[Rcpp-devel] Sparse matrices with RcppArmadillo

Le 08/12/12 09:45, S?ren H?jsgaard a ?crit :

Dear all,

I want to use a matrix (of type "dgCMatrix" from the Matrix package) in RcppArmadillo, so I do:

library(inline)
src <- '
using namespace arma;
using namespace Rcpp;
SpMat<double> X = as<SpMat<double> >(XX_);
'
foo <- cxxfunction(signature(XX_=""), body=src, plugin="RcppArmadillo")

- but this fails. It seems to me (browsing the web) that SpMat are supported, but I might be wrong here. I have no indication that dgCMatrix matrices can be "converted" to SpMat's. I know that I can work with dgCMatrix matrices with RcppEigen, but what I need is to extract submatrices and that is very easy to do with RcppArmadillo.

Any thoughts? Thanks in advance.
Regards
S?ren

Doug might know better about the internals of Matrix types. This is just 
following the recipee from how these are handled in RcppEigen:

#include <RcppArmadillo.h>
// [[Rcpp::depends(RcppArmadillo)]]
using namespace Rcpp ;

// [[Rcpp::export]]
void convert(S4 mat){
     IntegerVector dims = mat.slot( "Dim" ) ;
     IntegerVector i = mat.slot( "i" ) ;
     IntegerVector p = mat.slot( "p" ) ;
     NumericVector x = mat.slot( "x" ) ;

     int nrow = dims[0], ncol = dims[1] ;
     arma::sp_mat res( nrow, ncol) ;
     for(int j = 0; j < ncol; ++j) {
         for (int k = p[j]; k < p[j + 1]; ++k) res( i[k], j ) = x[k];
     }
     std::cout << res << std::endl ;

}


/*** R
     require(Matrix)
     i <- c(1,3:8); j <- c(2,9,6:10); x <- 7 * (1:7)
     ( A <- sparseMatrix(i, j, x = x) )

     convert(A)
***/

I don't think there is a better way to fill multiple values ina SpMat, 
maybe Conrad has insights.

Romain

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