InformIT: Sams Teach Yourself JavaScript in 24 Hours > Workshop: Working with the Math Object

Workshop: Working with the `Math` Object

The Math.random method, discussed earlier in this hour, generates a random number between 0 and 1. However, it's very difficult for a computer to generate a truly random number. (It's also hard for a human being to do so—that's why dice were invented.)

Today's computers do reasonably well at generating random numbers, but just how good is JavaScript's Math.random function? One way to test it is to generate many random numbers and calculate the average of all of them.

In theory, the average should be somewhere near .5, halfway between 0 and 1. The more random values you generate, the closer the average should get to this middle ground.

As an example of the use of the Math object, you can create a script that tests JavaScript's random number function. To do this, you'll generate 5,000 random numbers and calculate their average.

In case you skipped Hour 7, "Repeating Yourself: Using Loops," and are getting out your calculator, don't worry—you'll use a loop to generate the random numbers. You'll be surprised how fast JavaScript can do this.

To begin your script, you will initialize a variable called total. This variable will store a running total of all of the random values, so it's important that it starts at 0:

total = 0;

Next, begin a loop that will execute 5,000 times. Use a for loop because you want it to execute a fixed number of times:

for (i=1; i<=5000; i++) {

Within the loop, you will need to create a random number and add its value to total. Here are the statements that do this and continue with the next iteration of the loop:

    num = Math.random();
    total += num;
}

Depending on the speed of your computer, it might take a few minutes to generate those 5,000 random numbers. Just to be sure something's happening, the script will update the status line to tell the user how many numbers have been generated so far and the current total:

window.status = "Generated " + i + " numbers. Current total: " + total;

The final part of your script will calculate the average by dividing total by 5,000. Your script can also round the average to three decimal places, using the trick you learned earlier in this hour:

average = total / 5000;
average = Math.round(average * 1000) / 1000;
document.write("<H2>Average of 5000 numbers: " + average + "</H2>");

To test this script and see just how random those numbers are, combine the complete script with an HTML document and <script> tags. Listing 8.1 shows the complete random number testing script.

Example 8.1. A script to test JavaScript's random number function

<html>
<head>
<title>Math Example</title>
</head>
<body>
<h1>Math Example</h1>
<p>How random are JavaScript's random numbers?
Let's generate 5000 of them and find out.</p>
<script LANGUAGE="JavaScript" type="text/javascript">
total = 0;
for (i=1; i<=5000; i++) {
    num = Math.random();
    total += num;
    window.status = "Generated " + i + " numbers.";
}
average = total / 5000;
average = Math.round(average * 1000) / 1000;
document.write("<H2>Average of 5000 numbers: " + average + "</H2>");
</script>
</body>
</html>

To test the script, load the HTML document into a browser. The status line will immediately begin counting up to 5,000. After a short delay, you should see a result. If it's close to .5, the numbers are reasonably random. My result was .502, as shown in Figure 8.1.

Figure 8.1 The random number testing script in action.

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