Recipe 4.3. Rounding Numbers

Problem

You want to round a number to the nearest integer, decimal place, or interval (such as to the nearest multiple of five).

Solution

Use Math.round( ) to round a number to the nearest integer. Use Math.floor( ) and Math.ceil( ) to round a number down or up. Use a custom NumberUtilities.round( ) method to round a number to a specified number of decimal places or to a specified multiple.

Discussion

There are numerous reasons to round numbers. For example, when displaying the results of a calculation, you might display only the intended precision. Because all arithmetic in ActionScript is performed with floating-point numbers, some calculations result in unexpected floating-point numbers that must be rounded. For example, the result of a calculation may be 3.9999999 in practice, even though it should be 4.0 in theory.

The Math.round( ) method returns the nearest integer value of any parameter passed to it:

trace(Math.round(204.499));  // Displays: 204
trace(Math.round(401.5));    // Displays: 402

The Math.floor( ) method rounds down, and the Math.ceil( ) method rounds up:

trace(Math.floor(204.99));   // Displays: 204
trace(Math.ceil(401.01));    // Displays: 402

To round a number to the nearest decimal place:

Decide the number of decimal places to which you want the number rounded. For example, if you want to round 90.337 to 90.34, then you want to round to two decimal places, which means you want to round to the nearest .01.
Divide the input value by the number chosen in Step 1 (in this case, .01).
Use Math.round( ) to round the calculated value from Step 2 to the nearest integer.
Multiple the result of Step 3 by the same value that you used to divide in Step 2.

For example, to round 90.337 to two decimal places, you could use:

trace (Math.round(90.337 / .01) * .01);   // Displays: 9.34

You can use the identical math to round a number to the nearest multiple of an integer.

For example, this rounds 92.5 to the nearest multiple of 5:

trace (Math.round(92.5 / 5)  *  5);   // Displays: 95

As another example, this rounds 92.5 to the nearest multiple of 10:

trace (Math.round(92.5 / 10) * 10);   // Displays: 90

In practice you are likely to find it is much simpler to use a custom NumberUtilities.round( ) method that encapsulates this functionality. The custom method takes two parameters:

number: The number to round.
roundToInterval: The interval to which to round the number. For example, if you want to round to the nearest tenth, use 0.1 as the interval. Or, to round to the nearest multiple of six, use 6.

The NumberUtilities class is in the ascb.util package, so the first thing you'll want to add to any file that uses the class is an import statement. Here is an example of how to use the NumberUtilities.round( ) method (the following code assumes that you've imported ascb.util.NumberUtilities):

trace(NumberUtilities.round(Math.PI));          // Displays: 3
trace(NumberUtilities.round(Math.PI, .01));     // Displays: 3.14
trace(NumberUtilities.round(Math.PI, .0001));   // Displays: 3.1416
trace(NumberUtilities.round(123.456, 1));       // Displays: 123
trace(NumberUtilities.round(123.456, 6));       // Displays: 126
trace(NumberUtilities.round(123.456, .01));     // Displays: 123.46