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subtraction of floating point numbers
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Jaroslav Dobrek  
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 More options Feb 24, 3:41 am
Newsgroups: comp.lang.python
From: Jaroslav Dobrek <jaroslav.dob...@gmail.com>
Date: Fri, 24 Feb 2012 00:41:21 -0800 (PST)
Local: Fri, Feb 24 2012 3:41 am
Subject: subtraction of floating point numbers
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Hello,

when I have Python subtract floating point numbers it yields weird
results. Example:

4822.40 - 4785.52 = 36.8799999999992

Why doesn't Python simply yield the correct result? It doesn't have a
problem with this:

482240 - 478552 = 3688

Can I tell Python in some way to do this differently?

Jaroslav


 
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Alain Ketterlin  
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 More options Feb 24, 3:49 am
Newsgroups: comp.lang.python
From: Alain Ketterlin <al...@dpt-info.u-strasbg.fr>
Date: Fri, 24 Feb 2012 09:49:15 +0100
Local: Fri, Feb 24 2012 3:49 am
Subject: Re: subtraction of floating point numbers
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Jaroslav Dobrek <jaroslav.dob...@gmail.com> writes:
> when I have Python subtract floating point numbers it yields weird
> results. Example:

> 4822.40 - 4785.52 = 36.8799999999992

We've had this discussion here one or two days ago...

The usual answer is: please read "What Every Computer Scientist Should
Know About Floating Point Arithmetic", at:

http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.22.6768

and check the answers posted these last days. In brief: you're working
with floating point numbers, not reals (i.e., real "reals"). That's
life. Deal with it, or move to specialized packages, like decimal.

-- Alain.


 
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Chris Rebert  
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 More options Feb 24, 5:16 am
Newsgroups: comp.lang.python
From: Chris Rebert <c...@rebertia.com>
Date: Fri, 24 Feb 2012 02:16:29 -0800
Local: Fri, Feb 24 2012 5:16 am
Subject: Re: subtraction of floating point numbers
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On Fri, Feb 24, 2012 at 12:41 AM, Jaroslav Dobrek

<jaroslav.dob...@gmail.com> wrote:
> Hello,

> when I have Python subtract floating point numbers it yields weird
> results. Example:

> 4822.40 - 4785.52 = 36.8799999999992

> Why doesn't Python simply yield the correct result? It doesn't have a
> problem with this:

> 482240 - 478552 = 3688

> Can I tell Python in some way to do this differently?

Refer to this thread from 2 days ago:
http://mail.python.org/pipermail/python-list/2012-February/1288344.html

Regards,
Chris


 
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