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

Edit report at https://bugs.php.net/bug.php?id=69517&edit=1 ID: 69517 User updated by: leafface at gmail dot com Reported by: leafface at gmail dot com Summary: round() does something weird on result of pow() Status: Not a bug Type: Bug Package: Math related Operating System: Mac OS X PHP Version: 5.5.24 Assigned To: cmb Block user comment: N Private report: N New Comment: Okay I get it now. “echo $n” says 4, that's what confused me, but also “$n<4” is true. Previous Comments: ------------------------------------------------------------------------ [2015-04-23 21:34:38] bwoebi@php.net pow(8, 2/3) is 3.9999999999999956 … When you then do a round($n, 15) operation, it rounds to 3.999999999999996 which is the same as 3.9999999999999956 internally, thus returning true. Everything with second parameter smaller than 15 rounds to float(4), which isn't equal to $n. ------------------------------------------------------------------------ [2015-04-23 20:34:59] leafface at gmail dot com "Floating point values have a limited precision." - Actually what's really strange in my bug report is that "The bug doesn't occur if precision for round() is set and is greater than 14.", i.e. the bug doesn't only occur if there's no precision set for round(), but also if the precision is less than 15, even if it's 1! Meaning this statement for example results false: $n == round($n, 1) ------------------------------------------------------------------------ [2015-04-23 20:08:53] cmb@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. Never ever compare floating point values for equality. :) ------------------------------------------------------------------------ [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