Exclude values from Random.Range()?

Question

If you are using Random.Range() to generate values, is there any way to exclude some values within the range (for example: pick a number between 1 and 20, but not 6 through 8)?

Answer 1

The best way to do this is to use your favourite generator to generate an integer n between 1 and 17 then transform using

if (n > 5){
    n += 3;
}

If you sample between 1 and 20 then discard values you can introduce statistical anomalies, particularly with low discrepancy sequences.

Answer 2

So you actually want 17 (20 - 3) different values

  [1..5] U [9..20]

and you can implement something like this:

  // Simplest, not thread-safe
  private static Random random = new Random();

  ...  

  int r = (r = random.Next(1, 17)) > 5
    ? r + 3
    : r;

In general (and complicated) case I suggest generating an array of all possible values and then take the item from it:

  int[] values = Enumerable
    .Range(1, 100) // [1..100], but
    .Where(item => item % 2 == 1) // Odd values only
    .Where(item => !(item >= 5 && item <= 15)) // with [5..15] range excluded
    //TODO: Add as many conditions via .Where(item => ...) as you want
    .ToArray();

  ...

  int r = values[random.Next(values.Length)];