[Numpy-discussion] bug in round with negative number of decimals
Charles R Harris
charlesr.harris at gmail.com
Mon Sep 4 11:32:52 CDT 2006
On 9/4/06, Paulo J. S. Silva <pjssilva at ime.usp.br> wrote:
>
> Interesting, I was just reading about the round rule in IEEE standard
> last Friday.
>
> What numpy's "around" function does is called "round to even" (round is
> take to make the next digit even), instead of "round up". According to
> "What every computer scientist should know about floating-point
> arithmetic" (do a Google search), the reason to prefer "round to even"
> is exemplified by the following result from Reiser and Knuth:
>
> Theorem
>
> Let x and y be floating-point numbers, and define x0 = x, x1 = (x0 - y)
> + y, ..., xn = (xn-1 - y) + y. If + and - are exactly rounded using
> round to even, then either xn = x for all n or xn = x1 for all n >= 1.
>
> If you use "round up" the sequence xn can start increasing slowly "for
> ever", and as the paper says:
>
> "...This example suggests that when using the round up rule,
> computations can gradually drift upward, whereas when using round to
> even the theorem says this cannot happen."
>
> Best,
>
> Paulo
However, around does set the sign bit when rounding -.5 to zero:
>>> around(-.5).tostring()
'\x00\x00\x00\x00\x00\x00\x00\x80'
so that
>>> around(-.5)
-0.0
on the other hand
>>> around(-.5) == 0.0
True
I didn't know -0.0 was part of the IEEE spec.
Chuck
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