Question

I have a simple question and I’m a little rusty on random number generation. I want to generate large odd integers (I’m using doubles since my numbers could be outside the int range) and I can’t quite figure out how to get rid of the decimals in the random number generation and have the number be odd.

Right now I just have:

N = nMin + (nMax - nMin) * rand.nextDouble();

Which as I said gives me any random number (with decimals) between nMin and nMax. Any help would be much appreciated!

Answer 1

If your numbers can be out of the int range, then you should use long, or failing in that, BigInteger.

Use the information in this question to create a random BigInteger, and if it is even simply add 1 to it.

BigInteger randomOdd(BigInteger min, BigInteger max) {
    BigInteger range = max.subtract(min);

    // expected iterations: 2 - max iterations: infinite
    BigInteger tmp;
    do {
        tmp = new BigInteger(n.bitLength(), rng); // rng is your Random Number Generator
    } while (tmp.compareTo(range) >= 0);

    BigInteger result = min.add(tmp);

    // force the result to be odd
    // TODO: will this push it over max?
    result = result.or(BigInteger.ONE); 

    return result;
}

Alternatively, you could use a method on the BigInteger class: BigInteger.probablePrime():

public static BigInteger probablePrime(int bitLength,
Random rnd)

Returns a positive BigInteger that is probably prime, with the specified bitLength. The probability that a BigInteger returned by this method is composite does not exceed 2^100.

Parameters:

• bitLength – bitLength of the returned BigInteger.
• rnd – source of random bits used to select candidates to be tested for primality.

Returns:

• a BigInteger of bitLength bits that is probably prime

If it’s probably prime, it’s also probably odd.