#35896 [Opn->Bgs]: Round() not doing what it says on the tin

ID: 35896 Updated by: sniper@php.net Reported By: matt at equaliser dot net -Status: Open +Status: Bogus Bug Type: Math related Operating System: Linux PHP Version: 4.4.1 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://docs.sun.com/source/806-3568/ncg_goldberg.html Thank you for your interest in PHP. Previous Comments: ------------------------------------------------------------------------ [2006-01-04 20:37:21] matt at equaliser dot net Soory - I got the expected result and the actual result interposed in my previous posting. ------------------------------------------------------------------------ [2006-01-04 20:30:50] matt at equaliser dot net Description: ------------ I know *why* this is happening - so please don't point me to http://www.php.net/manual/en/language.types.float.php - I'm aware of this. This does not change the fact that a pure call to the round() function using a float value stored in memory can return the wrong result. The fact that this is easily fixed by rounding to one decimal place further along the float before performing the actual rounding, simply makes the continued existence of this bug more baffling. Why doesn't PHP round() do this internally? Do all PHP developers need to roll their own round() function if they are going to guarantee mathematical accuracy? Reproduce code: --------------- <?php $input_value = 45; $vat = $input_value * .175; echo $vat . '<br />'; echo round($vat , 2) . '<br />'; echo round(strval($vat) , 2) . '<br />'; echo round(round($vat , 3) , 2) . '<br />'; $input_value2 = 7.875; echo round($input_value2 , 2) . '<br />'; echo round(7.875 , 2) . '<br />'; echo $vat - $input_value2; ?> Expected result: ---------------- 7.875 7.87 7.88 7.88 7.88 7.88 -8.8817841970013E-16 Actual result: -------------- 7.875 7.88 7.88 7.88 7.88 7.88 -8.8817841970013E-16 ------------------------------------------------------------------------ -- Edit this bug report at http://bugs.php.net/?id=35896&edit=1