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

Ramon Diaz-Uriarte rdiaz02 at gmail.com
Mon Dec 10 16:49:26 CET 2012

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?


// [[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);


Ramon Diaz-Uriarte
Department of Biochemistry, Lab B-25
Facultad de Medicina 
Universidad Autónoma de Madrid 
Arzobispo Morcillo, 4
28029 Madrid

Phone: +34-91-497-2412

Email: rdiaz02 at gmail.com
       ramon.diaz at iib.uam.es


More information about the Rcpp-devel mailing list