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

Hanni Ali hanni.ali@gmail....
Tue Sep 9 02:57:04 CDT 2008


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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