Sparse matrices
In cpp11armadillo: An 'Armadillo' Interface

knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)

Limited support

cpp11armadillo, as of v0.5.0, supports the dgCMatrix class from the Matrix package. This class is a sparse matrix class that stores the matrix in a compressed column format.

The strategy for an efficient conversion from R to C++ and vice versa is to create an indexing of the non-zero elements of the input matrix and then use this indexing to create dgCMatrix (R) or SpMat (Armadillo/C++) object.

Note that cpp11 does not provide sparse matrices as it is the case for the dense data types doubles_matrix<> or integers_matrix<>. cpp11armadillo uses SEXP to provide a method to convert dgCMatrix objects to SpMat objects and vice versa using some properties of S4 objects.

Here is an example of how to convert a dgCMatrix object to a SpMat object and re-import it to R:

[[cpp11::register]] SEXP sum_matrices_(SEXP x) {
  // Convert from dgCMatrix to SpMat
  SpMat<double> A = as_SpMat(x);

  // Create a matrix B with a diagonal of random numbers
  SpMat<double> B(A.n_rows, A.n_cols);
  for (uword i = 0; i < A.n_rows; ++i) {
    B(i, i) = randu<double>();
  }

  A += B; // Add the two matrices

  // Convert back to dgCMatrix and return
  return as_dgCMatrix(A);
}