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

Sounds like what you really want is the Javascript random() function.

Answer 2

Look up how to get random numbers in JavaScript, and then depending on where that number falls, call each function.

Answer 3

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 4

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]+'()');