[Rcpp-devel] RcppArmadillo: passing matrix columns by reference

Dear All,

I am trying to pass columns from an Armadillo matrix to a function, but
I'd like to pass just a reference to the column, not a copy of the column
and I do not seem to be able to do it "elegantly".

The code below (function f1) I think shows that passing X.col to a
function creates a copy (X.unsafe_col does too). We can pass &X as
argument, and the index of the column, as in f2. And that will not create
a copy. But I think this is not the right way of doing what I want to do
(to begin with, I'd rather not pass the column index to the function).


What am I getting wrong?

Best,





// [[Rcpp::depends(RcppArmadillo)]]
#include <Rcpp.h>
#include <RcppArmadillo.h>

using namespace Rcpp;



double f1(arma::umat Z) {
   Z(0, 0) = 111;
   Z(9, 0) = 111;
   std::cout << "f1, this is Z " << std::endl << Z << std::endl;
   return 33.3;
}

double f2(arma::umat &Z, const int c1) {
   Z(1, 0) = 222;
   return 66.6;
}


double f3(arma::umat &Z) {
   Z(1, 0) = 223;
   return 99.9;
}


// [[Rcpp::export]]
List f0(IntegerVector s1_, IntegerVector c1_){
   const int  s1 = as<int>(s1_);
   const int  c1 = as<int>(c1_);

   arma::umat X(10, s1);
   for(int j = 0; j < s1; ++j) {
     for(int i = 0; i < 10; ++i) {
       X(i, j) = i * 10 + j;
     }
   }

   // in both cases, a copy seems to be made
   //double fitness = f1(X.col(c1));
   double outf1 = f1(X.unsafe_col(c1));
   std::cout << "f0, this is X after f1" << std::endl << X << std::endl;

   double outf2 = f2(X, c1);
   std::cout << "f0, this is X after f2" << std::endl << X << std::endl;


   // double outf3 = f3(X.unsafe_col(c1)); //will not work
   //double outf3 = f3(X.col(c1)); //will not work


   return List::create(Named("X") = wrap(X),
		      Named("of1") = outf1,
		      Named("of2") = outf2);
}