[Tutor] Is something wrong with my round() function?

[Lloyd Kvam]

This is not a Python issue.  Floating Point numbers are normally stored
in a base 2 format.  This means that only those fraactions that have
denominators that are powers of two can be stored exactly.  Python's
repr function shows a floating point number as exactly as it can (in
a decimal format).  the str function will do sensible rounding to show
what you expect.

 >>> x = .125
 >>> x
0.125
 >>> x = .3
 >>> x
0.29999999999999999
 >>> print x
0.3

.125 works exactly because it is 1/8 and 8 is a power of 2.
print uses the str function and displays what you would expect.
The underlying binary representation is not exactly equal to .3
because it can't hold it exactly.  It is the same problem we face
when trying to express 1/3 as a decimal.  We can't do it exactly.

Raymond Hettinger recently published a Python Cookbook recipe for
examining floating point arithmatic and tracking the accumulated
errors.
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/265894

hcohen2 wrote:

> The obvious answer is 'YES', but what's causing this strange behaviour?
> 
>  >>> ans = 48.936
>  >>> round(ans, 1)
> 48.899999999999999
>  >>> round(ans, 2)
> 48.939999999999998
>  >>> round(ans, 3)
> 48.936
>  >>> round(ans, 4)
> 48.936
>  >>> round(ans, 2)
> 48.939999999999998
>  >>> round(ans, 1)
> 48.899999999999999
>  >
> 
> I have another example were I was using a class to compute the total 
> room rates that were supposed to be rounded to the nearest cent and they 
> looked like the above sample, however, using the round(<week rate>, 1) 
> gave it properly to the nearest dime.
> 
> Using python ver 2.2.2 on Linux (Mandrake 9.1) on an IBM laptop.
> 
> TIA
> 
> 
> 
> _______________________________________________
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> 

-- 
Lloyd Kvam
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