[docs] possible dox bug in floating point tutorial

[Zachary Ware]

Hi Daniel,

On Mon, Jan 13, 2014 at 11:00 AM, Daniel Lichtblau <danl at wolfram.com> wrote:
> (I'm not sure if this is a bug but it seems mysterious.)
>
> http://docs.python.org/2.6/tutorial/floatingpoint.html
>
> Mentions that 0.1, under the hood, is a fairly long decimal.
> Specifically:
> "On most machines, if Python were to print the true
> decimal value of the binary approximation stored for 0.1, it would have to
> display
>
>>>> 0.1
>
> 0.1000000000000000055511151231257827021181583404541015625
>
> instead!"
>
> This seems to be too long for any hardware floating point representation,
> even quad. Also the length of that decimal string is 54. While that's not
> quite as suspicious as 53, it still makes one wonder if that should be
> the length of the corresponding display for a binary representation.

To understate it slightly, I'm not extremely familiar with the
intricacies of binary floating point.  However, I can tell you that
'0.1000000000000000055511151231257827021181583404541015625' was most
likely obtained like so:

   >>> from decimal import Decimal as d
   >>> str(d(0.1))
   '0.1000000000000000055511151231257827021181583404541015625'

Hopefully that takes some of the mystery out of it!  If you believe
this representation to be incorrect, do please open an issue on the
bug tracker.

Regards,

-- 
Zach