Question

I have an array which can have a max length of several hundred thousand rows. It looks like this:

arr[12]   = false
arr[334]  = true
arr[753]  = true
arr[1001] = false
arr[1222] = true

and so on…

I want to find the fastest way to randomly select the index of a row that is true…

My initial attempt was to do this:

for(k in arr) {
 if(arr[k]) {
  candidate.push(k);
 }
}

return Math.floor(Math.random() * candidate.length);

But it is quite slow.

Is there a better way to do it?

Ty

Answer 1

If the fraction of missing elements is sufficiently small it will be fast enough to just generate random indexes until you get a hit. Give it a try.

var chose_true = function(array) {
    while (true) {
        var index = Math.floor(Math.random() * array.length)
        if (array[index]) {
            return array[index];
        }
    }
}