JavaScript: Weighted Random Generator

Question

I'm attempting to create a function that will give me a "fish" at random, though a bias random depending on the weight value for the "fish".

var FISH = {
 "level1": [
 //["name", xp, weight] 
 ["Shrimp", 10, 95],
 ["Sardine", 20, 85],
 ["Herring", 30, 75],
 ["Anchovies", 40, 65],
 ["Mackerel", 40, 55]
 ]
};
function randomRange(min, max) {
 return Math.random() * (max - min) + min;
}
function getRandomFish(level) {
 var fishForLevel = FISH[level],
 numberOfFish = fishForLevel.length,
 chance = randomRange(0, 100);
 if (numberOfFish > 1) {
 var fish = fishForLevel[Math.floor(randomRange(0, numberOfFish))];
 if (chance <= fish[2]) {
 return fish;
 } else {
 return getRandomFish(level);
 }
 } else {
 return fishForLevel[0];
 }
}

Example of outcome: http://jsfiddle.net/qAvAs/

If this could be made more efficient, how would one do so?

Answer 1

I'd suggest you select a random number that's from 0 to the total weight of your fish. Then, each fish can have a piece of that random range according to it's weight which gives it the weighting you want. This ends up giving each fish a number of buckets that corresponds to it's weighting value and then you pick a random number from 0 to the total number of buckets and find out who's bucket the random number landed in.

It's the same idea as turning each weighting number into a percentage change and then picking a random number from 0 to 99.

This code should do that:

var FISH = {
 "level1": [
 //["name", xp, weight] 
 ["Shrimp", 10, 95],
 ["Sardine", 20, 85],
 ["Herring", 30, 75],
 ["Anchovies", 40, 65],
 ["Mackerel", 40, 55]
 ]
};
function getRandomFish(level) {
 var fishForLevel = FISH[level];
 var fishTotalWeight = 0, fishCumWeight = 0, i;
 // sum up the weights
 for (i = 0; i < fishForLevel.length; i++) {
 fishTotalWeight += fishForLevel[i][2];
 }
 var random = Math.floor(Math.random() * fishTotalWeight);
 // now find which bucket out random value is in
 for (i = 0; i < fishForLevel.length; i++) {
 fishCumWeight += fishForLevel[i][2];
 if (random < fishCumWeight) {
 return(fishForLevel[i]);
 }
 }
}

• Shaz: I like this. Certainly seems more efficient then what I was using. :)