Bug #51518 [Bgs]: should add to zero, but gets 1.1102230246252E-16 instead

Edit report at http://bugs.php.net/bug.php?id=51518&edit=1 ID: 51518 Updated by: rasmus@php.net Reported by: krenshala at koboldi dot net Summary: should add to zero, but gets 1.1102230246252E-16 instead Status: Bogus Type: Bug Package: Math related Operating System: Multiple PHP Version: 5.2.13 New Comment: The fix here is to decide on your precision and do: $output = round(0.9 + ($input * 0.3), 2); // 2-decimal precision Previous Comments: ------------------------------------------------------------------------ [2010-04-09 05:05:12] rasmus@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. . ------------------------------------------------------------------------ [2010-04-09 05:03:27] krenshala at koboldi dot net Description: ------------ Math error for code that should be returning zero, but instead returns 1.1102230246252E-16 on multiple systems using multiple versions of PHP. The problem is that when the input value is -3 the output value should be zero: 0.9 + (-3 * 0.3) = 0.9 - 0.9 = 0. If the calculated value is non-zero the error does not occur. I first saw the error on a WinXP (Home SP3, 32bit) system running PHP 5.2.9-1, but it also shows up on a MAC (10.6.2) with PHP 5.3.0 installed. I'm in the process of upgrading PHP (to 5.2.13) on my Gentoo box to test it there but haven't had a chance to do so yet. Test script: --------------- my_function(-3); function my_function($input){ $output = 1; if($input <= 0) $output = 0.9 + ($input * 0.3); // the above should give 0.9 - 0.9 = 0 echo "Verifying values: 0.9 - ".($input * 0.3)." is supposed to be zero?\n"; echo "Input: $input\tOutput: $output\n"; return $output; } Expected result: ---------------- should have received zero (0). Actual result: -------------- actually received: 1.1102230246252E-16 ------------------------------------------------------------------------ -- Edit this bug report at http://bugs.php.net/bug.php?id=51518&edit=1