Re: how to use fft in c++

[Pantxo]

Carlo de Falco-2 wrote
>> On 7 Jun 2017, at 21:27, Progressive <

> Progressive@

> > wrote:
>> 
>> Hi,
>> 
>> I have a c++ file with octave embedded in it. Calculating via Matrices
>> and
>> so on works fine but I don't get it how do I use functions, e.q. the fft
>> in
>> c++ / upon variables from/in c++ ?
>> 
>> The documentation
>> https://www.gnu.org/software/octave/doc/v4.0.1/Signal-Processing.html
>> just
>> tells fft(x).
>> 
>> Just briefly: I have an array in c++ and I want to apply fft on it. How
>> do I
>> do that?
>> 
>> Thank you very much!
>> 
> 
> you could invoke the interpreter doing something like this (untested):
> 
>  ColumnVector x, y;
>  octave_value_list in, out;
> 
>  in(0) = x;
>  out = feval ("fft", in, 1);
>  y = out(0).column_vector_value ();
> 
> but if you are looking for performance you are probably better off using
> fftw's API directly.
> 
> c.
> 
> 
> 
> 
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Since fft and related function are builtin you probably can include
<octave/builtin-defun-decls.h> and call Ffft directly (without the need for
feval) in your code. As an example the following code can be compiled with
mkocfile and works for me: 

//////////////////////////testfft.cc//////////////////
#include <octave/oct.h>
#include <octave/builtin-defun-decls.h>

DEFUN_DLD(testfft, args, nargout, "\
testfft\n\
")
{
  octave_value_list retval;
  int nargin = args.length ();
  
  retval = Ffft (args);
  return retval;
}
/////////////////////////////////////////////////////

Compile and test in Octave:

mkoctfile testfft.cc
x = 1:10;
all (testfft (x) == fft (x))
 
Pantxo



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