An earlier article discussed sparse matrix conversion but stopped short of showing how to create custom as<>() and wrap() methods or functions. This post starts to close this gap.

We will again look at sparse matrices from the Matrix package for R, as well as the SpMat class from Armadillo.
At least for now we will limit outselves to the case of double element types. These uses the sp_mat typedef which will be our basic type for sparse matrices at the C++ level.

At the time of writing, this code had just been added to the SVN repo RcppArmadillo as an extension header file spmat.h. Further integration is planned, but no concrete steps are planned just yet.

First, we look at the as method.

// [[Rcpp::depends(RcppArmadillo)]]

#include <RcppArmadillo.h>

namespace Rcpp {
    
    // converts an SEXP object from R which was created as a sparse
    // matrix via the Matrix package) into an Armadillo sp_mat matrix
    //
    // TODO: template'ize to allow for types other than double, though
    //       realistically this is all we need
    template <> arma::sp_mat as(SEXP sx) {
        S4 mat(sx);  
        IntegerVector dims = mat.slot("Dim");
        arma::urowvec i = Rcpp::as<arma::urowvec>(mat.slot("i"));
        arma::urowvec p = Rcpp::as<arma::urowvec>(mat.slot("p"));     
        arma::vec x     = Rcpp::as<arma::vec>(mat.slot("x"));
        
        int nrow = dims[0], ncol = dims[1];
        arma::sp_mat res(nrow, ncol);

        // create space for values, and copy
        arma::access::rw(res.values) = arma::memory::acquire_chunked<double>(x.size() + 1);
        arma::arrayops::copy(arma::access::rwp(res.values), x.begin(), x.size() + 1);

        // create space for row_indices, and copy 
        arma::access::rw(res.row_indices) = 
            arma::memory::acquire_chunked<arma::uword>(i.size() + 1);
        arma::arrayops::copy(arma::access::rwp(res.row_indices), i.begin(), i.size() + 1);
    
        // create space for col_ptrs, and copy 
        arma::access::rw(res.col_ptrs) = arma::memory::acquire<arma::uword>(p.size() + 2);
        arma::arrayops::copy(arma::access::rwp(res.col_ptrs), p.begin(), p.size() + 1);

        // important: set the sentinel as well
        arma::access::rwp(res.col_ptrs)[p.size()+1] = std::numeric_limits<arma::uword>::max();
    
        // set the number of non-zero elements
        arma::access::rw(res.n_nonzero) = x.size();

        return res;
    }

}

Next, we look at the corresponding wrap() method.

namespace Rcpp {

    // convert an Armadillo sp_mat into a corresponding R sparse matrix
    // we copy to STL vectors as the Matrix package expects vectors whereas the
    // default wrap in Armadillo returns matrix with one row (or col) 
    SEXP wrap(arma::sp_mat sm) {

        IntegerVector dim(2);
        dim[0] = sm.n_rows; 
        dim[1] = sm.n_cols;

        arma::vec  x(sm.n_nonzero);        // create space for values, and copy
        arma::arrayops::copy(x.begin(), sm.values, sm.n_nonzero);
        std::vector<double> vx = arma::conv_to< std::vector< double > >::from(x);

        arma::urowvec i(sm.n_nonzero);	// create space for row_indices, and copy & cast
        arma::arrayops::copy(i.begin(), sm.row_indices, sm.n_nonzero);
        std::vector<int> vi = arma::conv_to< std::vector< int > >::from(i);
 
        arma::urowvec p(sm.n_cols+1);	// create space for col_ptrs, and copy 
        arma::arrayops::copy(p.begin(), sm.col_ptrs, sm.n_cols+1);
        // do not copy sentinel for returning R
        std::vector<int> vp = arma::conv_to< std::vector< int > >::from(p);

        S4 s("dgCMatrix");
        s.slot("i")   = vi;
        s.slot("p")   = vp;
        s.slot("x")   = vx;
        s.slot("Dim") = dim;
        return s;
    }

}

We can now illustrate this with a simple example.

// [[Rcpp::export]]
arma::sp_mat doubleSparseMatrix(arma::sp_mat m) {
    Rcpp::Rcout << m << std::endl;  // use the i/o from Armadillo
    arma::sp_mat n = 2*m;
    return n;
}

First, we create a sparse matrix. We then the function we just showed to to a minimal (and boring) transformation: we double the values of the matrix. The key really in the seamless passage of matrix A from R down to the C++ code where it is accessed as m, and the return of the new matrix n which becomes B at the R level.

suppressMessages(library(Matrix))
i <- c(1,3:8)              # row indices
j <- c(2,9,6:10)           # col indices
x <- 7 * (1:7)             # values
A <- sparseMatrix(i, j, x = x)
A 


8 x 10 sparse Matrix of class "dgCMatrix"
                             
[1,] . 7 . . .  .  .  .  .  .
[2,] . . . . .  .  .  .  .  .
[3,] . . . . .  .  .  . 14  .
[4,] . . . . . 21  .  .  .  .
[5,] . . . . .  . 28  .  .  .
[6,] . . . . .  .  . 35  .  .
[7,] . . . . .  .  .  . 42  .
[8,] . . . . .  .  .  .  . 49

B <- doubleSparseMatrix(A) # this will print A from C++


[matrix size: 8x10; n_nonzero: 7; density: 8.75%]

     (0, 1)          7.0000
     (3, 5)         21.0000
     (4, 6)         28.0000
     (5, 7)         35.0000
     (2, 8)         14.0000
     (6, 8)         42.0000
     (7, 9)         49.0000

B


8 x 10 sparse Matrix of class "dgCMatrix"
                              
[1,] . 14 . . .  .  .  .  .  .
[2,] .  . . . .  .  .  .  .  .
[3,] .  . . . .  .  .  . 28  .
[4,] .  . . . . 42  .  .  .  .
[5,] .  . . . .  . 56  .  .  .
[6,] .  . . . .  .  . 70  .  .
[7,] .  . . . .  .  .  . 84  .
[8,] .  . . . .  .  .  .  . 98

identical(2*A, B)


[1] TRUE