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P. 199
Chapter 7: Functions
Recursion is another technique that you can use if you need to work on a set of values. Generally
speaking, it ’ s usually easier to use iteration than recursion; however, in certain situations recursion
makes more sense. Practically any loop can be converted to use recursion instead, and vice - versa.
So what is recursion, and how does it relate to functions? Well, in simple terms, recursion occurs when a
function calls itself. As you ’ d imagine, such a process would repeat indefinitely if not stopped, so the
recursion needs to have some sort of end condition — much like a loop. This end condition is known as
the base case , and the part of the function that calls itself is known as the recursive case .
Here ’ s a quick overview of how a recursive function operates:
❑ The recursive function is called by the calling code
❑ If the base case, or end condition, is met, the function does any processing required, then exits
❑ Otherwise, the function does any processing required, then calls itself to continue the recursion
Of course, you have to make sure that the base case is eventually reached, otherwise the function will
keep calling itself indefinitely (causing an infinite loop).
Try It Out Creating the Fibonacci Sequence with Recursion
Chapter 4 showed how to use looping to create the Fibonacci sequence of numbers. The following
script is similar to that shown in Chapter 4, except that it uses a recursive function to generate each
value, rather than computing the values iteratively.
<!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Strict//EN”
“http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd”>
<html xmlns=”http://www.w3.org/1999/xhtml” xml:lang=”en” lang=”en”>
<head>
<title>Fibonacci sequence using recursion</title>
<link rel=”stylesheet” type=”text/css” href=”common.css” />
<style type=”text/css”>
th { text-align: left; background-color: #999; }
th, td { padding: 0.4em; }
tr.alt td { background: #ddd; }
</style>
</head>
<body>
<h2>Fibonacci sequence using recursion</h2>
<table cellspacing=”0” border=”0” style=”width: 20em; border:
1px solid #666;”>
<tr>
<th>Sequence #</th>
<th>Value</th>
</tr>
<?php
$iterations = 10;
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