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

[John Swensen]

On Jun 27, 2008, at 9:44 AM, Jaroslav Hajek wrote:

> On Fri, Jun 27, 2008 at 3:01 PM, John Swensen  
> <address@hidden> wrote:
> > 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.
> > > --
> > > View this message in context: 
> > > http://www.nabble.com/Techniques-in-using-liboctave-and-liboctinterp-from-C%2B%2B-program-tp18137086p18139690.html
> > > Sent from the Octave - General mailing list archive at Nabble.com.
> > >
> > > _______________________________________________
> > > Help-octave mailing list
> > > address@hidden
> > > https://www.cae.wisc.edu/mailman/listinfo/help-octave
> > 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?
> I believe it is not, you just see a random error. I get a segfault
> with the same program.
> The problem is that you invoke binary ops on octave_value objects, but
> the typeinfo instance is not initialized. You can do this manually by
> calling the instance_ok static method. Your program still won't work
> as the types and ops must be registered first, but at least it will
> give you a proper error message. See install_types() and install_ops()
> for registering all common types and ops. I believe this also
> eliminates the need to initialize typeinfo manually.
>
>
> cheers
>
>
> > John Swensen
> > _______________________________________________
> > Help-octave mailing list
> > address@hidden
> > https://www.cae.wisc.edu/mailman/listinfo/help-octave
>
>
>
> --
> RNDr. Jaroslav Hajek
> computing expert
> Aeronautical Research and Test Institute (VZLU)
> Prague, Czech Republic
> url: www.highegg.matfyz.cz

That was exactly what I needed.  But, instead of trying to figure out  
all that needs initializing, I simply called octave_main() with the  
embedded flags set and then was able to do operations on Matrix and  
ColVectors and scalars just as I wanted to.  Thanks!

John Swensen