Thank you. The code in kmeans.c is indeed informative. One needs to remember that an R matrix is a vector with attributes.
-Christos -----Original Message----- From: Prof Brian Ripley [mailto:[EMAIL PROTECTED] Sent: Sunday, December 03, 2006 3:25 AM To: Christos Hatzis Cc: r-devel@r-project.org Subject: Re: [Rd] passing matrix as argument to a C function 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/> > > ______________________________________________ > R-devel@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel > -- Brian D. Ripley, [EMAIL PROTECTED] 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) Oxford OX1 3TG, UK Fax: +44 1865 272595 ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
