Using Fields within RcppArmadillo

Question

Greetings and Salutations,

I'm trying to use the field object type instead of the list data type to avoid having to issue a copy command. I'm trying to do this to decrease the amount of time associated with when removing a matrix from the list to be operated on by a matrix already defined in armadillo's data structures. (e.g. removing arma::mat * as<arma::mat>(NumericMatrix) ).

Specifically, I'm trying to pass a field into a function and have it operated upon. The error message I receive upon compile is:

No matching constructor for initialization of 'arma::field< arma::Mat >'

This errors on the function definition: #include <RcppArmadillo.h>

NOTE: The objective is to be able to use Field as an object in a function declaration. This function would NOT need to be accessed or called by R! Mainly, the function would act as a helper function to a larger RcppArmadillo function.

The code I am trying to use is:

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

// [[Rcpp::export]]
arma::mat get_matrice_dimensions(arma::field<arma::mat> x) {

  /* Get the length of the list */
  unsigned int bin = x.n_elem;

  /* Obtain matrix size for each element */
  arma::mat storage(bin,2);
  /* Temporary storage that does not need to be created each time */
  arma::rowvec cur_elem_dims(2);
  for(unsigned int i=0; i<bin; i++){
    /* Get dimensions */
    /* NumericMatrix uses .nrow(), .ncol() vs. .n_rows, .n_cols in arma */
    cur_elem_dims(0) = x(i).n_rows; 
    cur_elem_dims(1) = x(i).n_cols;
    /* Store dimensions */
    storage.row(i) = cur_elem_dims;
  }
  /* Delete object */
  cur_elem_dims.clear();

return storage;
}

I envision the following being passed into the function:

x = list(matrix(0,nrow=5,ncol=3),matrix(1,nrow=5,ncol=3))
get_matrice_dimensions(x)

Answer 1

It is easy enough to come up with support for as<field<...>>. I just sent a pull request with this Exporter implementation:

template <typename T> 
class Exporter< arma::field<T> > {
public: 
    Exporter(SEXP x) : data(x){}

    inline arma::field<T> get() {
        size_t n = data.size() ;
        arma::field<T> out( n ) ;
        for(size_t i=0; i<n; i++){
            out[i] = as<T>(data[i]) ;   
        }
        return out ;
    }

private:
    List data ;
}; 

With this, I get:

$ RcppScript /tmp/arma.cpp

> direct_sum(list(matrix(1:16, nc = 4), matrix(1:16,
+     nc = 4)))
     [,1] [,2]
[1,]    4    4
[2,]    4    4

Answer 2

Something does not mesh here. You write (in very loud bold)

This function would NOT need to be accessed or called by R!

and proceed to show code with a [[Rcpp::export]] tag which is the very definition of access from R.

So you need to supply as<>() and wrap() as I commented earlier.