ActionScript: The Definitive Guide

Recursive Functions

A recursive function is a function that calls itself (by using its own name within its function body). Here’s a simple example that shows the principle of recursion. But because the code tells the trouble( ) function to execute repeatedly (like an image reflected infinitely in two opposing mirrors), Flash will quickly run out of memory, causing an error:

function trouble( ) {
  trouble( );
}

Practical recursive functions call themselves only while a given condition is met (thus preventing infinite recursion). Example 9.4 used recursion to count from a specified number down to 1, but obviously that can be accomplished without recursion.

One classic use of recursion is to calculate the mathematical factorial of a number. The factorial of 3 (written as 3! in mathematical nomenclature) is 3*2*1=6. The factorial of 5 is 5*4*3*2*1=120. Example 9.8 shows a factorial function that uses recursion.

Example 9-8. Calculating Factorials Using Recursion

function factorial(x) {
    if (x < 0) {
      return undefined;  // Error condition
    } else if (x <= 1) {
      return 1;
    } else {
      return x * factorial(x-1);
    }
}
trace (factorial(3));  // Displays: 6
trace (factorial(5));  // Displays: 120

As usual, there is more than one way to skin a proverbial cat. Using a loop, we can also calculate a factorial without recursion, as shown in Example 9.9.

Example 9-9. Calculating Factorials Without Recursion

function factorial(x) { if (x < 0) { return undefined; // Error condition } else { var result = 1; for (var ...