[petsc-users] VecAXPY

[Matthew Knepley]

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/>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20190816/86433b62/attachment.html>