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

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: "Kevin W. Currey" <kevalier68 <at> hotmail.com>
Subject: bug#19025: closed (Re: bug#19025: 24.4; precision error when
 adding two floats)
Date: Wed, 12 Nov 2014 02:07:03 +0000

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Your bug report

#19025: 24.4; precision error when adding two floats

which was filed against the emacs package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 19025 <at> debbugs.gnu.org.

-- 
19025: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=19025
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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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

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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

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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
Configured using:
 `configure
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 --prefix=/usr --exec-prefix=/usr --bindir=/usr/bin --sbindir=/usr/sbin
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 -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
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Recent messages:
Load-path shadows:
None found.
 
 
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Memory information:
((conses 16 145217 10616)
 (symbols 48 23227 0)
 (miscs 40 133 446)
 (strings 32 31064 5702)
 (string-bytes 1 930985)
 (vectors 16 15771)
 (vector-slots 8 452262 7494)
 (floats 8 82 775)
 (intervals 56 640 31)
 (buffers 960 24))

 		 	   		  

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