note 90233 added to function.rand

rand(1,2) returns 1 about 25% of the time, and 2 75% of the time. If you want to use rand to choose evenly among a small range of numbers, you should be aware of uneven distribution in low ranges such as rand(1,2) and rand(1,4). Here's what I found, your mileage may vary, but please test before you assume you'll get a 50/50 distribution from rand(1,2) TEST CODE: $counts = array(1=>0,0,0,0,0,0,0,0,0); for($i=0; $i < 10000000; $i++) { $n = rand(1,4); $counts[$n]++; } echo "Counts: ".print_r($counts,true); RESULTS: rand(1,2) consistently returns a 25%/75% split [1] => 2502675 [2] => 7497325 rand(1,4) consistently returns a 6%, 31%, 31%, 31% split [1] => 625149 [2] => 3123425 [3] => 3127953 [4] => 3123473 It appears that rand chooses your "min" boundary less frequently than the rest of your range. If you test rand(1,9) you get similar results: the 1 will be chosen 123k times, and 2-8 will be chosen 1.233M times each. Of course, rand(1,2) is distributed the same as rand(8,9): the starting value always seems to be chosen less frequently. I tested and got the same distribution from mt_rand() ---- Server IP: 209.41.74.194 Probable Submitter: 206.71.67.6 ---- Manual Page -- http://www.php.net/manual/en/function.rand.php Edit -- https://master.php.net/note/edit/90233 Del: integrated -- https://master.php.net/note/delete/90233/integrated Del: useless -- https://master.php.net/note/delete/90233/useless Del: bad code -- https://master.php.net/note/delete/90233/bad+code Del: spam -- https://master.php.net/note/delete/90233/spam Del: non-english -- https://master.php.net/note/delete/90233/non-english Del: in docs -- https://master.php.net/note/delete/90233/in+docs Del: other reasons-- https://master.php.net/note/delete/90233 Reject -- https://master.php.net/note/reject/90233 Search -- https://master.php.net/manage/user-notes.php