Bug #62959 [Com]: Wrong math

Edit report at https://bugs.php.net/bug.php?id=62959&edit=1 ID: 62959 Comment by: nikic@php.net Reported by: lauris dot kuznecovs at laurisnet dot com Summary: Wrong math Status: Not a bug Type: Bug Package: Math related Operating System: Windows 7, Centos 5.8 PHP Version: 5.4.6 Block user comment: N Private report: N New Comment: Oh, sorry, I missed rasmus second reply ;) Previous Comments: ------------------------------------------------------------------------ [2012-08-28 19:00:48] 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 18:19:10] 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 18:11:12] 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 17:57:17] 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 17:50:56] 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 ------------------------------------------------------------------------ -- Edit this bug report at https://bugs.php.net/bug.php?id=62959&edit=1