Lisandro Dalcin wrote: > On 3/1/08, Charles R Harris <[EMAIL PROTECTED]> 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 > > http://www-unix.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-current/src/snes/examples/tutorials/ex5.c.html > > 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.
At work we noticed a significant difference in results occurring in two versions of our code, an earlier version written in C++, and a later version written in Python/numpy. The algorithms were structured about the same. My colleague found the cause of the discrepancy; it turned out to be a difference in the way numpy and the C++ program were compiled. One used -mfpmath=sse, and the other, -mfpmath=387. Keeping them both the same cleared the discrepancy. Damian _______________________________________________ Numpy-discussion mailing list Numpy-discussion@scipy.org http://projects.scipy.org/mailman/listinfo/numpy-discussion
