ActionScript: The Definitive Guide

Chapter 8. Loop Statements

In the previous chapter, we learned that a conditional causes a statement block to be executed once if the value of its test expression is true. A loop, on the other hand, causes a statement block to be executed repeatedly, for as long as its test expression remains true.

Loops come in a variety of tasty flavors: while, do-while, for, and for-in. The first three types have very similar effects, but with varying syntax. The last type of loop, for-in, is a specialized kind of loop used with objects. We’ll start our exploration of loops with the while statement, the easiest kind of loop to understand.

The while Loop

Structurally, a while statement is constructed much like an if statement: a main statement encloses a block of substatements that are executed only when a given condition is true:

while (condition) {
  substatements
}

If the condition is true, substatements are executed. But unlike the if statement, when the last substatement is finished, execution begins anew at the beginning of the while statement (that is the interpreter “loops” back to the beginning of the while statement). The second pass through the while statement works just like the first: the condition is evaluated, and if it is still true, substatements are executed again. This process continues until condition becomes false, at which point execution continues with any statements that follow the while statement in the script.

Here’s an example of a very simple loop:

var i = 3; while (i < 5) ...