why are nan being produced when assigning values in a cube?

Question

I'm having this weird issue with Armadillo and RcppArmadillo. I'm creating a cube filled with zeroes values, and I want specific elements to be turned into ones. However, when I used an assigment to do that, values of other elements changle slightly and often become equal to nan. Does anyone has any idea what could be causing that?

example:

#include <RcppArmadillo.h>

using namespace arma;


// [[Rcpp::depends(RcppArmadillo)]]
// [[Rcpp::export]]

cube testc() {
  cube tester = cube(10,10,2);
  uvec indexes = {25,125};
  for(unsigned int i=0; i<indexes.n_elem; i++) {
  tester(indexes(i))=1.0;
  };
  cout<< tester;
  return(tester);

} 

This error does not happen when i assign each element individually (tester(25)=1.0 followed by tester(125)=1.0), but this is impractical if I have a larger number of elements to replace. The nan show up in coutand in the R object, which makes me think the issue is independent of Rcpp.

Answer 1

Your cube object is not initialized with zeros, so it's possible to get NaN values.

From the documentation:

Constructors:

cube()
cube(n_rows, n_cols, n_slices)     (memory is not initialised)
cube(n_rows, n_cols, n_slices, fill_type)     (memory is initialised)
...

• When using the cube(n_rows, n_cols, n_slices) or cube(size(X)) constructors, by default the memory is uninitialised (ie. may contain garbage); memory can be explicitly initialised by specifying the fill_type, as per the Mat class (except for fill::eye)

Examples of explicit initialization with zeros:

cube A(10,10,2,fill::zeros);

cube B(10,10,2);
B.zeros();

cube C;
C.zeros(10,10,2);