Random Numbers in Flash written by
kirupa, code by
Scott Ruttencutter | 10 January 2009

In programming, it is often nice to be
able to work with numbers whose values are a bit
unpredictable. To create
numbers that occur with (almost) no set pattern, one would
need to use a function that returns a random value.

Before I go further, here is a simple example where you
will see a random number when you click on the button:

[ click on the Click Me button to see a
random number between 0 and 100 ]

Generating a random number is pretty straightforward
because a function for doing this already exists, and this
function is:

Math.random();

In ActionScript, Math.random() returns a number greater
than or equal to 0 but less than 1. Another way of stating
that is (0 <= n < 1) where n is the number you are
looking for.

To configure a number to randomly fall within a range of
numbers, use the following format:

Math.floor(Math.random()*(1+High-Low))+Low;

High refers to the largest
number in your range, and Low
refers to the lowest number in your range. For example, to
get a number between 10 and 50, my line of code would look
as follows:

Math.floor(Math.random()*41)+10;

A random
number between 10 and 50 will be displayed. Scott goes into
more detail in his blog post
True Random Integers in Flash CS3, but I'll summarize
this briefly.

Walking Through the Approach You may be wondering why getting the a random number
between a range of numbers is so...inelegant. The addition
of the 1 seems almost silly in the grand scheme of things.

The reason is that Math.random() returns a number greater than or equal to zero AND less than 1. The function will never
return a 1 as its value. It will return something like
.9999999, but it won't be a 1. Because we are rounding the
output by Math.floor where we round down to the nearest
integer, if we did not add the 1, you will never get the
maximum possible answer for
High - Low when it is multipled by Math.random().

Let's say we ignore the one. Your code would now look as
follows:

Math.floor(Math.random()*40)+10;

Math.random() * 40 will never return a 40 because that
would require Math.random() to return a 1. We know from the
AS3 documentation that Math.random() will get close to 1,
but it will never reach one. Let's be optimistic and say
that Math.random() returns a .9999, and we multiply that
value by 40. The answer you will receive will be 39.996.

Guess what Math.floor of 39.996 is going to be? It is
going to be 39! When this 39 gets added to 10, you get a
value of 49. As you can see, you have no conceivable way of
getting an answer of 50 which is the high number in your
range of values. The only solution is to add a 1 to the
operation where you are subtracting High from Low. Once you
do that, you will be able to generate all numbers within the
range of your high and low numbers with equal frequency.

Conclusion Well, this
wraps up my explanation of how random numbers can be
generated in Flash. If you would like to see what my example
looks like, download the source file from below:

If you have questions, need some assistance on this topic, or just want to
chat - post in the comments below or drop by our friendly forums
(where you have a lot more formatting options)
and post your question. There are a lot of knowledgeable and witty people who
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If you didn't like it, I always like to hear how I can do better next time.
Please feel free to contact me directly at kirupa[at]kirupa.com.