Update Rcpp::NumericMatrix passed by reference using RcppArmadillo submat()

Question

Following on this question, I am trying to understand how to efficiently update a subset of a Rccp::NumericMatrix data type.

I have the following scenario:<

Answer 1

The left side of your assignment in updateMatrix creates a temporary that is discarded after assignment. Therefore, m doesn't change at all. The code can't work as you expected as the that would mean the type of m would change. Look below:

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


// [[Rcpp::export]]
void updateMatrix(const Rcpp::NumericMatrix &m)
{
  std::cout << m << std::endl;

  std::cout << typeid(m).name() << std::endl;

  arma::mat m2 = Rcpp::as(m);

  std::cout << typeid(m2).name() << std::endl;

  m2.submat(0, 0, 3, 3) = Rcpp::as(m).submat(0, 0, 3, 3) + 1;

  std::cout << m2 << std::endl;
}

Running this code gives:

> m = matrix(0, 5, 5)
> updateMatrix(m)
0.00000 0.00000 0.00000 0.00000 0.00000
0.00000 0.00000 0.00000 0.00000 0.00000
0.00000 0.00000 0.00000 0.00000 0.00000
0.00000 0.00000 0.00000 0.00000 0.00000
0.00000 0.00000 0.00000 0.00000 0.00000

N4Rcpp6MatrixILi14ENS_15PreserveStorageEEE
N4arma3MatIdEE
   1.0000   1.0000   1.0000   1.0000        0
   1.0000   1.0000   1.0000   1.0000        0
   1.0000   1.0000   1.0000   1.0000        0
   1.0000   1.0000   1.0000   1.0000        0
        0        0        0        0        0