ActionScript 3.0 Cookbook: Chapter 1: ActionScript Basics
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Update expression

The update expression usually updates the value of the variable used in the test expression so that, at some point, the conditional test becomes false and the loop ends. The update expression is executed once each time through the loop. An infinite loop is often caused by failing to update the appropriate variable in the update expression (usually the same variable used in the test expression).

Statement body

The statement body is a block of substatements enclosed within curly braces that is executed each time through the loop. If the test expression is never true, the for statement's body isn't executed.

The for keyword should come first, and it should be followed by the initialization, test, and update expressions enclosed within parentheses. Semicolons must separate the three expressions from one another (although the initialization, test, and update statements are optional, the semicolons are mandatory). The remainder of the for loop is made up of the statement body enclosed in curly braces:

for (initialization; test; update) {
  statement body
}

Here is an example of a for statement that outputs the numbers from 0 to 999:

for (var i:int = 0; i < 1000; i++) {
  trace(i);
}
trace ("That's the end.");

To understand the for statement, you can follow along with the process the ActionScript interpreter uses to process the command. In the preceding example, the for keyword tells the interpreter to perform the statements within the for loop as long as the conditional expression is true. The process is as follows:

1. The initialization expression is executed only once, and it sets the variable i to 0.
2. Next, the interpreter checks the test expression (i < 1000). Because i is 0, which is less than 1,000, the expression evaluates to true and the trace( ) action within the for statement body is executed.
3. The ActionScript interpreter then executes the update statement, in this case i++, which increments i by 1.
4. The interpreter then repeats the process from the top of the loop (but skipping the initialization step). So the interpreter again checks whether the test expression is true and, if so, executes the statement body again. It then executes the update statement again.

This process repeats until the test expression is no longer true. The last value displayed in the Output window is 999 because once i is incremented to 1,000, the test expression no longer evaluates to true and the loop comes to an end. Once the loop terminates, execution continues with whatever commands follow the loop.

Both the initialization and update expressions can include multiple actions separated by commas. You should not, however, use the var keyword more than once in the initialization expression. The following example simultaneously increments i, decrements j, and displays their values in the Output window:

for (var i:int = 0, j:int = 10; i < 10; i++, j--) {
  trace("i is " + i);
  trace("j is " + j);
}

The preceding example is not the same as two nested for statements (which is shown in the next code block.)

It is also common to use nested for statements. When you use a nested for statement, use a different index variable than that used in the outer for loop. By convention, the outermost for loop uses the variable i, and the nested for loop uses the variable j. For example:

for (var i:int = 1; i <= 3; i++) {
  for (var j:int = 1; j <= 2; j++) {
    trace(i + " X " + j + " = " + (i * j));
  }
}

The preceding example displays the following multiplication table in the Output window:

1 X 1 = 1
1 X 2 = 2
2 X 1 = 2
2 X 2 = 4
3 X 1 = 3
3 X 2 = 6

It is possible to nest multiple levels of for statements. By convention, each additional level of nesting uses the next alphabetical character as the index variable. Therefore, the third level of nested for statements typically use k as the index variable:

for (var i:int = 1; i <= 3; i++) {
  for (var j:int = 1; j <= 3; j++) {
    for (var k:int = 1; k <= 3; k++) {
      trace(i + " X " + j + " X " + k + " = " + (i * j * k));
    }
  }
}

Additionally, you can use for statements to loop backward or to update the variable in ways other than by simply adding or subtracting one:

// Count backward from 10 to 1.
for (var i:int = 10; i > 0; i--) {
  trace(i);
}

// Display a sequence of square roots.
for (var i:Number = 50000; i > 2; i = Math.sqrt(i)) {
  trace(i);
}

In this case, the variable i winds up holding values other than integers, so it is best to declare it as a Number rather than an int.

You should not use a for statement to perform tasks over time.

Many programmers make the mistake of trying to use for statements to animate sprites; for example:

for (var i:int = 0; i < 20; i++) {
  _sprite.x += 10;
}

Whereas the preceding code moves the sprite 200 pixels to the right of its starting point, all the updates take place within the same frame. There are two problems with this. First, the Stage updates only once per frame, so only the last update is shown on the Stage (causing the sprite to jump 200 pixels suddenly rather than moving smoothly in 20 steps). Second, even if the Stage updates often, each iteration through the for loop takes only a few milliseconds, so the animation would happen too quickly. For actions that you want to take place over time, use the enterFrame event (see Recipe 1.5) or a timer (see Recipe 1.12).

Moreover, tight repeating loops should not be used to perform lengthy processes (anything that takes more than a fraction of a second). The Flash Player displays a warning whenever a single loop executes for more than 15 seconds. Using the other methods (just mentioned) avoids the warning message and allows Flash to perform other actions in addition to the repeated actions that are part of the loop.

See Also

Recipes 1.5 and 1.12. The for statement is used in many practical situations, and you can see examples in a great many of the recipes throughout this book. See Recipe 5.2 and Recipe 12.8 for some practical examples. Recipe 5.16 discusses for...in loops, which are used to enumerate the properties of an object or array.

Section 1.12: Repeating a Task over Time

Problem

You want to perform some action or actions over time.

Solution

Use the Timer class. Alternatively, listen for the enterFrame event of a sprite.

Discussion

The Timer class is new to ActionScript 3.0, and is recommended over the earlier setInterval( ) and setTimeout( ) functions. When you create an instance of the Timer class, it fires timer events at regular intervals. You can specify the delay between events and how many times you want the events to fire in the Timer constructor:

var timer:Timer = new Timer(delay, repeatCount);

You use addEventListener to set up a method to handle these events. After you create the timer and set up a listener, use its start( ) method to start it and stop( ) to stop it.

The Timer class is part of the flash.utils package, and there is also a TimerEvent class in the flash.events package, so those need to be imported:

package {
    import flash.display.Sprite;
    import flash.events.TimerEvent;
    import flash.utils.Timer;
    
    public class ExampleApplication extends Sprite {
        // Declare and initialize a variable to store the value
        // of the previous timer reading.
        private var _PreviousTime:Number = 0;
        
        public function ExampleApplication( ) {
            var tTimer:Timer = new Timer(500, 10);
            tTimer.addEventListener(TimerEvent.TIMER, onTimer);
            tTimer.start( );
        }
        
        private function onTimer(event:TimerEvent):void {
            // Output the difference between the current timer value and
            // its value from the last time the function was called.
            trace(flash.utils.getTimer( ) - _PreviousTime);
            _PreviousTime = flash.utils.getTimer( ); 
        }
    }
}

The getTimer( ) function (previously a top-level function), has been moved to the flash.utils package as well. This simply returns the number of milliseconds since the application started.

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