[Numpy-discussion] Floating Point Difference between numpy and numarray

Hanni Ali hanni.ali@gmail....
Tue Sep 9 07:32:54 CDT 2008


I already was...

2008/9/9 Matthieu Brucher <matthieu.brucher@gmail.com>

> Use 1./n instead of 1/n. If n is an integer, 1/n equals 0.
>
> Matthieu
>
> 2008/9/9 Hanni Ali <hanni.ali@gmail.com>:
> > Hi Matthieu,
> >
> > Interesting example thanks. I can't however seem to get anything other
> than
> > zero for the 100,000 to 1 sum.
> >
> > Cheers,
> >
> > Hanni
> >
> > 2008/9/9 Matthieu Brucher <matthieu.brucher@gmail.com>
> >>
> >> > I now have a distinct dislike of float values (it'll probably wear off
> >> > over
> >> > time), how can the sum of 100,000 numbers be anything other than the
> sum
> >> > of
> >> > those numbers. I know the reasoning, as highlighted by the couple of
> >> > other
> >> > e-mails we have had, but I feel the default should probably lean
> towards
> >> > accuracy than speed. 2.0+2.0=4.0 and 2.0+2.0.....=200,000.0 not
> >> > 2array.sum()
> >> > != 200,000...
> >>
> >> In that case, we should not use doubles, but long double or even
> >> better, the real numbers themselves. Which would mean that
> >> computations would be very very very slow.
> >> Numpy leans somehow towards accuracy. If you want more accuracy
> >> (because even with double, you can hit the limit very fast), use
> >> another type.
> >>
> >> You said :
> >>  how can the sum of 100,000 numbers be anything other than the sum of
> >> > those numbers
> >>
> >> This will always be a problem. With doubles, try to sum 1/n
> >> (1...100000), you'll be surprized. And then do sum 1/n (100000...1)
> >> with float values, and here the result should be better than when
> >> using doubles. Numerical issues in scientific computing are tricky.
> >> There is no single answer, it depends on your problem.
> >>
> >> Matthieu
> >> --
> >> French PhD student
> >> Information System Engineer
> >> Website: http://matthieu-brucher.developpez.com/
> >> Blogs: http://matt.eifelle.com and http://blog.developpez.com/?blog=92
> >> LinkedIn: http://www.linkedin.com/in/matthieubrucher
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> >
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> >
>
>
>
> --
> French PhD student
> Information System Engineer
> Website: http://matthieu-brucher.developpez.com/
> Blogs: http://matt.eifelle.com and http://blog.developpez.com/?blog=92
> LinkedIn: http://www.linkedin.com/in/matthieubrucher
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