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

[Romain Francois]

the .col method gives you a subview_col :

   arma_inline       subview_col<eT> col(const uword col_num);
   arma_inline const subview_col<eT> col(const uword col_num) const;

not an umat.

The name of the class implies that it is a "view" class, so a way to 
look a data from another class. hence, no data of its own, so cheap copy.

double f3(arma::subview_col<unsigned int> Z) {
   Z(1, 0) = 223;
   return 99.9;
}

Also, including RcppArmadillo.h after Rcpp.h is wrong. You should only 
include RcppArmadillo.h.

I should do something so that the compiler tells you this.

Romain


Le 10/12/12 16:49, Ramon Diaz-Uriarte a écrit :
>
>
> 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);
> }
>
>
>
>
>


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