[Mono-bugs] [Bug 359918] New: System.Random.Next(int, int) does not distribute values correctly

bugzilla_noreply at novell.com bugzilla_noreply at novell.com
Fri Feb 8 08:42:32 EST 2008


           Summary: System.Random.Next(int, int) does not distribute values
           Product: Mono: Class Libraries
           Version: 1.2.4
          Platform: i386
        OS/Version: Ubuntu
            Status: NEW
          Severity: Normal
          Priority: P5 - None
         Component: System
        AssignedTo: mono-bugs at lists.ximian.com
        ReportedBy: matthias.krebs at fhnw.ch
         QAContact: mono-bugs at lists.ximian.com
          Found By: Field Engineer

I have found out that the function System.Random.Next(int, int) does not
distribute the random values in the range it should.

For example: I would like to get random values in a range from -5 to 4, so I
use the function

int x = System.Random.Next(-5, 5);

repeatedly (like a million times).
Now, there should be an approximately uniform distribution from -5 to 4, but
instead I'm always getting a distribution like this:

-5: 0
-4: 1001832
-3: 1000609
-2: 999269
-1: 1000352
0: 1997431
1: 1001696
2: 1000023
3: 998896
4: 999892
5: 0

It is correct that the value 5 never occurs, but there are twice as many
occurrences of 0, but no occurrence of -5.

It also does not matter how big the range is or what the seed is, the lowest
value always gets a count of zero and the "middle" value gets a double count.

I also tested it with .NET 2.0 on Windows, it works as it should there,
therefore I assume there might be a bug in the Mono implementation.

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