[issue14245] float rounding examples in FAQ are outdated

Zbyszek Szmek report at bugs.python.org
Sat Mar 10 15:09:48 CET 2012


Zbyszek Szmek <zbyszek at in.waw.pl> added the comment:

On 03/10/2012 12:26 PM, Mark Dickinson wrote:
>
> Mark Dickinson<dickinsm at gmail.com>  added the comment:
>
> Proposed rewrite:

Hi,
thanks for the quick reply. If we were to rewrite the whole entry, some 
more changes could be done:

I think it would be useful to mention explicitly that Python simply uses 
the native floating-point implementation in hardware and thus behaves 
very similarly to other languages which do this, for instance C or Java. 
This should clear up a lot of the behaviour for people who know other 
programming languages. "how the underlying platform handles 
floating-point" says something very similar, but the reader needs to 
understand what the "underlying platform" exactly is.

It is easy to count, that exactly 17 digits are accurate.

I have to admit, that I'm completely lost here --- why would a vastly 
inaccurate number (with more than half of digits wrong) be ever stored?
If "1.2" is converted to a float (a C double in current implementation), 
it has 15.96 decimal digits of precision.

"Similarly, the result of a floating-point operation must be rounded to 
fit into the fixed precision, often resulting in another tiny error." ?

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