[issue41057] Division error

[Steven D'Aprano]

Steven D'Aprano  added the comment:

Further to what Mark said, I'm afraid you are mistaken when you thought that 
"the result was correct" on R. R cheats by not printing the full precision of 
the number, they just stop printing digits, giving a false impression of 
accuracy. You can prove this for yourself:

> 0.4 + 8/100
[1] 0.48
> (0.4 + 8/100) == 0.48
[1] FALSE

So even though the printed result *looks* like 0.48, it actually isn't. If you 
investigate carefully, you will probably find that the number R calculates is 
the same as Python.

And the same as Javascript:

js> 0.4 + 8/100
0.48004

and pretty much every programming language that uses 64-bit floats.


BTW, this is a FAQ:

https://docs.python.org/3/faq/design.html#why-are-floating-point-calculations-so-inaccurate

There are a ton of other resources on the web explaining this, since it occurs 
virtually everywhere, in every language with fixed-precision floating point 
numbers. For example:

https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html

https://randomascii.wordpress.com/2012/05/20/thats-not-normalthe-performance-of-odd-floats/

https://randomascii.wordpress.com/2012/04/05/floating-point-complexities/

--
nosy: +steven.daprano

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[issue41057] Division error

[Mark Dickinson]

Mark Dickinson  added the comment:

This isn't a bug in Python; it's a consequence of the 
what-you-see-is-not-what-you-get nature of binary floating-point.

The behaviour is explained in the tutorial, here: 
https://docs.python.org/3/tutorial/floatingpoint.html

--
nosy: +mark.dickinson
resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue41057] Division error

[Fenn Ehk]

New submission from Fenn Ehk :

When performing some basic calculations, the result is wrong.
0.4 + 8/100
Out[43]: 0.48004

0.3 + 8/100
Out[44]: 0.38

I thought it could be processor related and tried the same operation with R, 
but the result was correct. So I tried it on some online repls:
https://repl.it/languages/python3
https://www.learnpython.org/en/Basic_Operators

And the bug is there, it seems to exist in 3.7.6 and 3.8.3 (and probably all 
versions in between
Other examples of the error:
0.3 + 8/100
Out[50]: 0.38

0.4 + 8/100
Out[51]: 0.48004

0.4 + a
Out[52]: 0.48004

0.4 + 9/100
Out[53]: 0.49

0.7 + 9/100
Out[54]: 0.7899

0.7 + 10/100
Out[55]: 0.7999

0.7 + 10/100
Out[56]: 0.7999

0.7 + 11/100
Out[57]: 0.8099

0.7 + 12/100
Out[58]: 0.82

0.8 + 8/100
Out[59]: 0.88

0.8 + 9/100
Out[60]: 0.89

0.6 + 9/100
Out[61]: 0.69

0.7 + 9/100
Out[62]: 0.7899

--
components: Interpreter Core
messages: 371948
nosy: Fenn Ehk
priority: normal
severity: normal
status: open
title: Division error
type: behavior
versions: Python 3.7, Python 3.8

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