odeiv2 request

[Patrick Alken]

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
>
>