Get each row of an arma::mat matrix as arma::vec in for loop

Question

I am using RcppArmadillo to create a function using stochastic simulation. I have trouble pulling out each row of a arma::mat as an arma::vec.

Below is a simplified example of my problem. I have used R nomenclature to illustrate what I am trying to achieve.

I believe there should be a fairly simple way of achieving this in C++, but I'm afraid I haven't figured it out yet. Any help would be much appreciated.

#include <RcppArmadillo.h>   
using namespace Rcpp;

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

// [[Rcpp::export]]
List function(List params) {
  arma::mat c= params["c"];
  arma::mat I= params["I"];
  
  
  for (int istep = 0; istep < (I.n_elem); istep++) {
    arma::vec loopedrows = I[istep,] //Here I have used the R indexing method, but this does not work in C++

    double product= accu(c*loopedrows)

    arma:vec newvec = stochastic_simulation(product) 

    I[istep+1,] =  newvec // store the output of the in matrix I, again the nomenclature is in R.
  }  

  
  return wrap(I);
};

Answer 1

Here is a simple (and very pedestrian, going step by step in the loop) answer for you.

Code

#include <RcppArmadillo.h>

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

// [[Rcpp::export]]
arma::mat rowwiseAdd(arma::mat A, arma::mat B) {
    if (A.n_rows != B.n_rows || A.n_cols != B.n_cols)
        Rcpp::stop("Matrices must conform.");

    arma::mat C(A.n_rows, A.n_cols);

    for (size_t i=0; i < A.n_rows; i++) {
        arma::rowvec a = A.row(i);
        arma::rowvec b = B.row(i);
        arma::rowvec c = a + b;
        C.row(i) = c;
    }
    return C;
}

/*** R
A <- matrix(1:9, 3, 3)
B <- matrix(9:1, 3, 3)
rowwiseAdd(A, B)
*/

Output

> Rcpp::sourceCpp("~/git/stackoverflow/70251105/answer.cpp")

> A <- matrix(1:9, 3, 3)

> B <- matrix(9:1, 3, 3)

> rowwiseAdd(A, B)
     [,1] [,2] [,3]
[1,]   10   10   10
[2,]   10   10   10
[3,]   10   10   10
>