Floating point bug?

Diez B. Roggisch deets at nospam.web.de
Thu Feb 14 09:17:01 CET 2008


Preston Landers schrieb:
> marek.rocki at wp.pl(marek.rocki at wp.pl)@2008.02.13 15:13:20 -0800:
>> Not a bug. All languages implementing floating point numbers have the
>> same issue. Some just decide to hide it from you. Please read
>> http://docs.python.org/tut/node16.html and particularly
>> http://docs.python.org/tut/node16.html#SECTION0016100000000000000000
>>
> 
> 
> This is true.  Fortunately Python does provide a module which allows
> you to work with exact floating point quantities.

That's a misconception. The decimal-module has a different base (10 
instead of 2), and higher precision. But that doesn't change the fact 
that it will expose the same rounding-errors as floats do - just for 
different numbers.

 >>> import decimal as d
 >>> d = d.Decimal
 >>> d("1") / d("3") * d("3")
Decimal("0.9999999999999999999999999999")
 >>>

The advantage is that the rounding errors are the ones expected in 
monetary caluclations, which means that you can write correct programs 
for such purposes.

Diez



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