By: max

max — Fri, 06 Aug 2010 09:01:51 +0000

Your code for generating random integer number between 0 and a number different from 1 is not entirely correct. For example we want to generate an integer random number in the interal [0, 2]. Following yor example the probabilities for getting a 0 and a 1 are not equal. To get 0 (Math.random() * end) has to return a number in the interval [0, 0.5) so when rounded it will return 0. But to get 1, the number generated by (Math.random() * end) has to be in the interval [0.5, 1.5) which as you can see is two times bigger that the previous one. So the probability to get 1 compared to 0 is bigger. The same happens at the end of our wanted interval (in this example 2).
The easiest way to get equal probability is with this function:

private function rand(start:Number, end:Number):Number
{
return Math.ceil(Math.random()*(end-start+1)) % (end-start+1);
}

Note that this function too doesn't generate numbers with mathematically equal probability but it's close enough.
Note2: This function also work if you pass it start=0, end=1

To test that your example is not correct you can try this example:

private function rand(start:Number, end:Number):Number
{
return Math.ceil(Math.random()*(end-start+1)) % (end-start+1);
}

private function test_rand():void
{
var tmp:Array = new Array(0, 0, 0);
var i:Number;
var count:Number = 1000000;
for( i = 0; i < count; i++ ) {
//tmp[rand(0, 2)]++;
tmp[Math.round(Math.random()*2)]++;
}

lbl_test_rand.text = "";
for( i = 0; i

With your algorithm you will get probabilities such as:
0 -> 25%
1 -> 50%
2 -> 25%

with the function rand() (uncomment it in the loop and comment out the line below it) you will get probabilities such as:

0 -> 33%
1 -> 33%
2 -> 33%

Comments on: Random number in Flex

By: max