Re: Changing data type in octave_value_list [actually input args from DEFUN_DLD]

[Joshua Rigler]

<...original message deleted...>
Søren Hauberg wrote:
> Hi,
>   I'm not exactly sure if the following solve your problem, but it's
> worth a shot:
>
>   #include <octave/oct.h>
>   #include <cstdlib>
>   
>   DEFUN_DLD(bla, args, nargout, "...")
>   {
>     if (args(0).is_string()) {
>        args(0) = octave_value( atof(args(0).string_value().c_str()) );
>     }
>     return args;
>   }
>
> I didn't have convert_s2i, so I converted to a double instead. The real
> change is the use of the 'octave_value' constructor. Basicly you have to
> insert 'octave_value's in an octave_value_list.

Soren,

I think what you suggested *should* work, but it's just a slightly 
different version of something I already tried, which did not work.  So, 
I tried to come up with something even simpler in order to figure out 
what was going on, and discovered some very repeatable strangeness in 
Octave's dynamically loaded functions.  Would you, or other interested 
parties, look at the following short code snippet, which should compile 
easily using 'mkoctfile blah.cc', and consider a couple of questions?


  #include <octave/oct.h>

  DEFUN_DLD(blah, args, nargout, "...")
  {

    int i;
    octave_value_list rets;

    rets = args;

    for (i=0; i<args.length(); i++) {
      if (args(i).is_string()) {
        args(i) = octave_value (9999);
        rets(i) = octave_value (9999);
      }
    }
    //return rets;
    return args;
  }


I test this by [un]commenting the 'return rets;' line, recompiling, and 
re-running blah.oct with various inputs that are a mixture of strings 
and non-strings.  What I expect is that strings in both args(i) and 
rets(i) will get changed into the scalar 9999.  What actually happens is 
that *only* rets(i) gets modified, while args(i) remains unchanged.

What is so special about args here?  Why does it seem to be impossible 
to change any of its elements?  Is it not an octave_value_list like 
rets?  I'm sure the answer is related more to C++ than Octave, but I 
would love to be enlightened, nonetheless.

In any case, I think I've found a simple solution to my problem...just 
copy args(i) into a declared octave_value_list, and use that for all 
subsequent operations.  Thanks for your help.

-EJR