Well, I had a PDE which reduced to a set of ODEs via the method of
characteristics. I wanted to know the solution on a fixed grid, so for a
given grid point, I would integrate along the characteristic curve to
the boundary, and then use the boundary condition as the initial
condition to integrate backwards to the original grid point where I
wanted to find the solution. Changing the sign of driver->h worked very
nicely but it would be best to make a function to do this, and check
that its less than hmax, etc.
As a side note, does the _reset function reset the step size back to the
original hstart value given to the alloc routines? A quick glances
indicates that it doesn't. Does this mean that when you start a new
integration, it begins with the last step size from the previous
integration?
Also I was using driver_apply for both integrations. I kept a tight
tolerance on hmax to make sure it didn't go too far past the boundary or
too far past the original point.
Patrick
On 03/04/2012 12:02 AM, Tuomo Keskitalo wrote:
Hello,
that's an interesting use case. May I ask why you needed to integrate
backwards? Did you use driver_apply for integration?
I haven't really considered anyone "hotswapping" the direction of
integration using driver functions.. I think that the stepper at least
must be reset if integration direction is changed, to be safe with
multistep methods. This needs some thinking and testing. I can try to
see to this in a week or few.
Tuomo
On Fri, Mar 2, 2012 at 6:13 PM, Patrick Alken
<patrick.alken@colorado.edu> wrote:
I had a problem recently where I needed to integrate forward along a
curve
(positive step size h) and then integrate backward again (negative
h). But
there is currently no way to change the sign of the step size h using
the
driver routines, so I had to modify driver->h directly.
I propose adding a routine gsl_odeiv2_driver_set_h() to allow the
user to
reset the step size to what they want. Tuomo: do you see any negative
issues
with this? I can make the addition if you want.
Thanks,
Patrick