Probability of using javascript?

Question

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

I have absolutely no idea, but I would like to do something: from a 100% chance, perhaps a 0.7% chance to execute the e(); function e(); and a 30% chance to perform the d(); function d(); and so on - they will add up to 100% exactly with a different function for each, but I did not understand exactly how to do this in any form.

What I found is basically weird high school tutorials based on Javascriptkit or something like that.

Answer 1

For example, 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 

This universal function works for any number of functions, it executes it and returns 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 example, try with our 3 functions, 100,000 attempts:

 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); 

Result on my firefox

 a = 20039, b = 70055, c = 9906 

So, mileage is about 20%, b ~ 70% and c ~ 10%.

Change the 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 (lines)

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

then replace the end line of the randexec() function

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

with this new

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

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 a probability of 30.5%, func2() with 40% and func3() with 29.5%.

Perhaps you could do it more elegantly using a threshold dictionary for function pointers, and a loop that finds the first dictionary entry with a threshold greater than randomNumber .

Answer 3

It looks like you really want a Javascript random () function .

Answer 4

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

Answer 5

 // 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 for using 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 call code with 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); 

Answer 6

 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() };