difference between instantiating before the for loop and instantiating in the for loop

Question

so I've just started learning C# and I got across this exercise that rolls a die N times and then print the number of times each side is rolled. I got the answer however, my question lies on instantiating a Random number object before the loop and in the loop.

my code goes like this (this is instantiation before the loop):

static void DiceRoll(int RollTimes)
    {

        int roll = 0;

        int counter1 = 0;
        int counter2 = 0;
        int counter3 = 0;
        int counter4 = 0;
        int counter5 = 0;
        int counter6 = 0;

        Random rnd = new Random();

        for (int ctr = 0; ctr < RollTimes; ctr++)
        {
            roll = 0;
            roll = (int)rnd.Next(1, 7);

            switch (roll)
            {
                case 1:
                    counter1++;
                    break;

                case 2:
                    counter2++;
                    break;

                case 3:
                    counter3++;
                    break;

                case 4:
                    counter4++;
                    break;

                case 5:
                    counter5++;
                    break;

                case 6:
                    counter6++;
                    break;
            }
        }

        Console.WriteLine("1 is rolled {0} times.", counter1);
        Console.WriteLine("2 is rolled {0} times.", counter2);
        Console.WriteLine("3 is rolled {0} times.", counter3);
        Console.WriteLine("4 is rolled {0} times.", counter4);
        Console.WriteLine("5 is rolled {0} times.", counter5);
        Console.WriteLine("6 is rolled {0} times.", counter6);
    }

and the result is like this:

1 is rolled A times.
2 is rolled B times.
3 is rolled C times.
4 is rolled D times.
5 is rolled E times.
6 is rolled F times.

before I got that right, the instantiation line (Random rnd = new Random();) was in the loop.

for (int ctr = 0; ctr < RollTimes; ctr++)
        {
            roll = 0;
            roll = (int)rnd.Next(1, 7);
            Random rnd = new Random();
            // rest of the code

then the result (at random):

1 is rolled (N) times.
2 is rolled (N) times.
3 is rolled (N) times.
4 is rolled (N) times.
5 is rolled (N) times.
6 is rolled (N) times.

Can somebody explain or enlighten me why the position of instantiation alters the result? Thanks!

score 0 · Answer 1 · answered Jul 19 '15 at 13:58

The ideea is that the random object is not actually "random" and to be honest there is no such a thing in reality, except maybe in quantum mechanics. Ok, going back to your problem, to enlight you the ideea is that behind the scenes there is a series ( very complex one ) that for a given starting value ( the default value is zero) next values are computer by grabbing internally the next series value. The complexity of the internal series gives you the feeling that values are actually random. Whenever you instantiate a new Random object with the same seed you will have the same series values. The best pattern would be to use the overloaded constructor that takes a seed and use for the seed value something lime Datetime.Now.Ticks and try to cache your random object.

So the answer is to create a single instance outside the loop and even better use a different seed for each instantiation.

difference between instantiating before the for loop and instantiating in the for loop

1 Answers1