C# Random Question

Question

I have two different realization random function:

Version 1:

  public double myRnd()
        {
            Random rnd = new Random();
            return rnd.NextDouble();
        }                


            //-- Generate Random Number for Delaer
            double DealersCard = myRnd();

            //-- Generate Players  4 Random Number
            double Box1Card = myRnd();
            double Box2Card = myRnd();
            double Box3Card = myRnd();
            double Box4Card = myRnd();

version 2:

       //-- Create  Random  Object 
        var rnd = new Random();

        //-- Generate Random Number for Delaer
        double DealersCard = rnd.NextDouble();

        //-- Generate Players  4 Random Number
        double Box1Card = rnd.NextDouble();
        double Box2Card = rnd.NextDouble();
        double Box3Card = rnd.NextDouble();
        double Box4Card = rnd.NextDouble();

Which method is better ?

Old question about the problems in your first version: http://stackoverflow.com/questions/767999/random-number-generator-not-working-the-way-i-had-planned-c — CodesInChaos, Jul 10 '11 at 22:32
Well, it doesn't really matter which one you use. You'll get the exact same random number back in version 1. That cannot affect the outcome, you need to shuffle the deck first. So using the exact same random number from a shuffled deck doesn't matter. I recommend 4, chosen by a fair roll of a die. — Hans Passant, Jul 10 '11 at 22:43

score 9 · Answer 1 · edited May 23 '17 at 11:48

9

Version 2 is much better. Version 1 is totally broken.

Version 1 recreates Random for each number. Random is seeded by Environment.TickCount which changes only every 1-16ms. So you will get the same random number for all calls to myRnd in that time interval.

In addition version 2 is faster since it doesn't need to recreate Random.

Similar questions have been asked quite often, usually by people who implemented version 1 and wondered why it doesn't for. For example: Random number generator only generating one random number

The MSDN documentation for Random states:

The random number generation starts from a seed value. If the same seed is used repeatedly, the same series of numbers is generated. One way to produce different sequences is to make the seed value time-dependent, thereby producing a different series with each new instance of Random. By default, the parameterless constructor of the Random class uses the system clock to generate its seed value, while its parameterized constructor can take an Int32 value based on the number of ticks in the current time. However, because the clock has finite resolution, using the parameterless constructor to create different Random objects in close succession creates random number generators that produce identical sequences of random numbers.

http://msdn.microsoft.com/en-us/library/system.random.aspx

I've filed an improvement request on Microsoft Connect since this is a very common mistake.

edited May 23 '17 at 11:48

Community

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answered Jul 10 '11 at 22:26

CodesInChaos

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Your improvement is for sure an improvement; but how does one implement it without breaking "two runs of the same app gives different randoms"? – Captain Giraffe Jul 10 '11 at 22:49
@Captain I don't understand what you mean. – CodesInChaos Jul 11 '11 at 06:30
In c++ it is: rand() gets the same value for srand(time(Now)) thats well defined. I think the same contract is at the c# corner – Captain Giraffe Jul 11 '11 at 21:43
1

I only proposed changing the parameterless constructor of `Random`. This will not affect users who supply a seed to get deterministic sequences. I don't think there is a contractual obligation for `new Random()` to use the time for seeding. – CodesInChaos Jul 11 '11 at 21:47
I'm still very interested in any solution you might have – Captain Giraffe Jul 11 '11 at 21:51
I don't really see what your problem is. – CodesInChaos Jul 11 '11 at 21:53
@CodeInChaos let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/1334/discussion-between-captain-giraffe-and-codeinchaos) – Captain Giraffe Jul 11 '11 at 21:55

score 1 · Answer 2 · answered Jul 10 '11 at 22:29

The first one doesn't work very well.

The point in any random function is that it is pseudo-random, and the random value you get depends on the seed. When you create the Random object, it is seeded with the current time.

So when you create a new Random object every time you want a random number, you end up seeding with the current time again and again, which might result in getting the same numbers repeatedly (because the current time might not have changed, depending on the accuracy and resolution of the timer it uses).

The Random class is used to create one object, from which you can get a sequence of random numbers.

C# Random Question

2 Answers2