Question

I’d like to create a random boolean in JavaScript, but I want to take the previous value into account. If the previous value was true, I want it to be more likely for the next value to be true. At the moment I’ve got this (this is in the context of a closure – goUp and lastGoUp are locals to the containing scope):

function setGoUp() {
    goUp = getRandomBoolean();

    if(lastGoUp) {
        goUp = getRandomBoolean() || goUp;
    }
    else {
        goUp = getRandomBoolean() && goUp;
    }
    lastGoUp = goUp;
}

So, the algorithm goes:

1. Get a random boolean

2. If the random boolean from the previous call was True:

   a) get another random boolean, and or these two together

   b) else get another random boolean and and these together.

I’m sure this algorithm could be simplified. I wondered about doing:

if(lastGoUp && goUp) {
    goUp = goUp * (getRandomBoolean() || goUp);
}

but that seems really dirty.

There’s also a problem with this algorithm which means that I can only double the chance of getting the same boolean again – I can’t tweak it easily. Any ideas?

Answer 1

You should define the distribution you want, but maybe you are looking for the following?

if (lastGoUp) {
    goUp = Math.random() < 0.8;
} else {
    goUp = Math.random() < 0.2;
}