Bug #66132 [Com]: X*5/X where X is floating point number is greater than 5

Edit report at https://bugs.php.net/bug.php?id=66132&edit=1 ID: 66132 Comment by: utilmind at gmail dot com Reported by: utilmind at gmail dot com Summary: X*5/X where X is floating point number is greater than 5 Status: Not a bug Type: Bug Package: PHP-GTK related Operating System: Any, tested in Win32 and CentOS PHP Version: 5.4.22 Block user comment: N Private report: N New Comment: The "ini_set("precision", 17);" explains the origin of the problem, but does not solves it. The problem is not that floating point operations makes little inaccuracies. My point is that, if PHP with low precision setting rounds values for visual representation of inaccurate numbers, it should also round them on compare operations like < > and ==. Otherwise, we currently getting a value, which looks exactly like "5", but actually it little bit more than "5". Currently following type conversion operation fixes the problem: $n = (float)(string)(3.89880952381*5/3.89880952381); but it's not ideal. The "precision" option of php.ini should also affect on logical comparison operations. Previous Comments: ------------------------------------------------------------------------ [2013-11-21 14:41:11] bwoebi@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/ Thank you for your interest in PHP. php > ini_set("precision", 17); php > $n = (3.89880952381*5/3.89880952381); // result will be a very bit greater than 5 php > var_dump($n); float(5.0000000000000009) ------------------------------------------------------------------------ [2013-11-21 00:19:36] scorninpc@php.net This is an PHP main package bug Confirmed Version: PHP 5.5.1-2 (built: Aug 5 2013 14:10:28) OS: Linux version 3.10-2-amd64 #1 SMP Debian 3.10.5-1 (2013-08-07) ------------------------------------------------------------------------ [2013-11-20 23:53:46] utilmind at gmail dot com Description: ------------ It's probably bug with conversion from floating point number, or bug with comparision between types. We know that X*5/5 is 5. Even PHP returns 5, however, sometimes, returned value appears to be greater than 5. Test script: --------------- $n = (3.89880952381*5/3.89880952381); // result will be exactly 5 print $n.'!=5 ? '.($n>5); // here it appears that 5 is greater than 5 ------------------------------------------------------------------------ -- Edit this bug report at https://bugs.php.net/bug.php?id=66132&edit=1