Sort Multidimensional Arrays with PHP array_multisort

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Without question, the most common operation in the Web applications I build is retrieving gobs of structured data and displaying that data in some order.

Whether the source of this data is a database, a structured file, or a script, it most often takes the shape of a multidimensional (ususally 2-dimensional) array whose first dimension is simple numeric and whose second dimension consists of associative arrays.

Let’s suppose a Web application I’m buiding involves displaying player statistics for a fictional basketball team. The data elements for each player might include the following:

last_name
first_name
dob
points
assists
rebounds
fouls

Suppose I want the users of this Web application to be able to sort and view statistics by any or all of those data elements.

If I’m storing the data in, say, a simple MySQL database table, I could excecute separate SQL commands for each sort case. Here are some typical sort cases:

Sort by name:

SELECT * FROM statistics ORDER BY last_name ASC, first_name ASC

Sort by points scored (highest to lowest):

SELECT * FROM statistics ORDER BY points DESC, last_name ASC, first_name ASC

Sort by the number of personal fouls:

SELECT * FROM statistics ORDER BY fouls ASC, last_name ASC, first_name ASC

However…

  1. Making repeated database connections and executing repeated SQL commands just for the purpose of displaying data in a different sequence seems sub-optimal and ecologically irresponsible. (Those who are more knowlegeable on these matters are encouraged to comment.)
  2. Not all data sets come neatly from a single, simple database table that supports simple ORDER BY clauses. In fact, some data sets are arrays defined in scripts.

Here’s how to sort multidimensional arrays with php array_multisort, using only ONE SQL SELECT

After doing a SELECT sorted by name, we’d end up with an array that looks like this:

$players[] = array(
	'last_name' => 'Adams',
	'first_name' => 'Andy',
	'dob' => '1984-09-01',
	'points' => '10',
	'assists' => '20',
	'rebounds' => '5',
	'fouls' => '2',
);

$players[] = array(
	'last_name' => 'Bird',
	'first_name' => 'Bobby',
	'dob' => '1948-01-01',
	'points' => '35',
	'assists' => '10',
	'rebounds' => '15',
	'fouls' => '6',
);

$players[] = array(
	'last_name' => 'Carey',
	'first_name' => 'Charlie',
	'dob' => '1952-02-24',
	'points' => '20',
	'assists' => '1',
	'rebounds' => '14',
	'fouls' => '3',
);

The trick to using array_multisort to sort a multidimensional array like this is to create a new, temporary, utility array that maps to the original array and which will be the reference for sorting the original array.

The syntax of array_multisort for this purpose is:

array_multisort($array1, $array1_sort_order, $array2, $array1_sort_flags)

where

  • $array1 is the utility array
  • $array1_sort_order is the order to use when sorting $array2 (SORT_ASC or SORT_DESC)
  • $array2 is the original data array we want to sort
  • $array1_sort_flags tells PHP how to treat the sort values (e.g., SORT_NUMERIC)

Example 1: Sort players by points, highest to lowest

So let’s suppose we want to sort $players by points (highest to lowest). We’ll create a new array (call it $points) and populate it in a foreach loop:

$points = array();
foreach ( $players as $player ) {
	$points[] = $player['points'];
}

That will create a new array:

points:
Array
(
    [0] => 10
    [1] => 35
    [2] => 20
)

A glance at the $points array tells us that if we were to sort the $points array in descending order, element[1] (with a value of 35) would be the new element[0], element[2] (with a value of 20) would be the new element[1], and element[0] (with a value of 10) would be the new element[2]. In fact, if we could freeze the reindexing process, we would see this…

//points, in an imaginary freeze-frame, after sorting but before reindexing:
Array
(
    [1] => 35
    [2] => 20
    [0] => 10
)

…before we saw this, the final result of sorting $points in descending order:

//points sorted and reindexed:
Array
(
    [0] => 35
    [1] => 20
    [2] => 10
)

Now we run the array_multisort fucntion:

array_multisort($points, SORT_DESC, $players, SORT_NUMERIC);

The resulting reorganized $players array looks like this:

players:
Array
(
    [0] => Array
        (
            [last_name] => Bird
            [first_name] => Bobby
            [dob] => 1948-01-01
            [points] => 35
            [assists] => 10
            [rebounds] => 15
            [fouls] => 6
        )

    [1] => Array
        (
            [last_name] => Carey
            [first_name] => Charlie
            [dob] => 1952-02-24
            [points] => 20
            [assists] => 1
            [rebounds] => 14
            [fouls] => 3
        )

    [2] => Array
        (
            [last_name] => Adams
            [first_name] => Andy
            [dob] => 1984-09-01
            [points] => 10
            [assists] => 20
            [rebounds] => 5
            [fouls] => 2
        )

)

Huh? How/Why Does This Work?

This is one of those things I find easier to do than explain. And I’ll admit it took me a while to get my head around this. (Hat tip to Chris Shifflet who unlocked the door with this post from 2011.)

In each case, what we’ve done is create a “parallel” utility array that has the same number of top-level elements as the original array we want to sort. Both arrays have numeric indexes starting at zero. When we call the array_multisort function, what we’re saying is this:

  1. Sort $array1 according to the $array1_sort_order.
  2. Note how the numeric indexes are rearranged.
  3. Now sort $array2 such that its numeric indexes get rearranged in the same order as the numeric indexes of $array1.

It’s as if we’re connecting each array element of $array1 to each array element of $array2 with a rigid steel bar. Then, as we move the elements of $array1 to fall into the new sort order (e.g., moving 35 points — whose index is 1 — up to the first (i.e., zeroth) position in the first example), the corresponding 1 element of $players (Bobby Bird) gets dragged up to the top spot (zeroth) spot of its array.

If someone has a better explanation of this array_multisort phenomenon — or a better way to accomplish this result, for that matter — please comment below.

Here are some more examples.

Example 2: Sort players by rebounds, highest to lowest

$rebounds = array();
foreach ( $players as $player ) {
	$rebounds[] = $player['rebounds'];
}

Now:

array_multisort($rebounds, SORT_DESC, $players, SORT_NUMERIC);

Example 3: Sort players by fouls, lowest to highest

$fouls = array();
foreach ( $players as $player ) {
	$fouls[] = $player['fouls'];
}

Now:

array_multisort($fouls, SORT_ASC, $players, SORT_NUMERIC);

Comments Welcome

As I said, I welcome comments. If you have a better explanation for how array_multisort works — or a better way to accomplish the end result — I’d like to hear from you. And if this article helped you finally wrap your head around array_multisort, I’d like to hear that, too.

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About JeffCohan.com

The nSiteful Tech Blog (the official blog of nSiteful Web Builders, Inc. since January of 2013) is where I (Jeff Cohan) and (occasionally) associates will be posting articles of potential interest to like-minded techies, nSiteful clients who are playing active roles in the maintenance of their own Web sites and blogs, and pretty much anyone interested in how Web strategies and tools can help them reach their goals.

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About Jeff Cohan

Jeff and his wife, Margie, are the proud parents of Sarah and Jake. Jeff is the founder, president and chief cook and bottle washer of nSiteful Web Builders, Inc., a Web development and Internet Consulting firm. In his spare time, Jeff builds Web sites and Web applications, plays guitar, putters around in his basement woodworking shop, mercilessly spoils his grandchildren, and creates videos from more than two decades of home movies. His current video project is an extended montage of people (mainly family members) asking him to stop filming them.

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