[R] How can I pass a R matrix as parameter to C code?

Thomas Lumley tlumley at u.washington.edu
Mon Mar 20 22:38:00 CET 2006

On Mon, 20 Mar 2006, johan Faux wrote:

> Hello,
>  Is it possible to pass R matrix as a parameter to an internal C procedure?
>  From the documentation I got the impression that only 1-dim vectors can be passed.
>  Why the following wont work for me?
>  > a<-matrix(1:15,3,5)
>> .C("pr",as.integer(a))
>  void pr(int **a){
>  a[1][1]=100;
> }

Because a is passed as int* rather than int**.

Matrices in R are stored as one-dimensional vectors with a "dim" 
attribute.  They can be passed to .C, but you have to pass the dim 
information separately, eg

   void pr(int *a, int *dim){

      int nrow, ncol;

      a[1] = 100; /* a[1,1] */
      a[(10-1) + (5-1) * nrow] = 100; /* a[10,5] */


called as
   .C("pr",as.integer(a), as.integer(dim(a)))

In fact, int** wouldn't be the right type even for a C two-dimensional 
array. The rule that array parameters decay to pointers is not applied 
recursively, so int ** is the type for a one-dimensional vector of 
pointers to int. If a variable has type int[3][5] it has to be passed as 
*int[5] or int[][5] so that the compiler knows how far it is from a[1][1] 
to a[2][1].


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