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Bug #62959 Wrong math
Submitted: 2012-08-28 17:50 UTC Modified: 2012-08-31 04:41 UTC
Votes:1
Avg. Score:4.0 ± 0.0
Reproduced:0 of 0 (0.0%)
From: lauris dot kuznecovs at laurisnet dot com Assigned:
Status: Not a bug Package: Math related
PHP Version: 5.4.6 OS: Windows 7, Centos 5.8
Private report: No CVE-ID: None
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 [2012-08-28 17:50 UTC] lauris dot kuznecovs at laurisnet dot com 
Description:
------------
<? echo (443.292 - 193.126 - 250.166); ?>
Return: -2.8421709430404e-14

<? echo round(443.292 - 193.126 - 250.166, 2); ?>
Return: -0


But these calculations must return 0.
BTW -0 does not exist in MATH. Maybe some error in rounding function.

Test script:
---------------
<? echo (443.292 - 193.126 - 250.166); ?>
<? echo round(443.292 - 193.126 - 250.166, 2); ?>

Expected result:
----------------
0

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 [2012-08-28 17:57 UTC] rasmus@php.net 
-0 may not exist in traditional math, but it very much exists in computer 
representations. Computers can't represent floating point values with complete 
accuracy, so if you manipulate floating point you are going to get rounding 
errors. -0 means a value rounded to 0 from the negative direction. You can read 
http://en.wikipedia.org/wiki/Signed_zero for more details.
 [2012-08-28 17:57 UTC] rasmus@php.net
-Status: Open +Status: Not a bug
 [2012-08-28 18:11 UTC] lauris dot kuznecovs at laurisnet dot com 
Ok, lets say that the round() function works as expected, but still this 
calculation (443.292 - 193.126 - 250.166) return: -2.8421709430404 E-14
And that is not correct!
 [2012-08-28 18:19 UTC] rasmus@php.net 
Depends on your definition of correct. Like I said, computers cannot represent 
floating point with perfect accuracy. -2.8421709430404 E-14 is an extremely 
small number very close to 0, so it is as close to "correct" as you are going to 
get.
You should probably read through this http://floating-point-gui.de/ to get up to 
speed on how computers treat floating point values. This isn't a PHP-specific 
thing. For example:

Python 2.7.3 (default, Aug  1 2012, 05:14:39) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> (443.292 - 193.126 - 250.166)
-2.842170943040401e-14
 [2012-08-28 19:00 UTC] nikic@php.net 
Floating point values have a limited precision. Hence a value might 
not have the same string representation after any processing. That also
includes writing a floating point value in your script and directly 
printing it without any mathematical operations.

If you would like to know more about "floats" and what IEEE
754 is, read this:
http://www.floating-point-gui.de/
 [2012-08-28 19:09 UTC] nikic@php.net 
Oh, sorry, I missed rasmus second reply ;)
 [2012-08-31 04:41 UTC] lauris dot kuznecovs at laurisnet dot com 
I think this can be closed. No more questions.
 
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