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// -*- mode: poly-C++R; c-indent-level: 4; c-basic-offset: 4; indent-tabs-mode: nil; -*-
/**
* @title Sparse matrix in Armadillo
* @author Dirk Eddelbuettel
* @license GPL (>= 2)
* @tags armadillo matrix featured
* @summary This example shows how to create a sparse matrix in Armadillo
*
* The Matrix package in R supports sparse matrices, and we can use
* the S4 class in Rcpp to attach the different component row indices,
* column pointers and value which can then be used to initialize an
* Armadillo sparse matrix.
*
* Let's start with creating a sparse matrix.
*
*/
/*** R
suppressMessages(library(Matrix))
i <- c(1,3:8)
j <- c(2,9,6:10) a
x <- 7 * (1:7)
A <- sparseMatrix(i, j, x = x)
print(A)
*/
/**
* The following C++ function access the corresponding slots of the
* `sparseMatrix` object, and creates a `sp_mat` Armadillo object.
*/
#include <RcppArmadillo.h>
// [[Rcpp::depends(RcppArmadillo)]]
using namespace Rcpp ;
// [[Rcpp::export]]
void convertSparse(S4 mat) { // slight improvement with two non-nested loops
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);
// 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 -- so far in a lame loop
arma::access::rw(res.row_indices) =
arma::memory::acquire_chunked<arma::uword>(x.size() + 1);
for (int j=0; j<i.size(); j++)
arma::access::rwp(res.row_indices)[j] = i[j];
// create space for col_ptrs, and copy -- so far in a lame loop
arma::access::rw(res.col_ptrs) =
arma::memory::acquire<arma::uword>(p.size() + 2);
for (int j=0; j<p.size(); j++)
arma::access::rwp(res.col_ptrs)[j] = p[j];
// 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();
Rcout << "SpMat res:\n" << res << std::endl;
}
/**
* Running this example shows the same matrix printed to `stdout` by
* Armadillo.
*/
/***R
convertSparse(A)
*/
/**
* Support for sparse matrix is currently still limited in Armadillo,
* but expected to grow. Likewise, RcppArmadillo does not yet have
* `as<>()` and `wrap()` converters but we expect to add these
* eventually --- at which point this example will be much simpler.
*/
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