Generating random, unique values C#

Question

I've searched for a while and been struggling to find this, I'm trying to generate several random, unique numbers is C#. I'm using System.Random, and I'm using a DateTime.Now.Ticks seed:

public Random a = new Random(DateTime.Now.Ticks.GetHashCode());
private void NewNumber()
{
    MyNumber = a.Next(0, 10);
}

I'm calling NewNumber() regularly, but the problem is I often get repeated numbers. Some people suggested because I was declaring the random every time I did it, it would not produce a random number, so I put the declaration outside my function. Any suggestions or better ways than using System.Random ? Thank you

Answer 1

I'm calling NewNumber() regularly, but the problem is I often get repeated numbers.

Random.Next doesn't guarantee the number to be unique. Also your range is from 0 to 10 and chances are you will get duplicate values. May be you can setup a list of int and insert random numbers in the list after checking if it doesn't contain the duplicate. Something like:

public Random a = new Random(); // replace from new Random(DateTime.Now.Ticks.GetHashCode());
                                // Since similar code is done in default constructor internally
public List<int> randomList = new List<int>();
int MyNumber = 0;
private void NewNumber()
{
    MyNumber = a.Next(0, 10);
    if (!randomList.Contains(MyNumber))
        randomList.Add(MyNumber);
}

Answer 2

You might try shuffling an array of possible ints if your range is only 0 through 9. This adds the benefit of avoiding any conflicts in the number generation.

var nums = Enumerable.Range(0, 10).ToArray();
var rnd = new Random();

// Shuffle the array
for (int i = 0;i < nums.Length;++i)
{
    int randomIndex = rnd.Next(nums.Length);
    int temp = nums[randomIndex];
    nums[randomIndex] = nums[i];
    nums[i] = temp;
}

// Now your array is randomized and you can simply print them in order
for (int i = 0;i < nums.Length;++i)
    Console.WriteLine(nums[i]);

Answer 3

NOTE, I dont recommend this :). Here's a "oneliner" as well:

var result = Enumerable.Range(0,9).OrderBy(g => Guid.NewGuid()).ToArray();

Answer 4

I'm posting a correct implementation of a shuffle algorithm, since the other one posted here doesn't produce a uniform shuffle.

As the other answer states, for small numbers of values to be randomized, you can simply fill an array with those values, shuffle the array, and then use however many of the values that you want.

The following is an implementation of the Fisher-Yates Shuffle (aka the Knuth Shuffle). (Read the "implementation errors" section of that link (search for "always selecting j from the entire range of valid array indices on every iteration") to see some discussion about what is wrong with the other implementation posted here.)

using System;
using System.Collections.Generic;

namespace ConsoleApplication2
{
    static class Program
    {
        static void Main(string[] args)
        {
            Shuffler shuffler = new Shuffler();
            List<int> list = new List<int>{ 1, 2, 3, 4, 5, 6, 7, 8, 9 };
            shuffler.Shuffle(list);

            foreach (int value in list)
            {
                Console.WriteLine(value);
            }
        }
    }

    /// <summary>Used to shuffle collections.</summary>

    public class Shuffler
    {
        public Shuffler()
        {
            _rng = new Random();
        }

        /// <summary>Shuffles the specified array.</summary>
        /// <typeparam name="T">The type of the array elements.</typeparam>
        /// <param name="array">The array to shuffle.</param>

        public void Shuffle<T>(IList<T> array)
        {
            for (int n = array.Count; n > 1; )
            {
                int k = _rng.Next(n);
                --n;
                T temp = array[n];
                array[n] = array[k];
                array[k] = temp;
            }
        }

        private System.Random _rng;
    }
}

Answer 5

This is a unity only answer:

Check this ready-to-use method: Give in a range & count of number you want to get.

public static int[] getUniqueRandomArray(int min, int max, int count) {
    int[] result = new int[count];
    List<int> numbersInOrder = new List<int>();
    for (var x = min; x < max; x++) {
        numbersInOrder.Add(x);
    }
    for (var x = 0; x < count; x++) {
        var randomIndex = UnityEngine.Random.Range(0, numbersInOrder.Count);
        result[x] = numbersInOrder[randomIndex];
        numbersInOrder.RemoveAt(randomIndex);
    }

    return result;
}

Answer 6

Same as @Habib's answer, but as a function:

List<int> randomList = new List<int>();
int UniqueRandomInt(int min, int max)
{
    var rand = new Random();
    int myNumber;
    do
    {
       myNumber = rand.Next(min, max);
    } while (randomList.Contains(myNumber));
    return myNumber;
}

If randomList is a class property, UniqueRandomInt will return unique integers in the context of the same instance of that class. If you want it to be unique globally, you will need to make randomList static.

Answer 7

Depending on what you are really after you can do something like this:

using System;
using System.Collections.Generic;
using System.Linq;

namespace SO14473321
{
    class Program
    {
        static void Main()
        {
            UniqueRandom u = new UniqueRandom(Enumerable.Range(1,10));
            for (int i = 0; i < 10; i++)
            {
                Console.Write("{0} ",u.Next());
            }
        }
    }

    class UniqueRandom
    {
        private readonly List<int> _currentList;
        private readonly Random _random = new Random();

        public UniqueRandom(IEnumerable<int> seed)
        {
            _currentList = new List<int>(seed);
        }

        public int Next()
        {
            if (_currentList.Count == 0)
            {
                throw new ApplicationException("No more numbers");
            }

            int i = _random.Next(_currentList.Count);
            int result = _currentList[i];
            _currentList.RemoveAt(i);
            return result;
        }
    }
}

Answer 8

And here my version of finding N random unique numbers using HashSet. Looks pretty simple, since HashSet can contain only different items. It's interesting - would it be faster then using List or Shuffler?

using System;
using System.Collections.Generic;

namespace ConsoleApplication1
{
    class RnDHash
    {
        static void Main()
        {
            HashSet<int> rndIndexes = new HashSet<int>();
            Random rng = new Random();
            int maxNumber;
            Console.Write("Please input Max number: ");
            maxNumber = int.Parse(Console.ReadLine());
            int iter = 0;
            while (rndIndexes.Count != maxNumber)
            {
                int index = rng.Next(maxNumber);
                rndIndexes.Add(index);
                iter++;
            }
            Console.WriteLine("Random numbers were found in {0} iterations: ", iter);
            foreach (int num in rndIndexes)
            {
                Console.WriteLine(num);
            }
            Console.ReadKey();
        }
    }
}

Answer 9

I noted that the accepted answer keeps adding int to the list and keeps checking them with if (!randomList.Contains(MyNumber)) and I think this doesn't scale well, especially if you keep asking for new numbers.

I would do the opposite.

1. Generate the list at startup, linearly
2. Get a random index from the list
3. Remove the found int from the list

This would require a slightly bit more time at startup, but will scale much much better.

public class RandomIntGenerator
{
    public Random a = new Random();
    private List<int> _validNumbers;

    private RandomIntGenerator(int desiredAmount, int start = 0)
    {
        _validNumbers = new List<int>();
        for (int i = 0; i < desiredAmount; i++)
            _validNumbers.Add(i + start);
    }

    private int GetRandomInt()
    {
        if (_validNumbers.Count == 0)
        {
            //you could throw an exception here
            return -1;
        }
        else
        {
            var nextIndex = a.Next(0, _validNumbers.Count - 1);
            var number    = _validNumbers[nextIndex];
            _validNumbers.RemoveAt(nextIndex);
            return number;
        }
    }
}

Answer 10

It's may be a little bit late, but here is more suitable code, for example when you need to use loops:

            List<int> genered = new List<int>();

            Random rnd = new Random();

            for(int x = 0; x < files.Length; x++)
            {
                int value = rnd.Next(0, files.Length - 1);
                while (genered.Contains(value))
                {
                    value = rnd.Next(0, files.Length - 1);
                }
                genered.Add(value);

                returnFiles[x] = files[value];
            }

Answer 11

• with Functional way*

        static Func<int> GetNextUniqueIntegerFunc(int min, int max)
        {
            var list = new List<int>();

            var random = new Random();

            int getNextValue()
            {
                while (true)
                {
                    var random_number = random.Next(min, max);

                    if (!list.Contains(random_number))
                    {
                        list.Add(random_number);

                        return random_number;
                    }
                }
            }

            return getNextValue;
        }

Answer 12

Random r = new Random(); int[] v = new int[10];
        for (int i = 0; i < 10; i++)
        {
            v[i] = r.Next(1, 25); // random numbers between (1) and (25)
            for (int j = 0; j < i; j++)
            {
                if (v[j] == v[i]) // if it is a duplicated value, create new one!
                    i--;
            }
        }

        for (int i = 0; i < 10; i++)
            textBox1.Text += v[i].ToString() + " ";

Answer 13

unique random number from 0 to 9

      int sum = 0;
        int[] hue = new int[10];
        for (int i = 0; i < 10; i++)
        {

            int m;
            do
            {
                m = rand.Next(0, 10);
            } while (hue.Contains(m) && sum != 45);
            if (!hue.Contains(m))
            {
                hue[i] = m;
                sum = sum + m;
            }

        }

Answer 14

You could also use a dataTable storing each random value, then simply perform the random method while != values in the dataColumn

Answer 15

randomNumber function return unqiue integer value between 0 to 100000

  bool check[] = new bool[100001];
  Random r = new Random();
  public int randomNumber() {
      int num = r.Next(0,100000);
       while(check[num] == true) {
             num = r.Next(0,100000);
     }
    check[num] = true;
   return num;
 }

Answer 16

hi here i posted one video ,and it explains how to generate unique random number

  public List<int> random_generator(){

  Random random = new Random();

   List<int> random_container = new List<int>;

     do{

       int random_number = random.next(10);

      if(!random_container.contains(random_number){

       random_container.add(random_number)
  }
}
   while(random_container.count!=10);


     return random_container; 
  }

here ,,, in random container you will get non repeated 10 numbers starts from 0 to 9(10 numbers) as random.. thank you........

Answer 17

You can use basic Random Functions of C#

Random ran = new Random();
int randomno = ran.Next(0,100);

you can now use the value in the randomno in anything you want but keep in mind that this will generate a random number between 0 and 100 Only and you can extend that to any figure.

Answer 18

Try this:

private void NewNumber()
  {
     Random a = new Random(Guid.newGuid().GetHashCode());
     MyNumber = a.Next(0, 10);
  }

Some Explnations:

Guid : base on here : Represents a globally unique identifier (GUID)

Guid.newGuid() produces a unique identifier like "936DA01F-9ABD-4d9d-80C7-02AF85C822A8"

and it will be unique in all over the universe base on here

Hash code here produce a unique integer from our unique identifier

so Guid.newGuid().GetHashCode() gives us a unique number and the random class will produce real random numbers throw this

Sample: https://rextester.com/ODOXS63244

generated ten random numbers with this approach with result of:

-1541116401
7
-1936409663
3
-804754459
8
1403945863
3
1287118327
1
2112146189
1
1461188435
9
-752742620
4
-175247185
4
1666734552
7

we got two 1s next to each other, but the hash codes do not same.