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

[Romain Francois]

Le 10/12/12 17:06, Romain Francois a écrit :
> 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.

Done. The next version of RcppArmadillo will generate a compiler error 
when trying to compile a file that includes Rcpp.h before RcppArmadillo.h


> 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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Romain Francois
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