rounding errors?

Grant Edwards grante at visi.com
Sat Jul 3 07:16:49 CEST 2004


todd wrote:

> I've just started using Python, and am having an extraordinary
> experience.

Python's pretty cool, eh?

> One thing worries me, however, I'm planning on doing some
> mathematical research with Python, and it appears that it does
> funny thing with floating point numbers.

No, it isn't.  The problem is that you're expecting it to do
something funny with them and it isn't.

> Maybe It's superficial,

No, on the contrary, it's quite fundamental to the way floating
point works in computers.

> but here's what I'm getting,using the interpreter..
> 
>>>>>>.31
>>> 
>>> 0.31
>>> 
>>>>>>.32
>>> 
>>> 0.32000000000000001

[...]

And you'd prefer that the intrepreter lied to you and printed
"0.32" when you tell it to print the number 0.3200..001 --
that's a common reaction from people who don't understand
floating point numbers.  If you want, you can specify a format
when printing floating point numbers so that you only see as
many digits as you want to:

>>> "%0.2f" % 0.32
'0.32'
>>> 

> Now, I realize that this is really small errors.. Does anybody
> have an explanation why Python picks up or loses these?

Somebody else has already posted a pointer to the Python FAQ
about floating point, but if you're planning on doing numerical
research using floating point on computers, you really ought to
take a class on numerical methods or numerical analysis using
floating point on computers.

-- 
Grant Edwards                   grante             Yow!  PIZZA!!
                                  at               
                               visi.com            



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