Math.random() returns value greater than one?

Question

While playing around with random numbers in JavaScript I discovered a surprising bug, presumably in the V8 JavaScript engine in Google Chrome. Consider:

// Generate a random number [1,5].
var rand5 = function() {
  return parseInt(Math.random() * 5) + 1;
};

// Return a sample distribution over MAX times.
var testRand5 = function(dist, max) {
  if (!dist) { dist = {}; }
  if (!max) { max = 5000000; }
  for (var i=0; i<max; i++) {
    var r = rand5();
    dist[r] = (dist[r] || 0) + 1;
  }
  return dist;
};

Now when I run testRand5() I get the following results (of course, differing slightly with each run, you might need to set "max" to a higher value to reveal the bug):

var d = testRand5();
d = {
  1: 1002797,
  2: 998803,
  3: 999541,
  4: 1000851,
  5: 998007,
  10: 1 // XXX: Math.random() returned 4.5?!
}

Interestingly, I see comparable results in node.js, leading me to believe it's not specific to Chrome. Sometimes there are different or multiple mystery values (7, 9, etc).

Can anyone explain why I might be getting the results I see? I'm guessing it has something to do with using parseInt (instead of Math.floor()) but I'm still not sure why it could happen.

Skirting

Answer 1

The edge case occurs when you happen to generate a very small number, expressed with an exponent, like this for example 9.546056389808655e-8.

Combined with parseInt, which interprets the argument as a string, hell breaks loose. And as suggested before me, it can be solved using Math.floor.

Try it yourself with this piece of code:

var test = 9.546056389808655e-8;

console.log(test); // prints 9.546056389808655e-8
console.log(parseInt(test)); // prints 9 - oh noes!
console.log(Math.floor(test)) // prints 0 - this is better

Sibeal

Answer 2

Of course, it's a parseInt() gotcha. It converts its argument to a string first, and that can force scientific notation which will cause parseInt to do something like this:

var x = 0.000000004;
(x).toString(); // => "4e-9"
parseInt(x); // => 4

Silly me...

Skirting

wow, nice catch. I didn't know parseInt converted even a number to a string first. – Lima

Well the other alternative is to throw a TypeError, which is the Java way. Most APIs in javascript just try to work with invalid types rather than throw the error. – Embody

Answer 3

I would suggest changing your random number function to this:

var rand5 = function() {
  return(Math.floor(Math.random() * 5) + 1);
};

This will reliably generate an integer value between 1 and 5 inclusive.

You can see your test function in action here: http://jsfiddle.net/jfriend00/FCzjF/.

In this case, parseInt isn't the best choice because it's going to convert your float to a string which can be a number of different formats (including scientific notation) and then try to parse an integer out of it. Much better to just operate on the float directly with Math.floor().

Viceregal

How does his function end up generating numbers like 7,9, and 10? – Lima

@Lima - it likely has to do with parseInt without a radix parameter then operating on a decimal converted to a string. All of those are bad. – Viceregal