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Accuracy of math.sqrt(), was Re: Serious error in int() function?

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From Peter Otten <__peter__@web.de>
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Subject Accuracy of math.sqrt(), was Re: Serious error in int() function?
Date Thu, 14 Apr 2016 09:46:53 +0200
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ast wrote:

> 
> <martin.spichty@gmail.com> a écrit dans le message de
> news:52f7516c-8601-4252-ab16-bc30c59c8306@googlegroups.com...
>> Hi,
>>
>> there may be a serious error in python's int() function:
>>
>> print int(float(2.8/0.1))
>>
>> yields
>>
>> 27
>>
>> instead of 28!!
>>
>> I am using Python Python 2.7.6, GCC 4.8.2 on Linux Ubuntu.
>>
>> Is that known?
>> Best,
>> Martin
> 
> 
> I have a similar question, so I post my message here.
> Hope this will not annoy the OP

Well, you "annoy" me ;)

Would you please always take the time to come up with a subject line that 
matches your question. You should also start a new thread.

> Is it sure that the square root of a square number is always
> an integer ?
> 
> I would not like to get a result as 345.99999999
> 
> from math import sqrt
> 
>>>>sqrt(16)
> 4.0
>>>> sqrt(16).is_integer()
> True
> 
>>>> for n in range(1000000):
> ...         if not sqrt(n**2).is_integer():
> ...             print(sqrt(n**2))
> 
> it seems to work ... but does it work for all integers ?

Even if you get an integer it may not be the one you are expecting:

>>> sqrt((10**22)**2) == 10**22
True
>>> sqrt((10**23)**2) == 10**23
False
>>> sqrt((10**23)**2).is_integer()
True
>>> int(sqrt((10**23)**2))
99999999999999991611392

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Thread

Serious error in int() function? martin.spichty@gmail.com - 2016-04-13 00:41 -0700
  Re: Serious error in int() function? Marko Rauhamaa <marko@pacujo.net> - 2016-04-13 10:55 +0300
  Re: Serious error in int() function? Alain Ketterlin <alain@universite-de-strasbourg.fr.invalid> - 2016-04-13 10:00 +0200
  Re: Serious error in int() function? Peter Otten <__peter__@web.de> - 2016-04-13 10:03 +0200
  Re: Serious error in int() function? blindanagram@nowhere.net - 2016-04-13 09:04 +0100
  Re: Serious error in int() function? Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2016-04-13 23:51 +1200
  Re: Serious error in int() function? "ast" <nomail@com.invalid> - 2016-04-14 08:52 +0200
    Re: Serious error in int() function? Christian Gollwitzer <auriocus@gmx.de> - 2016-04-14 09:25 +0200
    Accuracy of math.sqrt(), was Re: Serious error in int() function? Peter Otten <__peter__@web.de> - 2016-04-14 09:46 +0200
    Re: Serious error in int() function? blindanagram@nowhere.net - 2016-04-14 08:59 +0100
      Re: Serious error in int() function? blindanagram@nowhere.net - 2016-04-14 09:13 +0100
        Re: Serious error in int() function? blindanagram@nowhere.net - 2016-04-14 13:07 +0100
          Re: Serious error in int() function? Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2016-04-14 20:36 -0400
            Re: Serious error in int() function? Christian Gollwitzer <auriocus@gmx.de> - 2016-04-15 04:38 +0200
              Re: Serious error in int() function? John Pote <johnhpote@o2.co.uk> - 2016-04-15 12:05 +0100
                Re: Serious error in int() function? alister <alister.ware@ntlworld.com> - 2016-04-15 11:50 +0000
            Re: Serious error in int() function? blindanagram@nowhere.net - 2016-04-15 09:41 +0100
              Re: Serious error in int() function? blindanagram@nowhere.net - 2016-04-15 09:41 +0100

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