[Rcpp-devel] Creating a multidimensional array

[Dirk Eddelbuettel]

On 31 July 2018 at 13:55, Barth Riley wrote:
| Dear list
| 
| I am trying to create a simple Rcpp function that creates a three-dimensional array based on the values of a four-dimensional array. The arrays are declared as NumericVectors with a dim attribute represent the number of levels of each dimension. Here is a sample of the code:
| 
| NumericVector sumby4(NumericVector X) {
|   Dimension dim = X.attr("dim");
|   if(dim.size() != 4) return NULL;
|   
|   NumericVector ret = NumericVector::create(dim[0],dim[1],dim[2]);
| 
|   for(int i = 0; i < dim[0]; i++) {
|     for(int j = 0; j < dim[1]; j++) {
|       for(int k = 0; k < dim[2]; k++) {
|         ret(i,j,k) = NA_REAL;
| …
| 
| When I attempt to compile the package in which this function resides, I get the error message associated with the “ret(I,j,k) = NA_REAL” line:
| 
| No match for call to Rcpp::NumericVector Rcpp::Vector<14 Rcpp::PreserveStorage>})(int&, int&, int&)
| 
| I take from this message that the NumericVector ret cannot be indexed by three values, even though it has been dimensioned as a 3D array. What am I doing wrong here?

Yes, the compiler barks because it sees no matching method.

Now, we _have_ examples for this somewhere I think.  One thing that came up in a
quick search is

   http://gallery.rcpp.org/articles/simple-array-class/

and you can see there that i) yes a multi-dim is used but ii) indexing games
need to be played.  Maybe that helps.

Else arma::cube is a real 3d data I have used happily in the past. And these
days CRAN has xtensor (if you can do C++14).

Dirk

-- 
http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org