Question

I’m looking for the best way of implementing random number generator, that will allow me to have control over probability from what range the generated number will be returned. To visualize what I’m trying to achieve I have a picture :

So to summarize :
Let’s say that my range is 400. At the beginning I’d like to have 5% probability of getting number 0-20. But at some moment in time I’d like to have this probability increased up to 50%. Hope you get the idea.

Answer 1

Hmm, working on your original I had a pretty simple algorithm to generate ranges in an array in the appropriate proportion, then randomly select a range and generate a random number within that range. No doubt it can be optimised if necessary, but it works for me.

It looks like a lot of code, but 3/4 of it is comments, test data and function, the actual randomRange function is only 17 lines of code.

<script type="text/javascript">

function randomRange(dataArray) {

  // Helper function
  function getRandomInRange(s, f) {
    return (Math.random() * (f-s+1) | 0) + s
  }

  // Generate new data array based on probability
  var i, j = dataArray.length;
  var oArray = [];
  var o;
  while (j--) {
    o = dataArray[j];

    // Make sure probability is an integer
    for (i=0, iLen=o.probability|0; i<iLen; i++) {  
      oArray.push([o.rangeStart, o.rangeEnd]);
    }
  }

  // Randomly select a range from new data array and
  // generate a random number in that range
  var oEnd = oArray.length;
  var range = oArray[getRandomInRange(0, oArray.length - 1)]; 
  return getRandomInRange(range[0], range[1]);
}

// Test data set. Probability just has to be
// representative, so 50/50 === 1/1
var dataArray = [
  {
    rangeStart: 0, 
    rangeEnd  : 20,
    probability: 1
  },
  {
    rangeStart: 21, 
    rangeEnd  : 400,
    probability: 1
  }
];

// Test function to show range and number is randomly
// selected for given probability
function testIt() {
  var el0 = document.getElementById('div0');
  var el1 = document.getElementById('div1');
  function run() {
    var n = randomRange(dataArray);
    if (n <= 20) {
      el0.innerHTML += '*';
    } else {
      el1.innerHTML += '*';
    }
  }
  setInterval(run, 500);
}


</script>

<button onclick="testIt();">Generate random number</button>

<div>Numbers 0 - 20</div>
<div id="div0"></div>
<div>Numbers 21 - 400</div>
<div id="div1"></div>