Avoiding random duplicates

Question

System.Random generator = new Random(DateTime.Now.Millisecond);
int[] lotteryNumber = new int[7];

Console.WriteLine(\"Your lottery numbers: \");
for (int i = 0; i&l         


        

Answer 1

HashSet<int> set = new HashSet<int>();
System.Random generator = new Random(DateTime.Now.Millisecond);

while(set.Count < 7){
    set.Add(generator.Next(1,37);
}

That should work, since a HashSet will automatically ignore duplicates. Just loop until the set reaches the number of units you need. Only potential problem is it has the POTENTIAL (unlikely) to loop for a long time, but it should eventually respond.

Answer 2

The simplest approach IMO would be to generate a sequence of all the possible numbers (i.e. 1-37), shuffle the collection, then take the first seven results.

Searching on Stack Overflow for "Fisher-Yates shuffle C#" will find lots of examples.

In fact, you could modify the Fisher-Yates shuffle to yield results as you took them, so you could write a method such as:

var numbers = Enumerable.Range(1, 37).Shuffle().Take(7).ToList();

Answer 3

const int nBalls = 37;
const int nPicks = 6;
int[] balls = new int[nPicks];
Random rnd = new Random(DateTime.Now.Millisecond);

int remainingBalls=nBalls;
int remainingPicks=nPicks;
for (int i = 1; i <= nBalls; i++)
{
    if (rnd.Next(1, remainingBalls+1) <= remainingPicks)
        balls[--remainingPicks]=i;
    remainingBalls--;
}

Console.WriteLine(string.Join(",",balls));

Will outperform Shuffle and HashSet methods as nPicks/nBalls gets larger.