How to get a random number from a range, excluding some values

you can use a do-while statement to pick another Random if it equals what number you want to exclude. this code is to exclude the number you picked before

    int newNumber;
do {
    newNumber = Random.Range (0, 100);
} while(number == newNumber);

number = newNumber;

Since no-one has posted any example code:

private int GiveMeANumber()
{
    var exclude = new HashSet<int>() { 5, 7, 17, 23 };
    var range = Enumerable.Range(1, 100).Where(i => !exclude.Contains(i));

    var rand = new System.Random();
    int index = rand.Next(0, 100 - exclude.Count);
    return range.ElementAt(index);
}

Here's the thinking:

1. Build a Hashset of numbers you want to exclude
2. Create a collection of all the numbers 0-100 which aren't in your list of numbers to exclude with a bit of LINQ.
3. Create a random object.
4. Use the Random object to give you a number between 0 and the number of elements in your range of numbers (inclusive).
5. Return the number at that index.

This is what I do in this situation, it's not perfect but works well for me. I usually do it only for 1 number but this is how it can be for a group of excluded numbers:

Let's say I want to exclude [5, 7, 17, 23] from a random between 1-100. I always have a substitution for each of the excluded numbers such as [6, 8, 18, 24]. If the random number falls into any of the excluded numbers, I replace it with its substitution.

I came here looking for a better solution but I couldn't find any so I ended up sharing mine.

If you care about Big O, check out this algorithm. It assumes that the excluded values array is sorted in ascending order and contains values within 0 and n-1 range (inclusive).

public static int random_except_list(int n, int[] x) 
{
    Random r = new Random();
    int result = r.Next(n - x.Length);

    for (int i = 0; i < x.Length; i++) 
    {
        if (result < x[i])
            return result;
        result++;
    }
    return result;
}

If you call it with:

random_except_list(8, new int[]{3,4,6})

it will return one of the following values: 0, 1, 2, 5, 7.