Fast and efficient coercion of standard matrix objects to sparse matrices.
Borrowed from http://gallery.rcpp.org/articles/sparse-matrix-coercion/.
INTERNAL USE ONLY.
as_dgCMatrix(XX_)
Arguments
XX_
An object of class Matrix that has a sparse structure
suitable for coercion to a sparse matrix format of dgCMatrix.
Value
An object of class dgCMatrix, coerced from input XX_.