When it comes to sorting items by comparing them merge sort is one very natural approach. It is natural, because simply divides the list into two equal sub-lists then sort these two partitions applying the same rule. That is a typical divide and conquer algorithm and it just follows the intuitive approach of speeding up the sorting process by reducing the number of comparisons. However there are other “divide and conquer” sorting algorithms that do not follow the merge sort scheme, while they have practically the same success. Such an algorithm is quicksort.

Overview

Back in 1960 C. A. R. Hoare comes with a brilliant sorting algorithm. In general quicksort consists of some very simple steps. First we’ve to choose an element from the list (called a pivot) then we must put all the elements with value less than the pivot on the left side of the pivot and all the items with value greater than the pivot on its right side. After that we must repeat these steps for the left and the right sub-lists. That is quicksort! Simple and elegant!

It is a pure divide and conquer approach as merge sort, but while merge sort’s tricky part was merging the sorted sub-lists, in quicksort there are other things to consider.

First of all obviously the choice of a pivot is the bottleneck. Indeed it all depends on that pivot. Imagine that you choose the greatest value from the list – than you’ve to put all the other items of the list into the “left” sub-list. If you do that on each step you’ll practically go into the worst scenario and that is no good. The thing is that in the worst case quicksort is not so effective and it’s practically as slow as bubble sort and insertion sort. The good thing is that in practice with randomly generated lists there is not a high possibility to go into the worst case of quicksort.

Choosing a pivot

Of course the best pivot is the middle element from the list. Thus the list will be divided into two fairly equal sub-lists. The problem is that there’s not an easy way to get the middle element from a list and this will slow down the algorithm. So typically we can get for a pivot the first or the last item of the list.

After choosing a pivot the rest is simple. Put every item with a greater value on the right and every item with a lesser value on the left. Then we must sort the left and right sub-lists just as we did with the initial list.

It’s clear that with this algorithm naturally we’re going into a recursive solution. Typically every divide and conquer approach is easy to implement with recursion. But because recursion can be heavy, there is an iterative approach.

Implementation

As I said above recursive approach is something very natural for quicksort as it follows the divide and conquer principles. On each step we divide the list in two and we pass those sub-lists to our recursive function. But recursion is dangerous sometimes, so an iterative approach is also available. Typically iterative approaches “model” recursion with extra memory and a model of a stack, which is our case. Here we have two examples of quicksort – recursive and iterative in PHP. Let’s go first with the recursion.

Recursive Quicksort

$list = array(5,3,9,8,7,2,4,1,6,5);
 
// recursive
function quicksort($array)
{
	if (count($array) == 0) {
    	return array();
	}
 
	$pivot = $array[0];
	$left = $right = array();
 
	for ($i = 1; $i < count($array); $i++) {
		if ($array[$i] < $pivot) {
			$left[] = $array[$i];
		} else {
			$right[] = $array[$i];
		}
	}
	
	return array_merge(quicksort($left), array($pivot), quicksort($right));
}

// 1, 2, 3, 4, 5, 5, 6, 7, 8, 9
print_r(quicksort($list));

Iterative Quicksort

$list = array(5,3,9,8,7,2,4,1,6,5);

// iterative
function quicksort_iterative($array)
{
    $stack = array($array);
    $sorted = array();
 
    while (count($stack) > 0) {
 
        $temp = array_pop($stack);
 
        if (count($temp) == 1) {
            $sorted[] = $temp[0];
            continue;
        }
 
        $pivot = $temp[0];
        $left = $right = array();
 
        for ($i = 1; $i < count($temp); $i++) {
            if ($pivot > $temp[$i]) {
                $left[] = $temp[$i];
            } else {
                $right[] = $temp[$i];
            }
        }
 
        $left[] = $pivot;
 
        if (count($right))
            array_push($stack, $right);
        if (count($left))
            array_push($stack, $left);
    }
 
    return $sorted;
}

// 1, 2, 3, 4, 5, 5, 6, 7, 8, 9
print_r(quicksort_iterative($list));

Complexity

The complexity of quicksort in the average case is O(n*log(n)) - same as Merge sort. The problem is that in the worst case it is O(n²) - same as bubble sort. Obviously the worst case is when we have an already sorted list, and we constantly take for a pivot the last element of the list. But we should consider that in practice we don’t quite use sorted lists that we have to sort again, right?

Application

Quicksort is a great sorting algorithm and developers often go for it, but let's see some pros and cons of it.

Why using quicksort

1. Recursive implementation is easy
2. In general its speed is same as merge sort - O(n*log(n))
3. Elegant solution with no tricky merging as merge sort

Why not using quicksort

1. As slow as bubble sort in the worst case!
2. Iterative implementation isn't easy
3. There are faster algorithms for some sets of data types

Quicksort is beautiful because of the elegant idea behind its principles. Indeed if you have two sorted lists one with items with a greater value from a given value and the other with items smaller form that given value you can simply concatenate them and you can be sure that the resulting list will be sorted with no need of special merge.

In fact quicksort is a very elegant general purpose sorting algorithm and every developer should be familiar with its principles.

The Route from $_GET to String

The global arrays in PHP contain request parameters. Either GET or POST. As you know if the page address is something like:

http://www.example.com/index.php?a=b&key=value

This means that you pass to the index.php file two parameters – “a” and “key” with their values: “b” and “value”. Now in this case you can dump the $_GET global array somewhere in index.php and you’ll receive something like this.

array(
	"a"   => "b",
	"key" => "value",
);

This is however pseudocode, but in fact $_GET will be very similar to this sample array.

$_GET to String

Very often when a developer need to process the $_GET array to a string, which means generating again the query string from $_GET, he often comes to some operation like this one.

$queryString = '';
foreach ($_GET as $key => $value) {
	$queryString .= $key . '=' . $value . '&';
}

However this will result in something quite ugly like a=b&key=value& which comes with a trailing & at the end of the string.

There is however another approach – using an array.

$queryString = array();
foreach ($_GET as $key => $value) {
	$queryString[] = $key . '=' . $value;
}
$queryString = implode('&', $queryString);

But that invokes one function more and this is still not the most elegant solution. As I said at the beginning PHP is full of useful functions and here comes the http_build_query.

http_build_query

This is exactly what you need. As it name describe you can build the query string even by using a different from & separator.

$queryString = http_build_query($_GET, '', '|');

Thus $queryString will contain a=b|key=value and at least the code will look pritier.

Most of the code chunks I’ve seen about getting a file extension from a string are based on some sort of string manipulation.

$filename = '/my/path/image.jpeg';
echo substr($filename, strrpos($filename, '.') + 1);

Howerver there is a more elegant solution.

$filename = '/my/path/image.jpeg';
echo strtolower(pathinfo($filename, PATHINFO_EXTENSION));

Thus you rely on PHP built in functions and it’s harder to overlook the exact string manipulation approach.

$data = array(...);
$this->_helper->json($data);

As you may know in PHP you can access everything in the request uri by accessing the global $_GET array. If there is something like that in the browser’s address field: www.example.com/index.php?controller=index&action=test, you can simply get the values by that:

echo $_GET['controller']; // this will print "index"
echo $_GET['action'];     // this will print "test"

Zend Framework, Uri & Request Params

If you code with Zend Framework, you should know already, and that’s perhaps the first thing you’ve learned about Zend, that $_GET params can be accessed by calling the requrest’s getParam() method. But first of all the request uri will be different. The Zend’s convetion is to place after the domain name first the module name (which is omited when there’s only one module), than the controller’s name followed by the action and the key value pairs of all parameters. In that scheme the request uri above will look like that:

http://www.example.com/index/test

Here the keywords “controller” and “action” are omitted. This is cool – it’s more user friendly and it definitely helps the SEO.

Get the $_GET

Once the uri is setup like so – /index/test you can access it via the Zend way:

echo $this->getRequest()->getParam('controller'); // this will print "index"

The cool thing is that in the first case you don’t have any prevention of a missing value, while in the second case there is a second parameter or the getParam() method that does this job. What if the uri is www.example.com/index.php?controller=&action=test than by printing the $_GET[‘controller’] you’ll get nothing. In other hand even this:

echo $this->getRequest()->getParam('action'); 

won’t return “test” if the uri is http://www.example.com/index/

Note: actually here the default “index” action will be referenced!

That’s where the power of the framework comes. In the first case the solution is:

echo (empty($_GET['controller']) ? 'default': $_GET['controller']);

while in Zend there’s more elegant solution:

echo $this->getRequest()->getParam('action', 'test');

Thus when the action param is missing the “test” value is considered as default! Very useful!

What’s the Goal?

You’ve a simple HTML form with several groups of form elements. Imagine the situation with title and link groups. You can have 1, 2 or more title/link pairs which you’d like to save in a database table, where perhaps there are only three columns – id, title, link.

What is the Shortest Path to the Solution?

In fact the task can be done by many ways, but there’s one really elegant solution. As it appears in many occasions PHP and HTML are born to work together!

1. First Step

Create your web form by simply modifying a bit the element names. Usually when you have an input you simply name it after the database column or something similar.

In reality PHP and HTML allows the name to be an array element, just like so:

2. Second Step

Than all this comes in the _POST array in PHP, but formatted in an array manner, so you can simply foreach it!

That is simply enough!

If you code PHP from couple of hours you are probably familiar with the following problem:

The first and most common solution is just by adding the isset() statement and the code will look like that:

However do you know there’s is another simple and elegant solution? Simply add the @ sign to suppress the element existence