Generate Random int number outside given List<int>

Question

I am currently trying to find a way to generate a random number outside given list.

I did this

public static int getNewNumberOutsideGivenList(List<int> _list, int _min, int _max)
{
    int a = 0;
    // I use "Unity.Random", but as C# user you guys can use System.Random function
    a = UnityEngine.Random.Range(_min, _max +1); 
    while (_list.Contains(a))
        a = UnityEngine.Random.Range(_min, _max + 1);

    return a;
}

It works(kinda) well (I got a short drop in FPS like 10ms after calling this function).

However i keep wondering, "is there a better way to do this?" or "does System.Linq has a function that is faster than that code?"

Can you please enlightened me if there exist a better way? Thank yoouuu for your insight. :)

Answer 1

IMHO you should pre intialize all possible values and than shuffle the list.

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

                    
public class Program
{
    static Queue<int> _randomNumbers;
    static Program()
    {
        var min = 0;
        var max = 10;
        var list = new List<int>();
        for (int i = min; i < max; i++)
            list.Add(i);
        
        _randomNumbers = new Queue<int>(list.OrderBy(x => Guid.NewGuid()));
    }
    
    public static void Main()
    {
        while (true)
        {
            var next = GetNext();
            if (next is null)
                break;
            
            Console.WriteLine(next);
        }
    }
    
    static int? GetNext() => _randomNumbers.Count == 0 ? null : _randomNumbers.Dequeue();
}

https://dotnetfiddle.net/pBsmyH

You may find a better shuffle alternative here:

Randomize a List<T>

Edit:

Here a version without the List<int> but instead it uses Enumerable.Range(start, count);

_randomNumbers = new Queue<int>(Enumerable.Range(min, max - min).OrderBy(x => Guid.NewGuid()));

https://dotnetfiddle.net/S8aDuE

Edit2:

By popular demand here a version with one of the better shuffle alternatives

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

                    
public class Program
{
    static Queue<int> _randomNumbers;
    static Random rng = new Random();  
    
    static Program()
    {
        var min = 0;
        var max = 10;
        var list = new List<int>();
        for (int i = min; i < max; i++)
            list.Add(i);
        
        Shuffle(list);
        _randomNumbers = new Queue<int>(list);
            
        static void Shuffle<T>(IList<T> list)  
        {  
            int n = list.Count;  
            while (n > 1) {  
                n--;  
                int k = rng.Next(n + 1);  
                (list[k], list[n]) = (list[n], list[k]); //Notice: I incorporated @MatthewWatson's idea, so it is slightly different than the one you find on the posted SO page
            }  
        }
    }
    
    public static void Main()
    {
        while (true)
        {
            var next = GetNext();
            if (next is null)
                break;
            
            Console.WriteLine(next);
        }
    }
    
    static int? GetNext() => _randomNumbers.Count == 0 ? null : _randomNumbers.Dequeue();
}

https://dotnetfiddle.net/FRKYyA