How can I generate a random BigInteger within a certain range?

Question

Consider this method that works well:

public static bool mightBePrime(int N) {
    BigInteger a = rGen.Next (1, N-1);
    return modExp (a, N - 1, N) == 1;
}
         


        

Answer 1

The following Range method will return an IEnumerable within the range you specify. A simple Extension method will return a random element within the IEnumerable.

public static IEnumerable Range(BigInteger from, BigInteger to)
{
    for(BigInteger i = from; i < to; i++) yield return i;
}

public static class Extensions
{
    public static BigInteger RandomElement(this IEnumerable enumerable, Random rand)
    {
        int index = rand.Next(0, enumerable.Count());
        return enumerable.ElementAt(index);
    }
}

usage:

Random rnd = new Random();
var big = Range(new BigInteger(10000000000000000), new BigInteger(10000000000000020)).RandomElement(rnd);

// returns random values and in this case it was 10000000000000003