Rcpp Update matrix passed by reference and return the update in R

问题

I checked many examples about how to pass by reference using Rcpp. I see for instance this very great. However I have one question. Suppose that I have a matrix as an object in R and I want to add 1 to the entry [1,1]. The examples i saw work if the matrix is in Cpp but i want to return the update in R without using return statement.

This is an example i did with a list and it works very well

//[[Rcpp::export]]
void test(List& a){
 a(0)=0;
}

I need to do similarely with a matrix. something like :

//[[Rcpp::export]]
 void test(arma::mat& a){
  a(0,0)=0;
 }

The second does not update my matrix in R but updates the list.

Can anyone help me ?

回答1:

Let's start by reiterating that this is probably bad practice. Don't use void, return your changed object -- a more common approach.

That said, you can make it work in either way. For RcppArmadillo, pass by (explicit) reference. I get the desired behaviour

> sourceCpp("/tmp/so.cpp")

> M1 <- M2 <- matrix(0, 2, 2)

> bar(M1)

> M1
     [,1] [,2]
[1,]   42    0
[2,]    0    0

> foo(M2)

> M2
     [,1] [,2]
[1,]   42    0
[2,]    0    0
> 

out of this short example:

#include <RcppArmadillo.h>

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

// [[Rcpp::export]]
void bar(Rcpp::NumericMatrix M) {
  M(0,0) = 42;
}

// [[Rcpp::export]]
void foo(arma::mat M) {
  M(0,0) = 42;
}

/*** R
M1 <- M2 <- matrix(0, 2, 2)

bar(M1)
M1

foo(M2)
M2
*/

来源：https://stackoverflow.com/questions/46087970/rcpp-update-matrix-passed-by-reference-and-return-the-update-in-r