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Re: ode45 gives solution in the "Dark Side," but not in Octave

From: John B. Thoo
Subject: Re: ode45 gives solution in the "Dark Side," but not in Octave
Date: Thu, 3 Sep 2009 11:33:48 -0700

On Sep 2, 2009, at 5:30 PM, John W. Eaton wrote:

On  2-Sep-2009, John B. Thoo wrote:

| On Sep 1, 2009, at 3:00 PM, John W. Eaton wrote:
| > On  1-Sep-2009, John B. Thoo wrote:
| >
| > | Hi. I have a question about using ode45 in Octave 3.2.2 (and OdePkg
| > | on Mac OS X 10.4.11) vs. in the "Dark Side" 7.4.0 (R2007a) (on a
| > | server ... Linux?).  My code can be found here at the moment:
| > |
| > | <>
| > |
| > | "run_twoway.m" executes the code.
| > |
| > | Both Octave and the Dark Side solve the problem (i.e., do not return
| > | an error) to produce a solution  u,  and both give
| > |
| > |  > size (u)
| > | ans =
| > |
| > |     8192    102
| > |
| > | However, while I can plot the solution in the Dark Side, in Octave I
| > | can plot only the first four frames.  In fact, in Octave I get
| > NaN +
| > | 0.00000i  from  u(:,5)  on.  Why is that?
| > |
| > | I would appreciate any hints.  Thanks.
| >
| > Have you tried using lsode in Octave instead of ode45 (which is not
| > part of Octave, BTW).
| I haven't tried lsode.  The reason for my using ode45 is my advisor
| uses the Dark Side only, and we need to run the same code.  Thanks.

Here's a simple wrapper for lsode that I think will provide a simple
ode45 interface.  It doesn't cover all the options, but it will
probably work in a lot of cases, provided that you just need a
solution, and not specifically a Runge-Kutta method.  It shoudl be
fairly easy to fix the interface if ti isn't compatible enough for


function [t_out, x] = ode45 (f, t, x0, options)

  if (nargin == 3 || nargin == 4)

    global __ode45s_rhs_fcn__;
    __ode45s_rhs_fcn__ = f;

    if (nargin == 4)
      if (isfield (options, "RelTol") && ! isempty (options.RelTol))
        lsode_options ("relative tolerance", options.RelTol)
      if (isfield (options, "AbsTol") && ! isempty (options.AbsTol))
        lsode_options ("absolute tolerance", options.AbsTol)

    x = lsode (@__lsode_rhs__, x0, t);
    t_out = t;

    print_usage ();


% ----------------------------------------------------------------------

function xdot = __lsode_rhs__ (x, t)

  global __ode45s_rhs_fcn__;

  xdot = __ode45s_rhs_fcn__ (t, x);


Hi, jwe.  Thanks for taking time on this.

I'm sorry, but I'm not familiar with using a "wrapper," so I haven't tried it, yet. Do I save the above to a .m file? Then how do I call on it?

In any case, I tried lsode directly in my code, in which, making a guess for how to proceed, I replaced the lines in "run_twoway.m"

options = odeset ();
[t, w] = ode45 (@F, tspan, w0, options, k, lx, ltrunc, epsilon, mu);

with the line

[w, istate, msg] = lsode (@F, tspan, w0, k, lx, ltrunc, epsilon, mu);

But when I executed the code, I got this:

octave-3.2.2:5> run_twoway
error: Invalid call to lsode.  Correct usage is:

 -- Loadable Function: [X, ISTATE, MSG] = lsode (FCN, X_0, T, T_CRIT)

Additional help for built-in functions and operators is
available in the on-line version of the manual.  Use the command
`doc <topic>' to search the manual index.

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Thanks again.


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