Field of matrices being returned with class `matrix` instead of `list`

[coatless]

Stumbled across an interesting hiccup in the exporter for arma::field<T> (the generic vector). In particular, the attributes associated with an arma::field<arma::mat> shows as matrix under a class(x) call instead of list. The only way to obtain the list type is to use a typeof() call.

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

// [[Rcpp::export]]
Rcpp::List test_case() {
  
  // Random dimensions
  int N = 5,
      P = 8,
      J = 10,
      K = 3,
      S = 2;

  // Create a 2d matrix
  arma::mat standard_2d_matrix(P, N);

  // Create a 3rd array
  arma::cube standard_3d_array(J, S, N);
  
  // Create a 4D Array
  arma::field<arma::cube> nonstandard_4d_array(N);
  
  // Fill it with cubes
  nonstandard_4d_array.fill(arma::ones<arma::cube>(J, K, S));
  
  return Rcpp::List::create(
    Rcpp::Named("d2", standard_2d_matrix),
    Rcpp::Named("d3", standard_3d_array),
    Rcpp::Named("d4", nonstandard_4d_array)
  );

}

The class issue is shown under d4 being matrix instead of list.

exporter_check = test_case() 

sapply(exporter_check, FUN = class)
#       d2       d3       d4 
# "matrix"  "array" "matrix" 
sapply(exporter_check, FUN = typeof)
#      d2       d3       d4 
# "double" "double"   "list" 

Moreover, the list is registering as a matrix with dim(x) of N x 1 instead of a list without dimensions.

sapply(exporter_check, FUN = dim)
# $d2
# [1] 8 5
# $d3
# [1] 10  2  5
# $d4
# [1] 5 1

And the object structure:

str(exporter_check)
# List of 3
# $ d2: num [1:8, 1:5] 1.49e-154 1.49e-154 2.57e-322 6.95e-310 6.95e-310 ...
# $ d3: num [1:10, 1:2, 1:5] 1.49e-154 1.49e-154 9.96e-320 1.63e-322 1.43e-322 ...
# $ d4:List of 5
#  ..$ : num [1:10, 1:3, 1:2] 1 1 1 1 1 1 1 1 1 1 ...
#  ..$ : num [1:10, 1:3, 1:2] 1 1 1 1 1 1 1 1 1 1 ...
#  ..$ : num [1:10, 1:3, 1:2] 1 1 1 1 1 1 1 1 1 1 ...
#  ..$ : num [1:10, 1:3, 1:2] 1 1 1 1 1 1 1 1 1 1 ...
#  ..$ : num [1:10, 1:3, 1:2] 1 1 1 1 1 1 1 1 1 1 ...
#  ..- attr(*, "dim")= int [1:2] 5 1

[coatless]

Yeah, so the offending piece of code is setting two dimensions and enforcing a matrix object return instead of a list. https://github.com/RcppCore/RcppArmadillo/blob/188beeae47cf354c4b00d8fdcdf6ec7255d36f92/inst/include/RcppArmadilloWrap.h#L133-L138

[mlampros]

For anyone else who might stumble upon this issue - as I did - the following worked for me:

// [[Rcpp::export]]
Rcpp::List conv_arma_field_to_rcpp_list(int init_arma_mat = 5,
                                        int dimensions = 6) {

  arma::field<arma::Mat<double> > Field_obj(init_arma_mat);

  for (unsigned int s = 0; s < init_arma_mat; s++) {

    int rows_cols = arma::randi<int>(arma::distr_param(dimensions, dimensions));
    Field_obj(s).zeros(rows_cols, rows_cols);
  }

  Rcpp::List list_obj = Rcpp::wrap( Rcpp::RcppArmadillo::FieldImporter< arma::Mat<double> >( Field_obj ) );
  return list_obj;
}

# res = conv_arma_field_to_rcpp_list()

[eddelbuettel]

This should now be fixed, albeit behind a #define that has to be turned on. See the new unit tests in tests_fields_new.R and the underlying C++ code (with the #define) in fields_new.cpp.