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- - **Re: Simple calculation error**
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Re: Simple calculation errorFrancois Liot wrote:
> > I observed a strange calculation answer, on both python 2.3.4 and 2.4.4 > >> >> print 753343.44 - 753361.89 > > -18.4500000001 > >> >> print ( (753361.89*100) - (753343.44*100) ) / 100 > > 18.45 > > Can somebody help me to play correctly with decimal values? A 64-bit binary floating point number can hold values between -1e308 and +1e308, in only 64 bits of memory. Since 1e308 is a *lot* larger than float(2**64) = ~1.8e19, it does this by splitting the number in a binary fraction, and a multiplier (stored as an exponent). Unfortunately, very few decimal fractions can be *exactly* represented as binary fractions, so you often get representation errors: http://docs.python.org/tut/node16.html This is usually not much of a problem, since you usually end up rounding things to a suitable number of decimals or significant digits when you print them anyway (see below), but repr() doesn't do that -- it always outputs enough digits to get back the *exact* binary representation if you convert the string back to a floating point value again. In practice, you can usually ignore this; just use the appropriate output methods, and things will just work: While pathological cases do exist, for most casual use of floating-point arithmetic you'll see the result you expect in the end if you simply round the display of your final results to the number of decimal digits you expect. str() usually suffices, and for finer control see the discussion of Python's % format operator: the %g, %f and %e format codes supply flexible and easy ways to round float results for display. (from the above link) If you really need full control over this, no matter what, use the Decimal type, as provided by the decimal module in the standard library. See the library reference for the full story. </F> |

Re: Simple calculation errorOn Jan 4, 12:30*pm, Fredrik Lundh <fred...@pythonware.com> wrote:
> Francois Liot wrote: > > > I observed a strange calculation answer, on both python 2.3.4 and 2.4.4 > > >> >> print 753343.44 - 753361.89 > > > -18.4500000001 > > >> >> print ( (753361.89*100) - (753343.44*100) ) / 100 > > > 18.45 > > > Can somebody help me to play correctly with decimal values? > If the OP is questioning the change in sign, the operands are reversed in the second statement. -- Paul |

RE: Simple calculation errorNo the change of sign is due to a fake copy and past,
My question was related to decimal calculation. Thanks, Francois Liot -----Original Message----- From: python-list-bounces+fliot=kyriba.com@python.org [mailto:python-list-bounces+fliot=kyriba.com@python.org] On Behalf Of Paul McGuire Sent: Friday, January 04, 2008 1:46 PM To: python-list@python.org Subject: Re: Simple calculation error On Jan 4, 12:30*pm, Fredrik Lundh <fred...@pythonware.com> wrote: > Francois Liot wrote: > > > I observed a strange calculation answer, on both python 2.3.4 and 2.4.4 > > >> >> print 753343.44 - 753361.89 > > > -18.4500000001 > > >> >> print ( (753361.89*100) - (753343.44*100) ) / 100 > > > 18.45 > > > Can somebody help me to play correctly with decimal values? > If the OP is questioning the change in sign, the operands are reversed in the second statement. -- Paul -- http://mail.python.org/mailman/listinfo/python-list |

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