Javascript Random (function) number distribution

The Javascript Math.random is very useful for generating random numbers. However, is the number distribution even across the range? This is important because a lot of websites implement this for various random things, such as banner rotation, etc. While it is best to do some of this logic at the server, sometimes its just easier to do this at the client using Javascript. Anyway this is a small test to check the distribution out. I did this because I created a samll random jeopardy website and wanted to make sure all of the “answers” had an even distribution.

To achieve this, I decided to use the command-line javascript as it is easier to do, without requiring a browser. So I installed spidermonkey for this.

See spidermonkey

Below is the script I wrote to test this. In this test I am generating random numbers between 1 and 320 a total of 100,000,000 times and counting their occurance.

var dist = [];
var rv;
for (loopi = 1; loopi <= 320; loopi++)
{
 dist[loopi]=0;
}
for (loopi = 0; loopi <= 100000000; loopi++)
{
 rv=Math.floor(Math.random()*319)+1;
 //print(rv);
 dist[rv]=dist[rv]+1;
}
for (loopi = 1; loopi <= 320; loopi++)
{
 print(loopi + " - " + dist[loopi]);
}

On running this very large sample set generation and distribution I found that the result was infact very evenly distributed. Here is the output to prove it. I thought of graphing it, but it is quite obvious just by observing the numbers that the distribution is quite even.

1 - 312999
2 - 313682
3 - 313696
4 - 313211
5 - 313773
6 - 313538
7 - 313230
8 - 314019
9 - 315107
10 - 313173
11 - 313186
12 - 312954
13 - 313650
14 - 313106
15 - 312534
16 - 313039
17 - 314425
18 - 311901
19 - 312643
20 - 313913
21 - 314109
22 - 312848
23 - 313484
24 - 312862
25 - 312730
26 - 314328
27 - 313142
28 - 313510
29 - 313830
30 - 313378
31 - 313414
32 - 314185
33 - 313059
34 - 313019
35 - 313368
36 - 312500
37 - 312859
38 - 313713
39 - 313394
40 - 313719
41 - 312837
42 - 312799
43 - 313907
44 - 312424
45 - 313071
46 - 313576
47 - 312796
48 - 313857
49 - 314417
50 - 313918
51 - 313099
52 - 313396
53 - 312824
54 - 313566
55 - 312437
56 - 313856
57 - 313413
58 - 313091
.....