note 74406 added to function.array-rand

Please note that (at least in PHP 4.3.11), while each key from the source array has a equal chance of being picked, the order in which they are returned is NOT random. In an array with keys from one to 10, where 2 random keys are picked, 0 is picked as the first number 18.8% of the time, and only as second number in 1.1% of the time: $n=100000; $a=array(1,2,3,4,5,6,7,8,9,10); $b=array(); for($i=0;$i<sizeof($a);$i++) { $b[$i]=array(0,0); } for($i=0;$i<$n;$i++) { $keys=array_rand($a,2); $b[$keys[0]][0]++; $b[$keys[1]][1]++; } for ($i=0;$i<sizeof($b);$i++){ printf("%d: %04.1f%% %04.1f%% %04.1f%%", $i, ($b[$i][0]/$n*100),($b[$i][1]/$n*100),($b[$i][0]+$b[$i][1])/$n*100); } The result is: 0: 18.8% 01.1% 19.9% 1: 17.0% 03.3% 20.3% 2: 14.3% 05.7% 20.0% 3: 12.2% 07.9% 20.1% 4: 10.0% 10.0% 20.0% 5: 07.8% 12.0% 19.8% 6: 05.5% 14.5% 20.0% 7: 03.3% 16.6% 19.9% 8: 01.2% 18.9% 20.1% 9: 10.0% 09.9% 19.9% The workaround is adding a shuffle command to shuffle the keys: 0: 10.0% 10.0% 20.0% 1: 10.0% 10.0% 20.0% 2: 10.0% 10.0% 20.0% 3: 10.0% 10.0% 20.0% 4: 10.0% 10.0% 20.0% 5: 10.0% 10.0% 20.0% 6: 10.1% 10.0% 20.1% 7: 10.0% 10.0% 20.0% 8: 10.0% 10.0% 20.0% 9: 10.0% 10.0% 20.0% ---- Server IP: 83.137.20.120 Probable Submitter: 82.204.44.50 ---- Manual Page -- http://www.php.net/manual/en/function.array-rand.php Edit -- https://master.php.net/note/edit/74406 Del: integrated -- https://master.php.net/note/delete/74406/integrated Del: useless -- https://master.php.net/note/delete/74406/useless Del: bad code -- https://master.php.net/note/delete/74406/bad+code Del: spam -- https://master.php.net/note/delete/74406/spam Del: non-english -- https://master.php.net/note/delete/74406/non-english Del: in docs -- https://master.php.net/note/delete/74406/in+docs Del: other reasons-- https://master.php.net/note/delete/74406 Reject -- https://master.php.net/note/reject/74406 Search -- https://master.php.net/manage/user-notes.php