How to prevent duplicate random numbers

Question

I write a simple code and I would like to prevent duplicate random numbers.

What is the simplest way to do it?

 Random rnd = new Random();

        int n1, n2, n3, n4, n5, n6, n7;

        n1 = rnd.Next(1, 37);
        n2 = rnd.Next(1, 37);
        n3 = rnd.Next(1, 37);
        n4 = rnd.Next(1, 37);
        n5 = rnd.Next(1, 37);
        n6 = rnd.Next(1, 37);
        n7 = rnd.Next(1, 7);

        Console.WriteLine("1 Number ({0})", n1);
        Console.WriteLine("2 Number ({0})", n2);
        Console.WriteLine("3 Number ({0})", n3);
        Console.WriteLine("4 Number ({0})", n4);
        Console.WriteLine("5 Number ({0})", n5);
        Console.WriteLine("6 Number ({0})", n6);
        Console.WriteLine("Additionl Number ({0})", n7);
    }
}

}

http://social.msdn.microsoft.com/Forums/en-US/csharplanguage/thread/21ce91f4-9763-4f97-b558-16120c0583d3 — unlimit, Jun 09 '13 at 15:39
You want to simulate a lotto draw or shuffling a deck of cards, but what you're actually simulating is rolling dice. — Matthew Watson, Jun 09 '13 at 15:41
And that `additional number` is independent of the others, right? — Matthew Watson, Jun 09 '13 at 15:44
@DARK-SPIRIT: this is what I mean. `rnd.Next(1, 37);`. Read this http://msdn.microsoft.com/en-us/library/2dx6wyd4(v=vs.90).aspx — Nikhil Agrawal, Jun 09 '13 at 15:48
[Unique number in random?](http://stackoverflow.com/questions/9592166/unique-number-in-random) — I4V, Jun 09 '13 at 15:49

score -1 · Answer 1 · answered Jun 09 '13 at 15:39

-1

I had the same problem. Use this instead:

private static readonly Random random = new Random();
private static readonly object syncLock = new object();
public int RandomNumber(int min, int max)
{
    lock (syncLock)
    { // synchronize
        return random.Next(min, max);
    }
}

Usage

int randomNumber = RandomNumber(0, 10); // random number between 0 and 10

answered Jun 09 '13 at 15:39

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3

You're assuming a multithreaded application, but even with one thread you can get duplicate numbers. – Caramiriel Jun 09 '13 at 15:39

How to prevent duplicate random numbers

1 Answers1