"new Random(x)" always generates the same numbers?

Question

I am trying to get a unique random number but i always get the same number every time i run the code. i will get 14 first then 6 but my list for holding all the used numbers doesn't seem to work.adding the 14 manually works but when i add randInt it doesn't work.

const int numCards = 32;
        List<int> usedCards = new List<int>();
        int randInt = 0;
        Random rand = new Random(numCards);
        usedCards.Add(14);
        do
        {

            randInt = rand.Next(0, numCards);
            MessageBox.Show(randInt.ToString(), "End Game", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);

        }
        while (usedCards.Contains(randInt));

        usedCards.Add(randInt);

@Alexei: that's a different mistake -- that's making a new `Random` on every use. — sarnold, Apr 20 '12 at 02:19
@AlexeiLevenkov "almost", but this is more ... specific. A good related question, however. — , Apr 20 '12 at 02:20

BrokenGlass · Accepted Answer · 2012-04-20T02:31:17.727

17

Replace:

Random rand = new Random(numCards);

with

Random rand = new Random();

Supplying a fixed seed value (numCards always has the same value) in the Random constructor call will result in a predictable, reproducible sequence which is always the same for the same seed value, just like this (not quite but still the point is valid):

enter image description here

For example using a fixed seed of 1 and drawing 10 numbers ranging from 0 to 100, on my machine always produces the sequence

24,11,46,77,65,43,35,94,10,64

Using no seed value on the other hand, the sequence becomes unpredictable.

Without a seed value passed in, Random will generate a new seed value based on the current time, that's what you want to get a new sequence of random numbers - provided you don't initialize it again in fast order, that's why you should re-use Random instances instead of re-creating them every time.

edited Apr 20 '12 at 02:31

answered Apr 20 '12 at 02:14

BrokenGlass

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Sorry, this needs to be included for situations like this: http://xkcd.com/221/ – David Z. Apr 20 '12 at 02:16
@David Z.: Why? That's not the same thing at all. – jason Apr 20 '12 at 02:27
thanks that works now i believe. now on to other problems with my code – user1345363 Apr 20 '12 at 02:33
Very good answer. Very nice picture :) You found the part of an algorithm of programs in Casino. :) – Samuel Dec 12 '13 at 16:17

score 3 · Answer 2 · answered Apr 20 '12 at 02:23

Random Class

If you specify a seed value to the Random class constructor, it will generate the same sequence every time. The algorithm to produce the random number sequence is deterministic.

If you use the constructor without parameters, the Random object will use a seed value on the current time, so each Random object instantiated using this constructor will have a different sequence of random numbers as long as the time seed value is different which is likely.

You can still get the same sequence of numbers from two different Random objects if they are initialized very close in time to each other.

Random Constructor

"new Random(x)" always generates the same numbers?

2 Answers2

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