#33576 [Bgs]: 19.6*100 != 19.6*10*10

ID: 33576 Updated by: rasmus@php.net Reported By: rick at ninjafoo dot com Status: Bogus Bug Type: Math related Operating System: * PHP Version: * New Comment: I am not sure why you think casting 1960 to a float suddenly would make 19.6*100 equal to exactly 1960.0. Floating point math is not exact. 19.6*100 is never going to be exactly 1960.0. It will be 1959.999999999999999999999 or 1960.00000000000000000001 regardless of any casting you do to it. You can round() it, or add a fuzz factor and floor() it. Previous Comments: ------------------------------------------------------------------------ [2005-07-06 10:42:38] rick at ninjafoo dot com Your right, this isnt a bug (double)(19.6*100) !== (double)1960 This is just plain broken. ------------------------------------------------------------------------ [2005-07-05 21:34:21] helly@php.net Thank you for taking the time to write to us, but this is not a bug. Please double-check the documentation available at http://www.php.net/manual/ and the instructions on how to report a bug at http://bugs.php.net/how-to-report.php . ------------------------------------------------------------------------ [2005-07-05 20:56:41] rick at ninjafoo dot com Yes, I understand that. My bug report was not to point out the joys of floating point math. My bug report was to illustrate a problem with php's internal casting. 19.6*100 != 1960 19.6*100 != (int)1960 19.6*100 != (float)1960 19.6*100 != (string)1960 echo gettype(19.6*100) returns 'double', However ..... 19.6*100 != (double)1960 19.6*100 cannot be compaired to anything without manually casting it as something else first. (string)(19.6*100) == (string)1960 ------------------------------------------------------------------------ [2005-07-05 20:13:07] derick@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://docs.sun.com/source/806-3568/ncg_goldberg.html Thank you for your interest in PHP. . ------------------------------------------------------------------------ [2005-07-05 20:11:18] rick at ninjafoo dot com Description: ------------ 100* certain values is not the same as 10*10* the same value. The following code should produce no output. I see output. Reproduce code: --------------- <?php $value1 = 19.6*100; $value2 = 19.6*10*10; if ($value1 != $value2) { echo 'funny, i thaught 100x was exactly the same as 10x10x....<br>'; echo 'value1='.$value1.', value2='.$value2.','; } ?> Expected result: ---------------- I expect nothing to happen. Actual result: -------------- funny, i thaught 100x was exactly the same as 10x10x.... value1=1960, value2=1960, ------------------------------------------------------------------------ -- Edit this bug report at http://bugs.php.net/?id=33576&edit=1