generate random number not including some numbers

Question

Is there a way to generate a random number from a group of numbers that wont include some numbers? For example - random number from 20-50 not including 25,27,34.

Answer 1

A scheme where you generate a number in the range 20 to 50 then discard the ones you don't want will introduce statistical bias . (You'll tend to increase the variance of the resulting distribution; particularly if your generator is linear congruential).

The best way is to generate in the range 20 - 47 (call a drawing x say) then make adjustments using

if (x >= 25) ++x; if (x >= 27) ++x; if (x >= 34) ++x;

Answer 2

LINQ implementation of @Yeldar Kurmangaliyev comment:

var allowed = Enumerable.Range(20, 31).Except(new int[] { 25, 27, 34 });

Random rand = new Random();

int randomNum = allowed.ElementAt(rand.Next(0, allowed.Count()));