How to create a Random Unique number in C# using a Recursive Method?

Question

I'm trying to generate 5 random numbers between 1-10 which do not have duplicate values. Therefore, I've created a recursive method which should check to see if the value created for whichever position in the array has been used already. If it has, then it will create a new random number and check again.

Here's my code:

    static Random randomObject = new Random();

    static void Main(string[] args)
    {

    long[] randomArr = new long[5];


    for (int i = 0; i < randomArr.Length; i++ )
    {
        if (randomArr[i] == randomArr[0])
        {
            randomArr[i] = randomObject.Next(1, 11);
        }
        else 
        {
            long check = randomObject.Next(1, 11);
            randomArr[i] = CheckIfUnique(check, randomArr);
        }
    }

    Console.WriteLine("\nPress the [enter] key to continue...");
    Console.ReadLine();

}

static long CheckIfUnique(long a, long[] b) 
{
    for (int i = 0; i <= b.GetUpperBound(0); i++) 
    {
        if (a == b[i])
        {
           a = randomObject.Next(1, 11);
           CheckIfUnique(a, b);
        }
    }
    return a;
}

But I'm still getting duplicate values. Does anyone know if there is an error in my logic, or if the compiler will crap out after so many recursive steps?

Answer 1

NOTE - THIS IS NOT A RECURSIVE ANSWER

---

You can make that much easier really:

static Random randomObject = new Random();

static void Main(string[] args)
{
    long[] randomArr = new long[5];

    for (int i = 0; i < randomArr.Length;)
    {
        long t = randomObject.Next(1, 11);
        if(CheckIfUnique(t, randomArr, i))
        {
              randomArr[i++] = t;
        }
    }
}

static bool CheckIfUnique(long a, long[] b, int length)
{
    for (int i = 0; i < length; i++) 
    {
        if (a == b[i])
        {
           return false
        }
    }
    return true;
}

This way, you're looping till you have enough random values, but you're only advancing the counter once a new unique value has been generated.

Answer 2

your approach is really heavy weight. As a variant you can see this approach. However, if you want I can try to correct your algorithm. The order of numbers are original which is created by Random class:

HashSet<int> origNumbers = new HashSet<int>();
Random rnd=new Random();            
do
{
   int k = rnd.Next(10);
   origNumbers.Add(k);
}
while (origNumbers.Count <5);

Update(thanks to @AlexeiLevenkov):

If order of added elements is important, it is preferable to use collection SortedSet<T>. The order of numbers created by Random class are sorted in ascending order, it is not the order created by Random class:

SortedSet<int> origNumbers = new SortedSet<int>();
Random rnd=new Random();            
do
{
   int k = rnd.Next(10);
   origNumbers.Add(k);
}
while (origNumbers.Count <5);

Answer 3

Here is a very simple solution:

var random = new System.Random(Guid.NewGuid().GetHashCode());  
var values = Enumerable.Range(0, 10).OrderBy(x => random.Next()).Take(5).ToArray();

Answer 4

It's not recursive, but unless it is homework, recursive would not work really well for your case

In this case, with the following assumptions:

1) No duplicates

2) Relatively small range of possible values (a few hundreds at most?)

I would populate a small initial dataset with all your possible values, and then select from those:

    static Random randomObject = new Random();
    static void Main(string[] args)
    {
        int MAX_NUMBER = 10;
        int MIN_NUMBER = 1;
        int NUMBER_OF_RESULTS_REQUIRED = 5;

        long[] randomArr = new long[NUMBER_OF_RESULTS_REQUIRED];
        //Use a pool of all possible values
        List<long> dataSet = new List<long>();


        for (int i = MIN_NUMBER; i <= MAX_NUMBER; ++i)
        {
            dataSet.Add(i);
        }

        for (int i = 0; i < NUMBER_OF_RESULTS_REQUIRED; ++i)
        {
            int t = randomObject.Next(0, dataSet.Count);
            randomArr[i] = dataSet[t];
            dataSet.RemoveAt(t);
        }
        for (int i = 0; i < randomArr.Length; ++i)
        {
            Console.WriteLine(randomArr[i]);
        }
        Console.ReadKey();
    }