[petsc-users] Example 1 of Vector section

[Fazlul Huq]

Got it.
Thanks a lot.

Best,
Huq

On Mon, Sep 24, 2018 at 8:26 PM Matthew Knepley <knepley at gmail.com> wrote:

> On Mon, Sep 24, 2018 at 9:02 PM Fazlul Huq <huq2090 at gmail.com> wrote:
>
>> Hello All,
>>
>> I am looking through the example problems given in PETSc. In the very
>> first example, ex1 of vector section, I don't understand the following
>> part, especially the term "PETSC_SMALL". What does this mean?
>>
>
> This checks that we get the norm we expect. PETSC_SMALL is a small
> difference we are willing to accept
> due to roundoff error.
>
>    Matt
>
>
>> Here, in the second line we already assign zero to v. What is the meaning
>> of "if" condition then?
>>
>> ierr = VecScale(x,two);CHKERRQ(ierr);
>>   ierr = VecNorm(x,NORM_2,&norm);CHKERRQ(ierr);
>>   v    = norm-2.0*PetscSqrtReal((PetscReal)n); if (v > -PETSC_SMALL && v
>> < PETSC_SMALL) v = 0.0;
>>   ierr = PetscPrintf(PETSC_COMM_WORLD,"VecScale
>> %g\n",(double)v);CHKERRQ(ierr);
>>
>>
>> All the best.
>> Sincerely,
>> Huq
>> --
>>
>> Fazlul Huq
>> Graduate Research Assistant
>> Department of Nuclear, Plasma & Radiological Engineering (NPRE)
>> University of Illinois at Urbana-Champaign (UIUC)
>> E-mail: huq2090 at gmail.com
>>
>
>
> --
> 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/>
>


-- 

Fazlul Huq
Graduate Research Assistant
Department of Nuclear, Plasma & Radiological Engineering (NPRE)
University of Illinois at Urbana-Champaign (UIUC)
E-mail: huq2090 at gmail.com
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20180924/e09bd98c/attachment-0001.html>