a Python bug report

[Peter J. Holzer]

On 2018-05-29 21:34:21 -0400, José María Mateos wrote:
> On Wed, May 30, 2018 at 01:07:38AM +0000, Ruifeng Guo wrote:
> > We encountered a bug in Python recently, we checked the behavior for Python version 2.7.12, and 3.1.1, both version show the same behavior. Please see below the unexpected behavior in "red text".
[...]
> In [3]: 1000 * 1.017
> Out[3]: 1016.9999999999999
> 
> So there you have it.

To expand a bit on that, the reason, why 1000 * 1.017 isn't 1017 isn't
that an x86 processor can't multiply, it is that 1017/1000 cannot be exactly
represented in a binary fraction (just like 1/7 cannot be exactly
represented in a decimal fraction).

So when you type 1.017, the computer really stores
1.016999999999999904076730672386474907398223876953125 and when you
multiply that by 1000, the result would be
1016.999999999999904076730672386474907398223876953125, but that needs a
few bits too much, so it rounded down to
1016.9999999999998863131622783839702606201171875.

        hp

-- 
   _  | Peter J. Holzer    | we build much bigger, better disasters now
|_|_) |                    | because we have much more sophisticated
| |   | hjp at hjp.at         | management tools.
__/   | http://www.hjp.at/ | -- Ross Anderson <https://www.edge.org/>
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