Re: How to use "do_colon_op" in an oct-file?

[John W. Eaton]

On 10/15/2015 11:29 AM, Lukas Reichlin wrote:

> I'm trying to substitute the following function with C++ using DEFUN_DLD:
>
> function [mat_idx, opt_idx] = __lti_input_idx__ (varargin)
>
>    str_idx = find (cellfun (@ischar, varargin));
>
>    if (isempty (str_idx))
>      mat_idx = 1 : nargin;
>      opt_idx = [];
>    else
>      mat_idx = 1 : str_idx(1)-1;
>      opt_idx = str_idx(1) : nargin;
>    endif
>
> endfunction
>
> In the file  libinterp/corefcn/data.cc,  I found the expression
>
>         retval = do_colon_op (args(0), args(1));
>
> on line 6322 (inside definition of DEFUN (colon, args, …) on lines 6305-6335) 
> and tried to use it in my C++ code.

I don't think you need do_colon_op for this job.  You just need to 
return Range or Matrix objects.

  Range mat_idx (1, idx);
  Range opt_idx (idx+1, len);

  retval(1) = opt_idx;
  retval(0) = mat_idx;

Note that you probably want to either set the size or retval when you 
create it, or assign the elements from N-1 to 0 so that resizing only 
happens once instead of N times.

jwe