Generate A Weighted Random Number

Question

I\'m trying to devise a (good) way to choose a random number from a range of possible numbers where each number in the range is given a weight. To put it simply: given the

Answer 1

I suggest to use a continuous check of the probability and the rest of the random number.

This function sets first the return value to the last possible index and iterates until the rest of the random value is smaller than the actual probability.

The probabilities have to sum to one.

function getRandomIndexByProbability(probabilities) {
    var r = Math.random(),
        index = probabilities.length - 1;

    probabilities.some(function (probability, i) {
        if (r < probability) {
            index = i;
            return true;
        }
        r -= probability;
    });
    return index;
}

var i,
    probabilities = [0.8, 0.1, 0.1],
    count = probabilities.map(function () { return 0; });

for (i = 0; i < 1e6; i++) {
    count[getRandomIndexByProbability(probabilities)]++;
}

console.log(count);

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