[Octave-bug-tracker] [bug #60531] dbstack(N) not omitting innermost frames as it should

[Rik]

Follow-up Comment #2, bug #60531 (project octave):

Perhaps jwe knows why this changed.  The code is in do_dbstack in the file
debug.cc.  The interesting bit is


  octave::tree_evaluator& tw = interp.get_evaluator ();

  if (nargout == 0)
    {
      octave_map stk = tw.backtrace (curr_frame);
      octave_idx_type nframes = stk.numel ();

      if (nframes > 0)
        {
          octave::preserve_stream_state stream_state (os);

          os << "stopped in:\n\n";

          Cell names = stk.contents ("name");
          Cell files = stk.contents ("file");
          Cell lines = stk.contents ("line");

          bool show_top_level = true;

          std::size_t max_name_len = 0;

          for (octave_idx_type i = nskip; i < nframes; i++)
            {
              std::string name = names(i).string_value ();

              max_name_len = std::max (name.length (), max_name_len);
            }

          for (octave_idx_type i = nskip; i < nframes; i++)
            {
              std::string name = names(i).string_value ();
              std::string file = files(i).string_value ();
              int line = lines(i).int_value ();

              if (show_top_level && i == curr_frame)
                show_top_level = false;

              os << (i == curr_frame ? "  --> " : "      ")
                 << std::setw (max_name_len) << name
                 << " at line " << line
                 << " [" << file << ']'
                 << std::endl;
            }

          if (tw.at_top_level () && show_top_level)
            os << "  --> top level" << std::endl;
        }
    }
  else
    {
      octave_map stk = tw.backtrace (curr_frame, false);

      // If current stack frame is not in the list curr_frame will be
      // -1 and either nskip caused us to skip it or we are at the top
      // level, which is not included in the list of frames.  So in the
      // interpreter, 0 will be our invalid frame index value.

      retval = ovl (stk, curr_frame + 1);
    }


When the true branch of the if statement is taken, nskip is used and only the
frames requested are printed.  This matches the behavior I mentioned in
comment #1.  When the else branch is taken, Octave just gets the entire stack
frame and returns it.

Maybe there is a way to call in to the interpreter and get just the number of
stack_frames desired.  I looked at pt-eval.h and call-stack.h, but I wasn't
sure.

Alternatively, all of the stack frames are returned in an octave_map (i.e., a
struct array).  One could check for nskip > 0 and if necessary, create a new
appropriately sized octave_map and copy over the desired stack frames in a for
loop.  Alternatively, one could use an index() call.  If this were in an
m-file, one would write


new_map = old_map(nskip:end);


It should be possible to do something similar in C++.

Or one could keep the old map and use delete_elements() to get rid of the
frames one didn't need.

I've added jwe because I think he might instantly have an intuition about the
best approach.

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