Generating random numbers in Javascript

Question

How can I generate a random whole number between two specified variables in Javascript, e.g. x=4 and y=8 would output any of 4,5,6,7,8?

Answer 1

There are some examples on the Mozilla Developer Center page:

/**
 * Returns a random number between min and max
 */
function getRandomArbitary (min, max) {
    return Math.random() * (max - min) + min;
}

/**
 * Returns a random integer between min and max
 * Using Math.round() will give you a non-uniform distribution!
 */
function getRandomInt (min, max) {
    return Math.floor(Math.random() * (max - min + 1)) + min;
}

---

Here's the logic behind it. It's a simple rule of three:

Math.random() returns a Number between 0 (inclusive) and 1 (exclusive). So we have an interval like this:

[0 .................................... 1)

Now, we'd like a number between min (inclusive) and max (exclusive):

[0 .................................... 1)
[min .................................. max)

We can use the Math.random to get the correspondent in the [min, max) interval. But, first we should factor a little bit the problem by subtracting min from the second interval:

[0 .................................... 1)
[min - min ............................ max - min)

This gives:

[0 .................................... 1)
[0 .................................... max - min)

We may now apply Math.random and then calculate the correspondent. Let's choose a random number:

                Math.random()
                    |
[0 .................................... 1)
[0 .................................... max - min)
                    |
                    x (what we need)

So, in order to find x, we would do:

x = Math.random() * (max - min);

Don't forget to add min back, so that we get a number in the [min, max) interval:

x = Math.random() * (max - min) + min;

That was the first function from MDC. The second one, returns an integer between min and max, both inclusive. In order to do that, a little trick is applied. The second interval is increased to max + 1 exclusive, so that max is a value inside the interval, thus a candidate for the final result:

                Math.random()
                    |
[0 .................................... 1)
[min ................................... max)
// increase interval
[min ..............................max.. max + 1)
// factor out min
[0 ................................max.. max - min + 1)
                    |
                    X (what we need)

Now, the above equation becomes:

x = Math.random() * (max - min + 1) + min;

Which may yield max as the value of x. But we want an integer, so we use Math.floor:

x = Math.floor(Math.random() * (max - min + 1)) + min;

Answer 2

function getRandomizer(a,b) {
    return function() {
        return (Math.floor( Math.random()* (b-a) ) ) + a;
    }
}

usage:

var rollDie = getRandomizer(1,6);
while (true) {
    alert( rollDie() );
}

breakdown:

We are returning a function (borrowing from functional programming) that when called, will return a random number between the the values a and b used to construct it.

(a is lower number, b is greater number)

Math.random() * (b-a)

Math.random returns a random double between 0 and 1. So multiplying it by the smaller number will give us a range we want (the difference between a and b).

Math.floor( Math.random() * (b-a) )

Math.floor rounds the number down to the nearest int. So we now have all the ints between 0 and b-a.

Math.floor( Math.random()* (b-a) ) + a

a is our lower value the difference between the two numbers, so after adding that, we will get numbers between a and b (inclusive). :D

If you pass in a non-int value or greater number first, you'll get undesirable behavior.

;D

Answer 3

Doing a Google search for "javascript random number" yields the following web page which explains how to do it:

http://www.javascriptkit.com/javatutors/randomnum.shtml

Answer 4

function getRandom(int lower, int upper)
{
    return lower + (Math.random() * (upper - lower));
}

Answer 5

var randomnumber = Math.floor(Math.random() * (maximum - minimum + 1)) + minimum;