Recursion in php

Recursion refers to a programming approach in which a function works by calling itself. A recursive function must have a case when no more recursive calls are needed, this is called a base case. When writing a recursive function, make sure that its base case(s) is(are) reached to avoid an inifinite sequence of recursive calls.

A recursive factorial function

A factorial is defined as:

The function below computes a factorial recursively. Note that in this case a loop solution is also possible.


<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<!--
Recursive factorial
Author: Elena Machkasova elenam@morris.umn.edu
Last modified: 4/15/10
-->
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html;charset=utf-8" />
<title>
CSci 1101: Recursive factorial
</title>
<?php
$n = $_GET["n"];

function factorial($n) {
   if ($n == 0) return 1;
   else return ($n * factorial($n-1));
 }
?>
</head>
<body>
<p>
<?php
print "factorial(5)=".factorial(5)."<br />";
print "factorial($n)=".factorial($n)."<br />";
?>
</p>
</body>
</html>
http://csci1101sp10.morris.umn.edu/~elenam/1101_spring10/recursion/factorial.php?n=10

Recursive sum of a nested array of numbers

The program shows a function that computes a sum of all numbers in an array that can have an arbitrary number of nested subarrays at any level of nesting. The way to distinguish a base case from a recursive step is to check if an array item is itself an array.

Note that in this case a loop solution is difficult to come up with. Recursion is a simple and natural approach to this problem.


<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<!-- 
Recursive nested arrays
Author: Elena Machkasova elenam@morris.umn.edu 
Last modified: 4/9/09
--> 
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html;charset=utf-8" />
<title>
CSci 1101: Recursive sum of a nested arrray
</title>
<?php
// the function is given an array that may 
// contain nested subarrays. The function
// assumes that all arrays have only 
// numbers in them. Arrays may be indexed 
// in any way
function nested_array_sum($array) {
   $sum = 0;
   foreach($array as $item) {    
     // check if the item is an array
     // recursive step (the item is itself an array):
     if (is_array($item)) {
       $sum = $sum + nested_array_sum($item);
     }
     // base case: the item is not an array, just add it to the sum
     else $sum = $sum + $item;
   }
   return $sum;
}
?>
</head>
<body>
<?php
$numbers = array(2, array(7, 8), array(3, array(5, 6)), 6);
print "<pre>";
print_r($numbers);
print "</pre>";
print "<p>The sum of the nested array is: ".nested_array_sum($numbers)."</p>";
?>
</body>
</html>
http://csci1101sp10.morris.umn.edu/~elenam/1101_spring10/recursion/array_sum.php

UMM CSci 1101

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