From help-request at octave dot org Sat Feb  5 03:08:53 2005
Subject: Re: octave-speed
From: "Paul Thomas" <paulthomas2 at wanadoo dot fr>
To: =?iso-8859-1?Q?Rafael_Rodr=EDguez_Velilla?= <rrv66 at tid dot es>,         <help@octave.org>
Date: Sat, 5 Feb 2005 10:13:36 +0100

Rafael,

I was curious as to just what performance improvement could be obtained with 
a little C++ routine, so....... here it is.  It si bare bones, has no error 
or parameter checking but it works.  The commented out line uses the cmath 
library sqrt, instead of calling the octave sqrt through feval.  The times 
are:

Original in octave         5.2s
C++ call octave sqrt     0.58s
C++ call cmath sqrt      0.16s

The octave part-

tic;
Ns = 1e5;
m = [1:(Ns-1)];
T = (sqrt (m .* (m+1)));
n = zeros (1, Ns);
n(1) = 8;
n = funct_iter ("sqrt", T, n);    %call to .oct function
printf ("time=%g\n", toc);
printf ("n=%g\n", n(Ns));

and funct_iter.cpp -

#include <octave/oct.h>
#include <octave/parse.h>
#include <cmath>
DEFUN_DLD (funct_iter, args,,
          "Function iteration demo")
{
  octave_value_list retval;
  std::string fcn = args( 0 ).string_value();
  ColumnVector T(args (1).vector_value ());
  ColumnVector n(args (2).vector_value ());
  int Ns = n.rows ();
  for (int i = 0; i < (Ns - 1); i++)
    {
      octave_value argi (T(i)*n(i));
      octave_value_list fargs (argi);

/* feval returns an octave_value_list, (0) picks out the 1st item on the 
list and
    scalar_value () converts it into a scalar */

      n(i+1) = feval (fcn,  fargs, 1)(0).scalar_value ();

/*      n(i+1) = sqrt (n(i)*T(i)); */

    }
  retval(0) = n;
  return retval;
} 




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