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

[Ramon Diaz-Uriarte]

On Mon, 10 Dec 2012 17:06:06 +0100,Romain Francois <romain at r-enthusiasts.com> wrote:
> 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;
> }


Aha! Great, thanks!

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

Oooops. Will not do that again.


Best,

R.


> 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);
> > }
> >
> >
> >
> >
> >


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

Phone: +34-91-497-2412

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

http://ligarto.org/rdiaz