Re: Techniques in using liboctave and liboctinterp from C++ program

[John Swensen]

On Jun 26, 2008, at 1:56 PM, dbateman wrote:

> John Swensen wrote:
> > I have started using the datatypes and operations of Octave from C++
> > for my research and have a few questions.
> >
> > 1) Can builtin functions and DLD-FUNCTIONS (e.g. find, eig, conv2,
> > etc) be used directly from C++?
>
> You can always use feval to call an m-file or oct-file. You can't  
> really
> call an oct-file directly from C++. What you can have for example is  
> if the
> oct-file calls another C++ function, you might declare that function  
> extern
> and call it as well, though the oct-file would have to be loaded first
>
>
>
> > 2) In the documentation, is there a list of overloaded operators for
> > octave_value objects of different types?  For example, I was trying  
> > to
> > do the following:
> > Matrix A(768,1024);
> > Matrix B(1,1);
> > Matric C = A*B;
> > This, however, didn't work.  Instead, I had to declare B as a scalar
> > double as follows:
> > Matrix A(768,1024);
> > double b;
> > Matrix C = A*B;
> > This isn't necessarily a problem, but it would be nice if there was a
> > list of possible overloads somewhere.
>
> That's not so easy for two reasons. Firstly someone has to do the  
> work of
> documenting them. However, the second reason is harder. Having  
> documented
> these functions they then define an API to the internals of Octave,
> something that has never really been stable. Therefore some thought  
> needs to
> go into this process of what functionality to document in an API.
>
>
>
> > 3) I often do image thresholding after I have processed it a bit.  In
> > the Octave interpreter I can simply type 'im2 = im1>0.5'.  I tried
> > doing the following, but it didn't work:
> > Matrix A(768,1024);
> > double B = 0.5;
> > Matrix C = A>B;
> > If I could find out where that bit of the interpreter was  
> > implemented,
> > I could figure it out, but I searched through the sources a bit and
> > couldn't find anything.
>
> I don't have a copy of Octave with me, but check the file
> src/OPERATORS/op-m-m.cc for the gt function and see what it does.
>
> D.
> --
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So I have kindof muddled my way through the operator business.  Once I  
got my head wrapped around all the #define's, it makes a lot of  
sense.  However, the comparator operators are still not working.  I  
can get it to compile fine, but every time I try to do a compare, I  
get a "fatal: T& Array<T>::checkelem (10, -1, -1): range error" error  
message.

Here is a very simple example that causes the problem:
// compile with 'mkoctfile --link-stand-alone test.cpp -o test'
#include "octave/config.h"
#include "octave.h"
#include <octave/ov.h>

// Standard C++ includes
#include <iostream>
#include <string>
#include <vector>
#include <map>
using namespace std;

int main (int argc, char* argv[])
{
  octave_value tmp1(0.5);
  octave_value tmp2(1.0);
  octave_value tmp3(0.5);

  tmp1 == tmp2;

  return 0;
}

I'm assuming this is some sort of indexing error, since the  
checkelem() function in Array.h appears to be checking indices.  What  
I don't understand is why it thinks these two scalar octave_values are  
somehow 3-dimensional.  Why isn't it using the checkelem() that  
appears to be tailored for single dimensional octave_values?

John Swensen