPROTECT(1);

	UNPROTECT(1);
	return alist;

}

-- 
Bob Wheeler --- http://www.bobwheeler.com/
         ECHIP, Inc. ---
Randomness comes in bunches.