Recursion
PHPWomen Contest Winner
ByPHPWomen.org recently held an article-writing contest on their Best Practices Forum. Authors of the two winning submissions each received copies of Zend Studio for Eclipse, a 1-year subscription to Linux Pro Magazine (which is called Linux Magazine outside North America), and the opportunity to feature their articles on the magazine websites. Congratulations goes to Rob Allen for his winning submission!
Recursion is the term used when an operation is repeated on the results of the same operation. That's a little confusing, but what it means to a programmer is that you have a function that calls itself.
Consider the process of adding up numbers in an array such as:
<?php
$array = array(10, 21, 4);
?>
All we need to do is iterate over the array and add each number to a variable as shown here:
<?php
function sumArray($array)
{
$total = 0;
foreach ($array as $element) {
$total += $element;
}
return $total;
}
$array = array(10, 21, 4);
$result = sumArray($array);
echo "result = $result\n";
?>
As you would expect, the output of the script is:
result = 35
But, what if the array is nested? For example, an array like this:
<?php
$array = array(10, 20, 5,
array(5, 2, 3)
);
?>
As we iterate over this new array, we will come to an element that is itself an array. We need to sum up the elements within this sub-array and, fortunately, we have just written a function that does just that (sumArray() !), so let's call it within the foreach() loop:
<?php
function sumArray($array)
{
$total = 0;
foreach ($array as $element) {
if(is_array($element)) {
$total += sumArray($element);
} else {
$total += $element;
}
}
return $total;
}
?>
This is recursion as we have called the sumArray() function from within the sumArray() function itself. That is, sumArray() is a recursive function.
We can now use our improved function to add up our nested array:
<?php
$array = array(10, 20, 5,
array(5, 2, 3)
);
$result = sumArray($array);
echo "result = $result\n";
?>
which will now output:
result = 45
As sumArray() is a recursive function, it will happily handle an array that is many nested levels deep. For example, consider an array that is nested 5 levels deep:
<?php
$array = array(10, 20, 5,
array(5, 2, 3,
array(5, 3,
array(2, 10,
array(19, 1)
),3
), 2, 7
), 3
);
$result = sumArray($array);
echo "result = $result\n";
?>
When this code is run, the output is:
result = 100
Clearly recursion is a powerful and flexible technique for solving problems involving nested data such as reading through XML structures or handling a tree within a database table (usually implemented with a parent_id column). It is also helpful when writing a sorting algorithm, but most PHP programmers don't need to do that! Recursive solutions also tend to be fairly compact and easy to debug as you only need to do it once!
Obviously, as we're programming, there are trade-offs involved! Recursive solutions are inefficient in terms of performance, so consider caching the result. Also, it's possible to get into situation where the recursion never stops. Always make sure that your function will end! Related to that, when you have deeply nested recursion, you can run out of "stack space" (this area reserved for the list of functions that are currently being called. In other words - make sure you test thoroughly :)
Go on.. write a recursive function today!
Comments
comments powered by DisqusIssue 230/2020
Buy this issue as a PDF
News
-
Elementary OS 5.1 Has Arrived
One of the most highly regarded Linux desktop distributions has released its next iteration.
-
Linux Mint 19.3 Will be Released by Christmas
The developers behind Linux Mint have announced 19.3 will be released by Christmas 2019.
-
Linux Kernel 5.4 Released
A number of new changes and improvements have reached the Linux kernel.
-
System76 To Design And Build Laptops In-House
In-house designed and built laptops coming from System76.
-
News and views on the GPU revolution in HPC and Big Data:
-
The PinePhone Pre-Order has Arrived
Anyone looking to finally get their hands on an early release of the PinePhone can do so as of November 15.
-
Microsoft Edge Coming to Linux
Microsoft is bringing it’s new Chromium-based Edge browser to Linux.
-
Open Invention Network Backs Gnome Project Against Patent Troll
OIN has deployed its legal team to find prior art.
-
Fedora 31 Released
The latest version of Fedora comes with new packages and libraries.
-
openSUSE OBS Can Now Build Windows WSL Images
openSUSE enables developers to build their own WSL distributions.
All pluses and no minuses ???