[petsc-users] VecAXPY

[José Lorenzo]

I tried that one as well but the result was the same as using eta-eta_old
when calling VecAXPY.

Thank you.

El vie., 16 ago. 2019 18:26, Matthew Knepley <knepley at gmail.com> escribió:

> On Fri, Aug 16, 2019 at 12:22 PM José Lorenzo via petsc-users <
> petsc-users at mcs.anl.gov> wrote:
>
>> Hello,
>>
>> I am struggling with a strange error when using VecAXPY. I have a ghost
>> vector H that needs to be updated as
>>
>> H = H + eta * dH - eta_old * dH
>>
>> However, for some reason I obtain different results when using
>>
>> call VecAXPY(H, eta - eta_old, dH, ierr)
>>
>> instead of
>>
>> call VecAXPY(H, - eta_old, dH, ierr)
>>
>> call VecAXPY(H, eta, dH, ierr)
>>
>> where eta and eta_old are PetscScalars.
>>
>> The first option seems to provide a wrong output, but I do not understand
>> what can go wrong in such simple operation.
>>
> Fortran is really unforgiving about inputs. Try declaring a new
> PetscScalar diff = eta - eta_old and trying the first option with that.
>
>   Thanks,
>
>     Matt
>
>> Thank you.
>>
> --
> What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> -- Norbert Wiener
>
> https://www.cse.buffalo.edu/~knepley/
> <http://www.cse.buffalo.edu/~knepley/>
>
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