Re: odeiv2 request

[Tuomo Keskitalo <Tuomo.Keskitalo@iki.fi>]

Hello,

OK. I've committed modifications to ode-initval2 to bzr trunk, which 
contains new function gsl_odeiv2_driver_reset_hstart function for this. 
Hopefully that does the trick (safely).

You are right, driver_reset does not reset the original step size, but 
uses the last suggested value. Now, with driver_reset_hstart, that is 
hopefully more clearly implied.

BR,
Tuomo

On 03/04/2012 09:26 AM, Patrick Alken wrote:

> 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
>>
>>
>
>


-- 
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