[Rd] passing matrix as argument to a C function

[Prof Brian Ripley]

The data in an R matrix is not stored as double **A, but as double *A: it 
is just a vector in R, with attributes.

There are lots of examples of manipulating R matrixes via .C: one is 
src/library/stats/src/kmeans.c.

The code for crossprod is not at all hard to find: it is in 
src/main/array.c.  However, is just a call to an LAPACK function.
The code there for matprod would be more informative.

On Sun, 3 Dec 2006, Christos Hatzis wrote:

> Hi,
>
> Although this is not directly an R-related question, it is relevant as I am
> trying to port some R code to C to speed things up in a computation.
>
> I am working through my first attempts to generate and link compiled C code
> in R.  I could make the 'convolve' function to work and similar functions
> that take vectors as arguments.  In my application I need to pass a couple
> of matrices to the C function and could not figure out how to make it work
> in C.
>
> As an example, I tried to code first a simple matrix-vector product
> function.  Here is my code:
>
> void matrix_mult( double **A, double *b, double *ab, int *nrow, int *ncol )
> {
>    int i, j, nr = *nrow, nc = *ncol;
>    for( i = 0; i < nr; i++ )
>    {
>        ab[i] = 0.0;
>        for( j = 0; j < nc; j++ )
>            ab[i] += A[i][j] * b[j];
>    }
> }
>
> As I understand, passing the matrix A as (double **) is not standard C and
> does not compile (to try things out I am using Microsoft Visual C++ compiler
> in Windows XP).  I tried to find the C code for crossprod for some hints,
> but could not locate it in the R source.  But probably this does not use the
> .C interface.
>
> Is there a way this can be done with .C?  I suspect it will use in some way
> the "vector of pointers to row vectors" concept, but I am not familiar
> enough in C yet to figure it out.  Any hints on how in can be done with
> .Call if easier?
>
> Thank you.
> -Christos
>
> Christos Hatzis, Ph.D.
> Nuvera Biosciences, Inc.
> 400 West Cummings Park
> Suite 5350
> Woburn, MA 01801
> Tel: 781-938-3830
> www.nuverabio.com <http://www.nuverabio.com/>
>
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>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
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