probability in javascript help?

Question

Sorry, I'm new to JS and can't seem to figure this out: how would I do probability?

I have absolutely no idea, but I'd like to do something: out of 100% chance, maybe 0.7% chance to execute function e(); and 30% chance to execute function d(); and so on - they will add up to 100% exactly with a different function for each, but I haven't figured out exactly how to do this in any form.

What I found is mostly strange high school math tutorials "powered by" Javascriptkit or something.

Answer 1

var decide = function(){ 
    var num = parseInt(Math.random() * 10) + 1; // assigns num randomly (1-10)
    num > 7 ? d() : e(); // if num > 7 call d(), else call e() 
};

Answer 2

Something like this should help:

var threshhold1 = 30.5;
var threshhold2 = 70.5;
var randomNumber = random() * 100;
if (randomNumber < threshhold1) {
   func1()
}
else if (randomNumber < threshhold2) {
   func2()
}
else {
   func3()
}

This will execute func1() with 30.5% probability, func2() with 40%, and func3() with 29.5%.

You could probably do it more elegantly using a dictionary of threshholds to function pointers, and a loop that finds the first dictionary entry with a threshhold greater than randomNumber.

Answer 3

For instance we define a number of functions

function a () { return 0; }
function b () { return 1; }
function c () { return 2; }

var probas = [ 20, 70, 10 ]; // 20%, 70% and 10%
var funcs = [ a, b, c ]; // the functions array

That generic function works for any number of functions, it executes it and return the result:

function randexec()
{
  var ar = [];
  var i,sum = 0;


  // that following initialization loop could be done only once above that
  // randexec() function, we let it here for clarity

  for (i=0 ; i<probas.length-1 ; i++) // notice the '-1'
  {
    sum += (probas[i] / 100.0);
    ar[i] = sum;
  }


  // Then we get a random number and finds where it sits inside the probabilities 
  // defined earlier

  var r = Math.random(); // returns [0,1]

  for (i=0 ; i<ar.length && r>=ar[i] ; i++) ;

  // Finally execute the function and return its result

  return (funcs[i])();
}

For instance, let's try with our 3 functions, 100000 tries:

var count = [ 0, 0, 0 ];

for (var i=0 ; i<100000 ; i++)
{
  count[randexec()]++;
}

var s = '';
var f = [ "a", "b", "c" ];

for (var i=0 ; i<3 ; i++)
  s += (s ? ', ':'') + f[i] + ' = ' + count[i];

alert(s);

The result on my Firefox

a = 20039, b = 70055, c = 9906

So a run about 20%, b ~ 70% and c ~ 10%.

Edit following comments.

If your browser has a cough with return (funcs[i])();, just replace the funcs array

var funcs = [ a, b, c ]; // the old functions array

with this new one (strings)

var funcs = [ "a", "b", "c" ]; // the new functions array

then replace the final line of the function randexec()

return (funcs[i])(); // old

with that new one

return eval(funcs[i]+'()');

Answer 4

// generate cumulative distribution function from weights
function cdf(weights) {
    // calculate total
    var total = 0;
    for(var i=0; i<weights.length; i++) {
        total += weights[i];
    }
    // generate CDF, normalizing with total
    var cumul = [];
    cumul[0] = weights[0]/total;
    for(var i=1; i<weights.length; i++) {
        cumul[i] = cumul[i-1] + (weights[i]/total);
    }
    return cumul;
}

// pick the index using the random value
function selectInd(cumul,rand) {
    for(var i=0; (i < cumul.length) && (rand > cumul[i]); ++i) {};
    return i;
}

Code block to use the above

// setup (do this once)
var weights = [70,20,10];
var cumul = cdf(weights)

// get the index and pick the function
var ran = Math.random(); // 0 : 1
var func = funcs[selectInd(cumul,ran)]; 

// call the function
var someArgVal = 5;
var myResult = func(someArgVal);

// do it in one line
var myResult = (funcs[selectInd(cumul,Math.random())])(someArgVal);

Simplify calling code with a reusable object

function CumulDistributor(cumul,funcs) {
    var funcArr = funcs;
    var cumulArr = cumul;
    function execRandomFunc(someArg) {
        var func = funcArr[selectInd(cumulArr,Math.random())];
        return func(someArg);
    }
}

// example usage
var cdistor = new CumulDistributor(cumul,funcs);
var myResult = cdistor.execRandomFunc(someArgValue);