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Re: Floating point multiplication in python

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  Re: Floating point multiplication in python Gary Herron <gherron@islandtraining.com> - 2011-09-06 00:00 -0700
    Re: Floating point multiplication in python Duncan Booth <duncan.booth@invalid.invalid> - 2011-09-06 08:30 +0000

#12814 — Re: Floating point multiplication in python

On 09/05/2011 10:57 PM, xyz wrote:
> hi all:
>
> As we know ,  1.1 * 1.1 is 1.21 .
> But in python ,I got following :
>
>>>> 1.1 * 1.1
> 1.2100000000000002
>
> why python get wrong result? Who can tell me  where's the 0.0000000000000002 from?

It's not a python error    It's the nature of floating point arithmetic 
to be inaccurate on *ANY* computer.    Python just allows you to see the 
inaccuracies.

(But try:
   print 1.1*1.1
and see that the print statement does hide the roundoff error from you.)

Read this for more info:
     
http://pyfaq.infogami.com/why-are-floating-point-calculations-so-inaccurate

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#12821

Gary Herron <gherron@islandtraining.com> wrote:

> (But try:
>    print 1.1*1.1
> and see that the print statement does hide the roundoff error from you.)

That varies according to the version of Python you are using. On my system:

Python 2.7 (r27:82525, Jul  4 2010, 09:01:59) [MSC v.1500 32 bit (Intel)] 
on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> print(1.1*1.1)
1.21
>>> ^Z


C:\Python27>cd \python32

C:\Python32>python
Python 3.2.1 (default, Jul 10 2011, 21:51:15) [MSC v.1500 32 bit (Intel)] 
on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> print(1.1*1.1)
1.2100000000000002

-- 
Duncan Booth http://kupuguy.blogspot.com

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