Problem with xtensor xt::where index related function

Question

Here I'm trying to do a very basic operation with the xtensor library in C++. I have the xarray a, and with the index related function xt::where, I want to get an array of indexes where the condition holds True (beware, there is another xt::where function, but it is an operator function and I don't want it). When I try to compile it, with this line, I get a lot of errors:

g++ -I/usr/include/xtensor -I/usr/local/include/xtl getindx.cpp -o getindx

Curiously, when I try to use the other xt::where function (the operator function), it works and compiles and runs. I'm clearly missing something; I search for it, but I can't get through, please help me! Thank you.

Here is the code:

#include "xtensor/xarray.hpp"
#include "xtensor/xio.hpp"
#include "xtensor/xview.hpp"
#include "xtensor/xoperation.hpp"
#include "xtensor/xtensor.hpp"


using namespace std;

int main(int argc, char** argv){

  xt::xarray<double> arr {5.0, 6.0, 7.0};
  auto idx = xt::where(arr >= 6);

  std::cout << idx << std::endl;

 return 0;

}

EDIT: the error.

error: no match for ‘operator<<’ (operand types are ‘std::ostream {aka std::basic_ostream<char>}’ and ‘std::vector<std::vector<long unsigned int>, std::allocator<std::vector<long unsigned int> > >’)
   std::cout << idx << std::endl;

EDIT2: solved without xtensor. Maybe it will be a bit slower.

int main(int argc, char** argv){

 
  std::vector<double> arr{5.0,6.0,7.0};
  std::vector<unsigned int> indices;
  
  auto ptr = &bits[0];
  for (int i = 0; i<arr.size(); i++, ptr++)
    {
      if (*ptr>=6) indices.push_back (i);    
   }

  for (int i=0; i<indices.size(); i++){
    cout << "indices= "indices[i] << endl;
  } //output: indices=1, indices=2.
 return 0;
}

Answer 1

The problem is that xt::where (or maybe the syntax xt::argwhere is more descriptive here) returns an std::vector or array indices for which there is no operator<< overload e.g. for printing.

To deal with this xt::from_indices was created. From the relevant docs page:

int main()
{
    xt::xarray<size_t> a = xt::arange<size_t>(3 * 4);

    a.reshape({3,4});

    auto idx = xt::from_indices(xt::argwhere(a >= 6));

    std::cout << idx << std::endl;
}

In this case idx can also be typed xt::xarray<size_t> or xt::xtensor<size_t, 2> if you'd want to be more verbose that with auto.