[Numpy-discussion] numpy and roundoff(?)

Lisandro Dalcin dalcinl@gmail....
Sat Mar 1 16:19:37 CST 2008

On 3/1/08, Charles R Harris <charlesr.harris@gmail.com> wrote:
> So they differ in the least significant bit. Not surprising, I expect the
> Fortran compiler might well perform operations in different order,
> accumulate in different places, etc. It might also accumulate in higher
> precision registers or round differently depending on hardware and various
> flags.

Of course, but a completely unrelated but equivalent C implementation
of this problem, as you can check in line 313 at this link


behaves almost the same that my F90 implemented residual. Perhaps
Fortran compiler (gfortran) will generate the same code as the C one,
but I'm not sure, Fortran compilers can be smarter that C compilers
for this kind of looping.

> The exp functions in Fortran and C might also return slightly
> different results.

I believe this is not the source of the problem, I've tried commenting
that term, and differences are still there.

> I don't think the differences are significant, but if you
> really want to compare results you will need a higher precision solution to
> compare against.

I agree, the differences are not significant, but they end up having a
noticeable impact. I'm still surprised!.

Let's stop all this now. I'll be back as soon as I can produce some
self-contained code to show and reproducing the problem.

Lisandro Dalcín
Centro Internacional de Métodos Computacionales en Ingeniería (CIMEC)
Instituto de Desarrollo Tecnológico para la Industria Química (INTEC)
Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)
PTLC - Güemes 3450, (3000) Santa Fe, Argentina
Tel/Fax: +54-(0)342-451.1594

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