bug#19025: 24.4; precision error when adding two floats

bug#19025: 24.4; precision error when adding two floats

[Kevin W. Currey]

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(message "%s" (+ 379.744 57.93))

The above prints 437.67400000000004.  Same if I use 57.930, but if I use 57.931 (below) I get the correct answer 437.675.
 
(message "%s" (+ 379.744 57.931))

I was able to work around by using format:
 
(message "%s" (format "%f" (+ 379.744 57.93)))

which prints 437.674000.
 
FYI, I can replicate the issue with Emacs 21.3.1.
 
Thanks,
-Kevin
 
 
In GNU Emacs 24.4.1 (x86_64-unknown-cygwin)
 of 2014-10-21 on desktop-new
Windowing system distributor `Microsoft Corp.', version 6.1.7601
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bug#19025: 24.4; precision error when adding two floats

[Alexis]

Kevin W. Currey writes:

> (message "%s" (+ 379.744 57.93))
>
> The above prints 437.67400000000004.  Same if I use 57.930, but if I use 57.931 (below) I get the correct answer 437.675.
>  
> (message "%s" (+ 379.744 57.931))
>
> I was able to work around by using format:
>  
> (message "%s" (format "%f" (+ 379.744 57.93)))
>
> which prints 437.674000.
>  
> FYI, I can replicate the issue with Emacs 21.3.1.

Isn't this an artifact of the nature of floating-point arithmetic, rather than
an issue with Emacs per se? For example, in Python:

    Python 2.7.3 (default, Mar 13 2014, 11:03:55) 
    [GCC 4.7.2] on linux2
    Type "help", "copyright", "credits" or "license" for more information.
    >>> 379.744 + 57.93
    437.67400000000004
    >>>


Alexis.

bug#19025: 24.4; precision error when adding two floats

[Jay Belanger]

>> (message "%s" (+ 379.744 57.93))
>>
>> The above prints 437.67400000000004.  Same if I use 57.930, but if I
>> use 57.931 (below) I get the correct answer 437.675.
>>  
>> (message "%s" (+ 379.744 57.931))
>>
>> I was able to work around by using format:
>>  
>> (message "%s" (format "%f" (+ 379.744 57.93)))
>>
>> which prints 437.674000.
>>  
>> FYI, I can replicate the issue with Emacs 21.3.1.
>
> Isn't this an artifact of the nature of floating-point arithmetic, rather than
> an issue with Emacs per se?

Yes, although it probably wouldn't be a bad idea to point out this issue
in the Elisp documentation.

bug#19025: 24.4; precision error when adding two floats

[Alexis]

Jay Belanger writes:

>> Isn't this an artifact of the nature of floating-point arithmetic,
>> rather than an issue with Emacs per se?
>
> Yes, although it probably wouldn't be a bad idea to point out this
> issue in the Elisp documentation.

*nod* And perhaps mention Calc functions, which can work with a
specified precision?

(Assuming we can't simply link to e.g. http://floating-point-gui.de/ :-) )


Alexis.

bug#19025: 24.4; precision error when adding two floats

[Stefan Monnier]

tags 19025 notabug
thanks

> (message "%s" (+ 379.744 57.93))
> The above prints 437.67400000000004.

That's the difference between floating-point numbers and real or
rational numbers.

> (message "%s" (format "%f" (+ 379.744 57.93)))

You can cut the middle man: (message "%f" (+ 379.744 57.93))

But note that this prints something "incorrect":

     M-: (message "%f" (+ 379.744 57.93))
     => 437.674000.
     M-: (equal (+ 379.744 57.93) 437.674000)
     => nil

It's just that %f rounds differently.



        Stefan