#50825 [Bgs]: rand () and mt_rand () do not properly calculate when min=0

ID: 50825 Updated by: pajoye@php.net Reported By: xeriar at gmail dot com Status: Bogus Bug Type: Math related Operating System: Debian Squeeze PHP Version: 5.2.12 New Comment: /dev/urandom is indeed better that MT or the rather basic rand. Previous Comments: ------------------------------------------------------------------------ [2010-01-22 17:23:08] xeriar at gmail dot com As I mentioned, simply passing mt_rand (1,2) nets the 50/50 results desired. Which you would have noted if you had bothered reading the description. It is in fact fixable, and I've created my own implementation piping from /dev/urandom to cope with this since I obviously can't trust the php code to. ------------------------------------------------------------------------ [2010-01-22 16:47:53] pajoye@php.net As both implemenations are not ideal, this specific is not fixable and won't be. Btw, if you use better random sources, getting a 50/50 as you expect is very unlikely .) ------------------------------------------------------------------------ [2010-01-22 16:42:56] xeriar at gmail dot com Description: ------------ Whenever rand () or mt_rand () are passed with a min=0, 0 gets reported less often than it should be. mt_rand (0,1) returns 1 75% of the time, for example, while mt_rand (1,2) works as expected. Reproduce code: --------------- <?php $arr = array (); for ($i = 0; $i < 100000; $i++) { $c = mt_rand (0,1); if (!isset ($arr[$c])) $arr[$c] = 1; else $arr[$c]++; } ksort ($arr); foreach ($arr as $key => $value) echo '<b>'.$key.':</b> '.$value.'<br />'; Expected result: ---------------- 0 and 1 should each get ~50,000 results Actual result: -------------- 0 gets ~25k results and 1 gets ~75k ------------------------------------------------------------------------ -- Edit this bug report at http://bugs.php.net/?id=50825&edit=1