Random numbers for dice game

Question

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random string generation - two generated one after another give same results

I am writing a simple dice game for windows phone 7, that involves rolling two dice at the same time. Here is my Dice Roll Code:

 private int DiceRoll()
    {
        int result;
        Random rnd = new Random();

        result = rnd.Next(1, 7);
        return result;
    }

I then have this code that rolls the dice when a button is clicked:

   private void roll_Click(object sender, RoutedEventArgs e)
    {
        roll1 = DiceRoll();
        roll2 = DiceRoll();}

My problem is that both die get the same result.

Any idea how I can get a rolling algorithm that will usually return different results, but occasionally return the same?

score 12 · Accepted Answer · answered Mar 19 '11 at 02:12

The default seed for Random is based on the current time. To quote the documentation,

As a result, different Random objects that are created in close succession by a call to the default constructor will have identical default seed values and, therefore, will produce identical sets of random numbers. This problem can be avoided by using a single Random object to generate all random numbers.

That is exactly what you should do: create one instance of Random and use it to generate all your random numbers.

score 4 · Answer 2 · answered Mar 19 '11 at 02:10

4

You need to keep one Random object around and reuse it; every time you create a new Random object, you effectively reset the sequence of numbers to begin in the same place. Store the Random object as a member variable someplace. You'll also want to seed it with a different value each time you run the program -- for example, a value based on the system clock time.

answered Mar 19 '11 at 02:10

Ernest Friedman-Hill

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Doesn't the random function in C# automatically seed from the system time though? – BigPete Mar 19 '11 at 02:11
Correct, except that you don't have to seed it, it's already seeded with a time dependent value (http://msdn.microsoft.com/en-us/library/h343ddh9.aspx) – steinar Mar 19 '11 at 02:12
1

@BigPete: You are correct, the default seed is based on the system time. – Anomie Mar 19 '11 at 02:13
2

@BigPete: correct, but if you create two Random objects in quick succession they are likely to have the same seed because of the finite resolution of the system time. – Gary Kerr Mar 19 '11 at 02:16

score 4 · Answer 3 · answered Mar 19 '11 at 02:11

The clear majority of 'random number' tools I've seen fail badly if you allocate two or more random objects in a single application. You're allocating a new Random object for every invocation, and each time they are going to be seeded with something pretty weak, and maybe even identical seeds.

So, generate a single Random object and use it over the life of your application.

Random numbers for dice game

3 Answers3