GNU bug report logs - #19025
24.4; precision error when adding two floats

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: "Kevin W. Currey" <kevalier68 <at> hotmail.com>
To: "bug-gnu-emacs <at> gnu.org" <bug-gnu-emacs <at> gnu.org>
Subject: 24.4; precision error when adding two floats
Date: Tue, 11 Nov 2014 19:36:39 -0500

[Message part 1 (text/plain, inline)]

(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
Configured using:
 `configure
 --srcdir=/home/kbrown/src/cygemacs/emacs-24.4-1.x86_64/src/emacs-24.4
 --prefix=/usr --exec-prefix=/usr --bindir=/usr/bin --sbindir=/usr/sbin
 --libexecdir=/usr/libexec --datadir=/usr/share --localstatedir=/var
 --sysconfdir=/etc --libdir=/usr/lib --datarootdir=/usr/share
 --docdir=/usr/share/doc/emacs --htmldir=/usr/share/doc/emacs/html -C
 --with-w32 --without-file-notification --enable-checking 'CFLAGS=-ggdb
 -O2 -pipe -Wimplicit-function-declaration -Og -g3
 -fdebug-prefix-map=/home/kbrown/src/cygemacs/emacs-24.4-1.x86_64/build=/usr/src/debug/emacs-24.4-1
 -fdebug-prefix-map=/home/kbrown/src/cygemacs/emacs-24.4-1.x86_64/src/emacs-24.4=/usr/src/debug/emacs-24.4-1'
 CPPFLAGS= LDFLAGS='
 
 
Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix
 
 
Major mode: Fundamental
 
 
Minor modes in effect:
  shell-dirtrack-mode: t
  display-time-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  line-number-mode: t

Recent messages:
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None found.
 
 
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Memory information:
((conses 16 145217 10616)
 (symbols 48 23227 0)
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 (strings 32 31064 5702)
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[Message part 2 (text/html, inline)]

Message #8 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Alexis <flexibeast <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: Re: bug#19025: 24.4; precision error when adding two floats
Date: Wed, 12 Nov 2014 12:38:12 +1100

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.

Message #15 received at 19025-done <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: "Kevin W. Currey" <kevalier68 <at> hotmail.com>
Cc: 19025-done <at> debbugs.gnu.org
Subject: Re: bug#19025: 24.4; precision error when adding two floats
Date: Tue, 11 Nov 2014 21:06:35 -0500

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

Message #18 received at 19025 <at> debbugs.gnu.org (full text, mbox):

From: Jay Belanger <jay.p.belanger <at> gmail.com>
To: 19025 <at> debbugs.gnu.org
Cc: jay.p.belanger <at> gmail.com
Subject: Re: bug#19025: 24.4; precision error when adding two floats
Date: Tue, 11 Nov 2014 20:28:49 -0600

>> (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.

Message #21 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Alexis <flexibeast <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: Re: bug#19025: 24.4; precision error when adding two floats
Date: Wed, 12 Nov 2014 13:42:48 +1100

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.

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