[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.
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