C#•15mo ago

❔ Random problem

When choosing a random number in c#, I don't want the same number to come across again, how can I do it?

40 Replies

Do you mean 1, 2, 3, 4 is okay; But 1, 2, 3, 4, 1 is not okay?

mtreit•15mo ago

Yes, define your exact requirements. How many random numbers do you expect to generate and does it matter what range of values they fall into? There are probably several ways of solving this depending on your exact needs.

wwww•15mo ago

List<int> list = new List<int>();
Random rnd = new Random();

const int randmomNumbersAmount = 10;
const int randomLeftBorder = 0, randomRightBorder = 10;

while(list.Count < randmomNumbersAmount)
{
    int number = rnd.Next(randomLeftBorder, randomRightBorder);
    if (!list.Contains(number))
        list.Add(number);
}

List<int> list = new List<int>();
Random rnd = new Random();

const int randmomNumbersAmount = 10;
const int randomLeftBorder = 0, randomRightBorder = 10;

while(list.Count < randmomNumbersAmount)
{
    int number = rnd.Next(randomLeftBorder, randomRightBorder);
    if (!list.Contains(number))
        list.Add(number);
}

You can try something like this if I understand correctly what you want.

mtreit•15mo ago

A HashSet would be a much better data structure than a List for keeping track of the values that have already been used

Mayor McCheese•15mo ago

That's what I was thinking but I didn't want to type it out on my phone without being sure of reqs

Google•15mo ago

Fisher-Yates shuffle and call it a day

hiyosilver•15mo ago

How does shuffling help?

Mayor McCheese•15mo ago

Doesn't that require a known set of values first?

Google•15mo ago

1..int.maxvalue

!ScofieldOP•15mo ago

yes i want this I'm sorry for the late reply i will try thank you all in advance

Mayor McCheese•15mo ago

like @mtreit said, a HashSet will better handle the case of I've already got it

mtreit•15mo ago

How many random numbers do you need to generate? Because by the pigeon hole principle you will eventually have to repeat.

!ScofieldOP•15mo ago

estimated 45-50

mtreit•15mo ago

Do they need to be within a certain range?

!ScofieldOP•15mo ago

OK, let it be repeated, but for example, coming back to 5 after 5 is a problem for me. yes

mtreit•15mo ago

What's the range?

Mayor McCheese•15mo ago

1-2

mtreit•15mo ago

Mayor McCheese•15mo ago

There's an infinite set of numbers between 1 and 2 3.14 to 3.15 is interesting too

Mayor McCheese•15mo ago

https://tenor.com/view/ralph-wiggum-simpsons-hi-bye-gif-16529059407582436389

Tenor

!ScofieldOP•15mo ago

I don't understand much from c#. I'm working on Unity and I want to turn off the random number doubles in my game. It doesn't feel right for me to share because there are Unity codes rndm = Random.Range(1, 45);

mtreit•15mo ago

So you want to hand out 50 random numbers in the range of 1 to 45 without duplicates?

Mayor McCheese•15mo ago

https://tenor.com/view/sweating-nervous-wreck-gif-24688521

Tenor

!ScofieldOP•15mo ago

I can explain like this. 34,23,43,43 I don't want the same number to appear side by side again. I want the incoming number to be less likely to come back Im really sorry if I cant explain but my english is not that clear or if a number comes up once, number must wait for the other 50 numbers to finish before it comes back

mtreit•15mo ago

I think you can achieve this by simply generating a pool of numbers, shuffling them, and then keeping a pointer into that shuffled set and using that to hand out the next value. Do you want to hand out more numbers than is in the range (like the example I gave?) Because that's obviously a problem since duplicates are guaranteed 🙂

!ScofieldOP•15mo ago

Yes it makes sense. I will do it. Thanks for your help No, I know I can't explain my problem fully, but I think I'd better do what you just said.

mtreit•15mo ago

I'm not sure it's the best approach, you could still end up with duplicates next to each other in the sequence. @!Scofield here is a completely brute force way to do this that will definitely work. It has theoretical performance problems if the same number that has already been sent keeps getting generated, but in your example it's probably fine.

wwww•15mo ago

like, 34, 23, 43, 20, 43 is ok?

mtreit•15mo ago

using System;
using System.Collections.Generic;

namespace Test
{
    class TestClass
    {
        static void Main()
        {
            var gen = new RandomGenerator(1, 45);
            for (int i = 0; i < 100; i++)
            {
                Console.Write(gen.Next());
                Console.Write(" ");
            }
        }
    }

    internal class RandomGenerator
    {
        int _numberSeen;
        int _minVal;
        int _maxVal;
        HashSet<int> _previous;

        public RandomGenerator(int minVal, int maxValInclusive)
        {
            _previous = new HashSet<int>();
            _minVal = minVal;
            _maxVal = maxValInclusive;
        }

        public int Next()
        {
            while (true)
            {
                var candidate = Random.Shared.Next(_minVal, _maxVal + 1);
                var maxAllowed = _maxVal + 1 - _minVal;

                if (_numberSeen >= maxAllowed)
                {
                    _numberSeen = 0;
                    _previous.Clear();
                    Console.WriteLine();
                }

                if (!_previous.Contains(candidate))
                {
                    _previous.Add(candidate);
                    _numberSeen++;
                    return candidate;
                }
            }
        }
    }
}

using System;
using System.Collections.Generic;

namespace Test
{
    class TestClass
    {
        static void Main()
        {
            var gen = new RandomGenerator(1, 45);
            for (int i = 0; i < 100; i++)
            {
                Console.Write(gen.Next());
                Console.Write(" ");
            }
        }
    }

    internal class RandomGenerator
    {
        int _numberSeen;
        int _minVal;
        int _maxVal;
        HashSet<int> _previous;

        public RandomGenerator(int minVal, int maxValInclusive)
        {
            _previous = new HashSet<int>();
            _minVal = minVal;
            _maxVal = maxValInclusive;
        }

        public int Next()
        {
            while (true)
            {
                var candidate = Random.Shared.Next(_minVal, _maxVal + 1);
                var maxAllowed = _maxVal + 1 - _minVal;

                if (_numberSeen >= maxAllowed)
                {
                    _numberSeen = 0;
                    _previous.Clear();
                    Console.WriteLine();
                }

                if (!_previous.Contains(candidate))
                {
                    _previous.Add(candidate);
                    _numberSeen++;
                    return candidate;
                }
            }
        }
    }
}

Sample output:

44 6 31 5 19 29 17 43 35 34 2 1 39 23 27 13 11 45 40 26 37 22 36 18 24 16 20 7 32 28 12 25 15 8 10 30 41 42 14 3 33 4 38 9 21
4 34 13 1 43 19 42 39 26 32 5 2 15 28 33 44 38 21 35 41 10 25 3 18 9 37 36 45 27 29 11 7 12 17 24 8 23 20 16 30 14 22 31 40 6
44 28 20 26 25 43 6 34 33 39

44 6 31 5 19 29 17 43 35 34 2 1 39 23 27 13 11 45 40 26 37 22 36 18 24 16 20 7 32 28 12 25 15 8 10 30 41 42 14 3 33 4 38 9 21
4 34 13 1 43 19 42 39 26 32 5 2 15 28 33 44 38 21 35 41 10 25 3 18 9 37 36 45 27 29 11 7 12 17 24 8 23 20 16 30 14 22 31 40 6
44 28 20 26 25 43 6 34 33 39

No number will ever repeat until all possible values have been sent Performance of this seems totally fine after testing, I think it's a much better solution than my original suggestion.

Google•15mo ago

Fisher-Yates shuffle wins!

Mayor McCheese•15mo ago

Fyaseh-Resti Shuffle

mtreit•15mo ago

No, it was rejected

!ScofieldOP•15mo ago

Im really grateful, sorry if I stole your time

SinFluxx•15mo ago

https://tenor.com/view/time-theif-fire-him-the-office-dwight-schrute-rainn-wilson-gif-15464769

Tenor

mtreit•15mo ago

Not at all, it was a fun problem to think about.

!ScofieldOP•15mo ago

https://tenor.com/view/no-god-please-no-no-please-no-no-god-no-yelling-gif-26334010

Tenor

mtreit•15mo ago

Actually I guess if you re-shuffled after the entire sequence has been produced you would avoid repeating the same exact number sequence over and over again.

Google•15mo ago

Yep. It's deterministic.

mtreit•15mo ago

using System;

namespace Test
{
    class TestClass
    {
        static void Main()
        {
            var gen = new RandomGenerator(1, 10);
            for (int i = 0; i < 100; i++)
            {
                Console.Write(gen.Next());
                Console.Write(" ");
            }
        }
    }

    internal class RandomGenerator
    {
        int _numberSeen;
        int _minVal;
        int _maxVal;
        int[] _values;

        public RandomGenerator(int minVal, int maxValInclusive)
        {
            _minVal = minVal;
            _maxVal = maxValInclusive;
            var maxAllowed = _maxVal + 1 - _minVal;
            _values = new int[maxAllowed];
            for (int i = minVal; i < _values.Length; i++)
            {
                _values[i] = i;
            }

            FisherYates(_values);
        }

        public int Next()
        {
            if (_numberSeen == _values.Length)
            {
                FisherYates(_values);
                Console.WriteLine();
                _numberSeen = 0;
            }

            return _values[_numberSeen++];
        }

        public void FisherYates<T>(T[] values)
        {
            for (int i = values.Length - 1; i >= 0; --i)
            {
                int n = Random.Shared.Next(i, values.Length);
                Swap(i, n);
            }

            void Swap(int x, int y)
            {
                T tmp = values[x];
                values[x] = values[y];
                values[y] = tmp;
            }
        }
    }
}

using System;

namespace Test
{
    class TestClass
    {
        static void Main()
        {
            var gen = new RandomGenerator(1, 10);
            for (int i = 0; i < 100; i++)
            {
                Console.Write(gen.Next());
                Console.Write(" ");
            }
        }
    }

    internal class RandomGenerator
    {
        int _numberSeen;
        int _minVal;
        int _maxVal;
        int[] _values;

        public RandomGenerator(int minVal, int maxValInclusive)
        {
            _minVal = minVal;
            _maxVal = maxValInclusive;
            var maxAllowed = _maxVal + 1 - _minVal;
            _values = new int[maxAllowed];
            for (int i = minVal; i < _values.Length; i++)
            {
                _values[i] = i;
            }

            FisherYates(_values);
        }

        public int Next()
        {
            if (_numberSeen == _values.Length)
            {
                FisherYates(_values);
                Console.WriteLine();
                _numberSeen = 0;
            }

            return _values[_numberSeen++];
        }

        public void FisherYates<T>(T[] values)
        {
            for (int i = values.Length - 1; i >= 0; --i)
            {
                int n = Random.Shared.Next(i, values.Length);
                Swap(i, n);
            }

            void Swap(int x, int y)
            {
                T tmp = values[x];
                values[x] = values[y];
                values[y] = tmp;
            }
        }
    }
}

Here's the same program but using Fisher-Yates

Accord•15mo ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ Random problem