weighted random selection in java?

Question

Can I do the following in java without using external libraries? Maybe with if-else statements? thanks

import random

print(random.choices(['A', 'B', 'C'], [0.5, 0.3, 0.2]))

Answer 1

Sure. The tools required are:

• an instance of java.util.Random
• call its nextDouble method.

The algorithm is something like:

• First calculate, once-off, the incremental weighting. In your example that would be [0.5, 0.8, 1.0].
• Multiply the output of nextDouble with the final weight (here the final weight is 1.0, so not needed. Multiplying by 1.0 doesn't hurt, of course).
• loop through the incremental weights and check if the random number you have is less than it. If yes, that's your choice.

Example:

public class WeightedList {
    private final char[] choices;
    private final double[] weights;
    private final Random rnd = new Random();

    public WeightedList(char[] choices, double[] weights) {
        if (choices.length != weights.length) throw new IllegalArgumentException();
        this.choices = Arrays.copyOf(choices);
        this.weights = new double[weights.length];
        double s = 0.0;
        for (int i = 0; i < weights.length; i++) {
            this.weights[i] = (s += weights[i]);
        }
    }

    public char get() {
        double v = rnd.nextDouble() * weights[weights.length - 1];
        for (int i = 0; i < weights.length - 1; i++) {
            if (v < weights[i]) return choices[i];
        }
        return weights[weights.length - 1];
    }
}