CODEWITHSUNDEEP
c#random
Helen
Helen

Reputation: 357

Functioning of Random, C#

I have a question about working of Random in C#. Say I want to call some function if variable i == 0. I have the following code:

Random rnd = new Random();
int i = rnd.Next(5);
if (i == 0){
   myFunction();
}

So, I would call myFunction() one time per 5 launches of the program. And what if I had another code:

Random rnd = new Random();
for (int j = 0; j < 10; j++){
    int i = rnd.Next(50);
    if (i == 0){
       myFunction();
    }
}

Would I have the same result in the final? (calling of myFunction() one time per 5 launches of the program)

Upvotes: 0

Views: 135

Answers (2)

lemon
lemon

Reputation: 346

If you give it a try, running this several times :

class Program
{
    static int _caseOneCount = 0;
    static int _caseTwoCount = 0;

    static Random _rnd = new Random();

    static void Main( string[] args )
    {
        var max = 100000;

        for ( var i = 0 ; i < max ; i++ )
        {
            CaseOne();
            CaseTwo();

            Console.WriteLine( _caseOneCount.ToString() + "/" + _caseTwoCount.ToString() );
        }
    }

    static void CaseOne()
    {
        if ( _rnd.Next( 5 ) == 0 )
            _caseOneCount++;
    }

    static void CaseTwo()
    {
        for ( var i = 0 ; i < 10 ; i++ )
            if ( _rnd.Next( 50 ) == 0 )
                _caseTwoCount++;
    }
}

You will see that the results are nearly equivalent and close to 20% as expected.

Edit : Now if you run CaseOne and CaseTwo only once, you can have :

  • CaseOne : only 0 or 1,
  • CaseTwo : a value from 0 to 10

Edit 2 : following the comments of @Jean-ClaudeColette. The second case corresponds to a binomial distribution (https://en.wikipedia.org/wiki/Binomial_distribution).

So as results, the probability to have :

  • 0 call is 81.7%
  • 1 call is 16.7%
  • 2 calls is 1.5%
  • more is 100% minus the above which is around 0.086%

But the average value stays 20%.

Which means indeed that applying the second case only once will lead to a different result compared to the first case.

Random and its details are in the documentation : https://msdn.microsoft.com/fr-fr/library/system.random(v=vs.110).aspx

And the description of the inner algorithm (Knuth subtractive random generator) is described here (with a C# implementation which is not the .Net implementation but a way to see how it works) : https://rosettacode.org/wiki/Subtractive_generator

Upvotes: 3

sidecus
sidecus

Reputation: 752

Actually both your above statements were wrong.

For your first loop, there is no guarantee that your function will get called one time per 5 launches - but if you run it enough times the probability of your function got called is 1/5.

For your second code sample, the probability is 1/50 instead. And your outer (j) loop just controls how many "launches" you are going to run using your words - it doesn't change the probability.

Upvotes: 0

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