UgridPy wrappers for inquiring for variable names on a particular mesh

[arthurvd]

Is your feature request related to a problem? Please describe. All UgridPy API functions to get data from a file currently require a string variable name passed as a argument. This now can only be hardcoded by the user. Rather, I'd like to inquire which variables are in the file on a particular mesh.

Describe the solution you'd like

• A Py wrapper around the already existing ug_topology_get_data_variables_names() p.s.: Question: does that C++ function now return an array of single chars? Or an array of strings (one for each variable name?)
• A new function in UGrid and UGridPy that does something like this: In pseudocode:

  function ug_inq_varid(int file_id, int topology_type, int topology_id, char* variable_name)
  // check direct name: ncvar = ncfile.getVar(variable_name)
  // if ncvar has matching topology attributes (:mesh == topology_id..name) then success
  //
  // else, try ncvar = ncfile.getVar(mesh/topology_name + "_" + variable_name)
  // if ncvar has matching topology attributes (:mesh == topology_id..name) then success
  //
  // else, fallback:
  // success = ug_topology_get_data_variables_names(file_id, topology_type, topology_id, location, allvarnames)
  // const auto it = vars.find(name);
  // if (it == vars.end()) // etcetera

Describe alternatives you've considered Hardcoding is not an option, especially when data variable occur multiple times in the same file, for example: "mesh1d_waterdepth", "mesh2d_waterdepth"

Additional context This arose in a discussion with @d2hydro.

[arthurvd]

@lucacarniato answered this question:

does that C++ function now return an array of single chars? Or an array of strings (one for each variable name?)

The C api returns an error code, but the results are always the last parameters of the function, in this case a char *, so i flatten the results in vector_of_strings_to_char_array(). There is no separator, but it is assumed the individual data variable names do not exceed the ugrid::name_long_length.

To use this, you can count the variables using ug_topology_count_data_variables(), then allocate the memory on the client side, and loop throught it in these same-sized blocks of substrings.

If we would consider a char** to be the result instead, then we should test whether one can pass an array of char * (not only in python, but also to fortran).