Coding pattern for random percentage branching?

Question

So let\'s say we have a code block that we want to execute 70% of times and another one 30% of times.

if(Math.random() < 0.7)
    70percentmethod();
else
         


        

Answer 1

You could compute the cumulative probability for each class, pick a random number from [0; 1) and see where that number falls.

class WeightedRandomPicker {

    private static Random random = new Random();

    public static int choose(double[] probabilties) {
        double randomVal = random.nextDouble();
        double cumulativeProbability = 0;
        for (int i = 0; i < probabilties.length; ++i) {
            cumulativeProbability += probabilties[i];
            if (randomVal < cumulativeProbability) {
                return i;
            }
        }
        return probabilties.length - 1; // to account for numerical errors
    }

    public static void main (String[] args) {
        double[] probabilties = new double[]{0.1, 0.1, 0.2, 0.6}; // the final value is optional
        for (int i = 0; i < 20; ++i) {
            System.out.printf("%d\n", choose(probabilties));
        }
    }
}