Bug #69517 [Opn->Nab]: round() does something weird on result of pow()

Edit report at https://bugs.php.net/bug.php?id=69517&edit=1 ID: 69517 Updated by: cmb@php.net Reported by: leafface at gmail dot com Summary: round() does something weird on result of pow() -Status: Open +Status: Not a bug Type: Bug -Package: Unknown/Other Function +Package: Math related Operating System: Mac OS X PHP Version: 5.5.24 -Assigned To: +Assigned To: cmb Block user comment: N Private report: N New Comment: 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. Never ever compare floating point values for equality. :) Previous Comments: ------------------------------------------------------------------------ [2015-04-23 18:45:21] leafface at gmail dot com Description: ------------ round() acts weird in some cases when pow() theoretically returns a round number. The bug doesn't occur if precision for round() is set and is greater than 14. (Sorry, I actually have PHP 5.5.14, but it's pretty unlikely this has been resolved since.) Test script: --------------- <pre> <? $n = pow(8, 2/3); echo ($n == round($n) ? 'true' : 'false'). "\n". ($n == round($n, 14) ? 'true' : 'false'). "\n". ($n == round($n, 15) ? 'true' : 'false'); Expected result: ---------------- true true true Actual result: -------------- false false true ------------------------------------------------------------------------ -- Edit this bug report at https://bugs.php.net/bug.php?id=69517&edit=1