R internals matrix manipulations

Rinternals_create.cpp

#include <RcppArmadillo.h>
#include <Rinternals.h>
#include <Rcpp.h>

#include <iostream>

SEXP createMat(const SEXP& input)
{
  arma::mat matrix(REAL(input), Rf_nrows(input), Rf_ncols(input), false);
  
  // Create new SEXP
  unsigned int ncols = Rf_ncols(input);
  unsigned int nrows = Rf_nrows(input);
  SEXP res = Rf_allocMatrix(REALSXP, ncols, nrows);
  // Protect it from garbage collector
  res = Rf_protect(res);
  // Pass it to armadillo
  arma::mat result(REAL(res), Rf_nrows(res), Rf_ncols(res), false);
  // Do some processing
  //...
   
  // return the new array
  return res;
}

Rinternals_process_inplace.cpp

#include <RcppArmadillo.h>
#include <Rinternals.h>
#include <Rcpp.h>

#include <iostream>

SEXP readFromMat(const SEXP& mat)
{
  
  // Cast it as a double vector
  double* p = REAL(mat);
  
  //This outputs Rf_nrows(mat) * Rf_ncols(mat)
  std::cout << <<LENGTH(mat) << std::endl;
  
  // pass matrix to armadillo to process the array
  arma::mat matrix(p, Rf_nrows(mat), Rf_ncols(mat), false);
  
  // do some processing
  //  ...
  
  return mat;
}