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arrays object php sorting spl

How can I sort arrays and data in PHP?

321

This question is intended as a reference for questions about sorting arrays in PHP. It is easy to think that your particular case is unique and worthy of a new question, but most are actually minor variations of one of the solutions on this page.

If your question is closed as a duplicate of this one, please ask for your question to be reopened only if you can explain why it differs markedly from all of the below.

How do I sort an array in PHP?
How do I sort a complex array in PHP?
How do I sort an array of objects in PHP?


  1. Basic one-dimensional arrays; Incl. Multidimensional arrays, incl. arrays of objects; Incl. Sorting one array based on another

  2. Sorting with SPL

  3. Stable sort

For the practical answer using PHP’s existing functions see 1., for the academic in-detail answer on sorting algorithms (which PHP’s functions implement and which you may need for really, really complex cases), see 2.

0

    189

    Basic one dimensional arrays

    $array = array(3, 5, 2, 8);
    

    Applicable sort functions:

    • sort
    • rsort
    • asort
    • arsort
    • natsort
    • natcasesort
    • ksort
    • krsort

    The difference between those is merely whether key-value associations are kept (the “a” functions), whether it sorts low-to-high or reverse (“r“), whether it sorts values or keys (“k“) and how it compares values (“nat” vs. normal). See http://php.net/manual/en/array.sorting.php for an overview and links to further details.

    Multi dimensional arrays, including arrays of objects

    $array = array(
        array('foo' => 'bar', 'baz' => 42),
        array('foo' => ...,   'baz' => ...),
        ...
    );
    

    If you want to sort $array by the key ‘foo’ of each entry, you need a custom comparison function. The above sort and related functions work on simple values that they know how to compare and sort. PHP does not simply “know” what to do with a complex value like array('foo' => 'bar', 'baz' => 42) though; so you need to tell it.

    To do that, you need to create a comparison function. That function takes two elements and must return 0 if these elements are considered equal, a value lower than 0 if the first value is lower and a value higher than 0 if the first value is higher. That’s all that’s needed:

    function cmp(array $a, array $b) {
        if ($a['foo'] < $b['foo']) {
            return -1;
        } else if ($a['foo'] > $b['foo']) {
            return 1;
        } else {
            return 0;
        }
    }
    

    Often, you will want to use an anonymous function as the callback. If you want to use a method or static method, see the other ways of specifying a callback in PHP.

    You then use one of these functions:

    Again, they only differ in whether they keep key-value associations and sort by values or keys. Read their documentation for details.

    Example usage:

    usort($array, 'cmp');
    

    usort will take two items from the array and call your cmp function with them. So cmp() will be called with $a as array('foo' => 'bar', 'baz' => 42) and $b as another array('foo' => ..., 'baz' => ...). The function then returns to usort which of the values was larger or whether they were equal. usort repeats this process passing different values for $a and $b until the array is sorted. The cmp function will be called many times, at least as many times as there are values in $array, with different combinations of values for $a and $b every time.

    To get used to this idea, try this:

    function cmp($a, $b) {
        echo 'cmp called with $a:', PHP_EOL;
        var_dump($a);
        echo 'and $b:', PHP_EOL;
        var_dump($b);
    }
    

    All you did was define a custom way to compare two items, that’s all you need. That works with all sorts of values.

    By the way, this works on any value, the values don’t have to be complex arrays. If you have a custom comparison you want to do, you can do it on a simple array of numbers too.

    sort sorts by reference and does not return anything useful!

    Note that the array sorts in place, you do not need to assign the return value to anything. $array = sort($array) will replace the array with true, not with a sorted array. Just sort($array); works.

    Custom numeric comparisons

    If you want to sort by the baz key, which is numeric, all you need to do is:

    function cmp(array $a, array $b) {
        return $a['baz'] - $b['baz'];
    }
    

    Thanks to The PoWEr oF MATH this returns a value < 0, 0 or > 0 depending on whether $a is lower than, equal to or larger than $b.

    Note that this won’t work well for float values, since they’ll be reduced to an int and lose precision. Use explicit -1, 0 and 1 return values instead.

    Objects

    If you have an array of objects, it works the same way:

    function cmp($a, $b) {
        return $a->baz - $b->baz;
    }
    

    Functions

    You can do anything you need inside a comparison function, including calling functions:

    function cmp(array $a, array $b) {
        return someFunction($a['baz']) - someFunction($b['baz']);
    }
    

    Strings

    A shortcut for the first string comparison version:

    function cmp(array $a, array $b) {
        return strcmp($a['foo'], $b['foo']);
    }
    

    strcmp does exactly what’s expected of cmp here, it returns -1, 0 or 1.

    Spaceship operator

    PHP 7 introduced the spaceship operator, which unifies and simplifies equal/smaller/larger than comparisons across types:

    function cmp(array $a, array $b) {
        return $a['foo'] <=> $b['foo'];
    }
    

    Sorting by multiple fields

    If you want to sort primarily by foo, but if foo is equal for two elements sort by baz:

    function cmp(array $a, array $b) {
        if (($cmp = strcmp($a['foo'], $b['foo'])) !== 0) {
            return $cmp;
        } else {
            return $a['baz'] - $b['baz'];
        }
    }
    

    For those familiar, this is equivalent to an SQL query with ORDER BY foo, baz.
    Also see this very neat shorthand version and how to create such a comparison function dynamically for an arbitrary number of keys.

    Sorting into a manual, static order

    If you want to sort elements into a “manual order” like “foo”, “bar”, “baz”:

    function cmp(array $a, array $b) {
        static $order = array('foo', 'bar', 'baz');
        return array_search($a['foo'], $order) - array_search($b['foo'], $order);
    }
    

    For all the above, if you’re using PHP 5.3 or higher (and you really should), use anonymous functions for shorter code and to avoid having another global function floating around:

    usort($array, function (array $a, array $b) { return $a['baz'] - $b['baz']; });
    

    That’s how simple sorting a complex multi-dimensional array can be. Again, just think in terms of teaching PHP how to tell which of two items is “greater”; let PHP do the actual sorting.

    Also for all of the above, to switch between ascending and descending order simply swap the $a and $b arguments around. E.g.:

    return $a['baz'] - $b['baz']; // ascending
    return $b['baz'] - $a['baz']; // descending
    

    Sorting one array based on another

    And then there’s the peculiar array_multisort, which lets you sort one array based on another:

    $array1 = array( 4,   6,   1);
    $array2 = array('a', 'b', 'c');
    

    The expected result here would be:

    $array2 = array('c', 'a', 'b');  // the sorted order of $array1
    

    Use array_multisort to get there:

    array_multisort($array1, $array2);
    

    As of PHP 5.5.0 you can use array_column to extract a column from a multi dimensional array and sort the array on that column:

    array_multisort(array_column($array, 'foo'), SORT_DESC, $array);
    

    You can also sort on more than one column each in either direction:

    array_multisort(array_column($array, 'foo'), SORT_DESC,
                    array_column($array, 'bar'), SORT_ASC,
                    $array);
    

    As of PHP 7.0.0 you can also extract properties from an array of objects.


    If you have more common cases, feel free to edit this answer.

    6

    • The numeric comparison function doesn’t work for float values; I’m sure you know what I mean 🙂

      – Ja͢ck

      Jun 28, 2013 at 12:07

    • 2

      For the static order, I would apply array_flip() to make use of speedier position lookup, e.g. $order[$a['foo']] instead of array_search($a['foo'], $order).

      – Ja͢ck

      Jun 28, 2013 at 23:58

    • Might be a bit of a big edit: gist.github.com/Rizier123/24a6248758b53245a63e839d8e08a32b but if you think it is an improvement and I included everything essential I can apply it.

      – Rizier123

      Jun 2, 2016 at 16:30

    • @Rizier123 I certainly applaud the effort, it’s a very good writeup; but I would prefer it if you posted it as separate answer, even if it’s very similar. Your rewrite contains a lot of details (pass by reference, big table etc.), but that detail distracts from the smooth introduction to the core topic of the workings of the comparison function, IMHO. I explicitly refer to the manual several times on purpose, because that’s where such detail should be looked up; no need to repeat it here and distract from the core idea I’m trying to convey.

      – deceze

      Jun 3, 2016 at 1:49

    • @deceze The main challenge, since it is a reference Q&A, is to display the information as compact and readable as possible and make it easy for users to find their sorting function. I tweaked a few things: gist.github.com/Rizier123/24a6248758b53245a63e839d8e08a32b but I still have to think about it, if it is useful and valuable to post it as separate answer, since it is very similar content

      – Rizier123

      Jun 3, 2016 at 15:13


    142

    Well most basic methods are already covered by deceze I would try to look at other types of sort

    Sorting with SPL

    SplHeap

    class SimpleHeapSort extends SplHeap {
        public function compare($a, $b) {
            return strcmp($a, $b);
        }
    }
    
    // Let's populate our heap here (data of 2009)
    $heap = new SimpleHeapSort();
    $heap->insert("a");
    $heap->insert("b");
    $heap->insert("c");
    
    echo implode(PHP_EOL, iterator_to_array($heap));
    

    Output

    c
    b
    a
    

    SplMaxHeap

    The SplMaxHeap class provides the main functionalities of a heap, keeping the maximum on the top.

    $heap = new SplMaxHeap();
    $heap->insert(1);
    $heap->insert(2);
    $heap->insert(3);
    

    SplMinHeap

    The SplMinHeap class provides the main functionalities of a heap, keeping the minimum on the top.

    $heap = new SplMinHeap ();
    $heap->insert(3);
    $heap->insert(1);
    $heap->insert(2);
    

    Other Types of Sort

    Bubble Sort

    From the Wikipedia article on Bubble Sort:

    Bubble sort, sometimes incorrectly referred to as sinking sort, is a simple sorting algorithm that works by repeatedly stepping through the list to be sorted, comparing each pair of adjacent items and swapping them if they are in the wrong order. The pass through the list is repeated until no swaps are needed, which indicates that the list is sorted. The algorithm gets its name from the way smaller elements “bubble” to the top of the list. Because it only uses comparisons to operate on elements, it is a comparison sort. Although the algorithm is simple, most of the other sorting algorithms are more efficient for large lists.

    function bubbleSort(array $array) {
        $array_size = count($array);
        for($i = 0; $i < $array_size; $i ++) {
            for($j = 0; $j < $array_size; $j ++) {
                if ($array[$i] < $array[$j]) {
                    $tem = $array[$i];
                    $array[$i] = $array[$j];
                    $array[$j] = $tem;
                }
            }
        }
        return $array;
    }
    

    Selection sort

    From the Wikipedia article on Selection sort:

    In computer science, selection sort is a sorting algorithm, specifically an in-place comparison sort. It has O(n2) time complexity, making it inefficient on large lists, and generally performs worse than the similar insertion sort. Selection sort is noted for its simplicity, and it has performance advantages over more complicated algorithms in certain situations, particularly where auxiliary memory is limited.

    function selectionSort(array $array) {
        $length = count($array);
        for($i = 0; $i < $length; $i ++) {
            $min = $i;
            for($j = $i + 1; $j < $length; $j ++) {
                if ($array[$j] < $array[$min]) {
                    $min = $j;
                }
            }
            $tmp = $array[$min];
            $array[$min] = $array[$i];
            $array[$i] = $tmp;
        }
        return $array;
    }
    

    Insertion sort

    From the Wikipedia article on Insertion sort:

    Insertion sort is a simple sorting algorithm that builds the final sorted array (or list) one item at a time. It is much less efficient on large lists than more advanced algorithms such as quicksort, heapsort, or merge sort. However, insertion sort provides several advantages:

    function insertionSort(array $array) {
        $count = count($array);
        for($i = 1; $i < $count; $i ++) {
    
            $j = $i - 1;
            // second element of the array
            $element = $array[$i];
            while ( $j >= 0 && $array[$j] > $element ) {
                $array[$j + 1] = $array[$j];
                $array[$j] = $element;
                $j = $j - 1;
            }
        }
        return $array;
    }
    

    Shellsort

    From the Wikipedia article on Shellsort:

    Shellsort, also known as Shell sort or Shell’s method, is an in-place comparison sort. It generalizes an exchanging sort, such as insertion or bubble sort, by starting the comparison and exchange of elements with elements that are far apart before finishing with neighboring elements.

    function shellSort(array $array) {
        $gaps = array(
                1,
                2,
                3,
                4,
                6
        );
        $gap = array_pop($gaps);
        $length = count($array);
        while ( $gap > 0 ) {
            for($i = $gap; $i < $length; $i ++) {
                $tmp = $array[$i];
                $j = $i;
                while ( $j >= $gap && $array[$j - $gap] > $tmp ) {
                    $array[$j] = $array[$j - $gap];
                    $j -= $gap;
                }
                $array[$j] = $tmp;
            }
            $gap = array_pop($gaps);
        }
        return $array;
    }
    

    Comb sort

    From the Wikipedia article on Comb sort:

    Comb sort is a relatively simple sorting algorithm originally designed by Wlodzimierz Dobosiewicz in 1980. Later it was rediscovered by Stephen Lacey and Richard Box in 1991. Comb sort improves on bubble sort.

    function combSort(array $array) {
        $gap = count($array);
        $swap = true;
        while ( $gap > 1 || $swap ) {
            if ($gap > 1)
                $gap /= 1.25;
            $swap = false;
            $i = 0;
            while ( $i + $gap < count($array) ) {
                if ($array[$i] > $array[$i + $gap]) {
                    // swapping the elements.
                    list($array[$i], $array[$i + $gap]) = array(
                            $array[$i + $gap],
                            $array[$i]
                    );
                    $swap = true;
                }
                $i ++;
            }
        }
        return $array;
    }
    

    Merge sort

    From the Wikipedia article on Merge sort:

    In computer science, a merge sort (also commonly spelled mergesort) is an O(n log n) comparison-based sorting algorithm. Most implementations produce a stable sort, which means that the implementation preserves the input order of equal elements in the sorted output

    function mergeSort(array $array) {
        if (count($array) <= 1)
            return $array;
    
        $left = mergeSort(array_splice($array, floor(count($array) / 2)));
        $right = mergeSort($array);
    
        $result = array();
    
        while ( count($left) > 0 && count($right) > 0 ) {
            if ($left[0] <= $right[0]) {
                array_push($result, array_shift($left));
            } else {
                array_push($result, array_shift($right));
            }
        }
        while ( count($left) > 0 )
            array_push($result, array_shift($left));
    
        while ( count($right) > 0 )
            array_push($result, array_shift($right));
    
        return $result;
    }
    

    Quicksort

    From the Wikipedia article on Quicksort:

    Quicksort, or partition-exchange sort, is a sorting algorithm developed by Tony Hoare that, on average, makes O(n log n) comparisons to sort n items. In the worst case, it makes O(n2) comparisons, though this behavior is rare.

    function quickSort(array $array) {
        if (count($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));
    }
    

    Permutation sort

    From the Wikipedia article on Permutation sort:

    Permutation sort, which proceeds by generating the possible permutations of the input array/list until discovering the sorted one.

    function permutationSort($items, $perms = array()) {
        if (empty($items)) {
            if (inOrder($perms)) {
                return $perms;
            }
        } else {
            for($i = count($items) - 1; $i >= 0; -- $i) {
                $newitems = $items;
                $newperms = $perms;
                list($foo) = array_splice($newitems, $i, 1);
                array_unshift($newperms, $foo);
                $res = permutationSort($newitems, $newperms);
                if ($res) {
                    return $res;
                }
            }
        }
    }
    
    function inOrder($array) {
        for($i = 0; $i < count($array); $i ++) {
            if (isset($array[$i + 1])) {
                if ($array[$i] > $array[$i + 1]) {
                    return False;
                }
            }
        }
        return True;
    }
    

    Radix sort

    From the Wikipedia article on Radix sort:

    In computer science, radix sort is a non-comparative integer sorting algorithm that sorts data with integer keys by grouping keys by the individual digits which share the same significant position and value.

    // Radix Sort for 0 to 256
    function radixSort($array) {
        $n = count($array);
        $partition = array();
    
        for($slot = 0; $slot < 256; ++ $slot) {
            $partition[] = array();
        }
    
        for($i = 0; $i < $n; ++ $i) {
            $partition[$array[$i]->age & 0xFF][] = &$array[$i];
        }
    
        $i = 0;
    
        for($slot = 0; $slot < 256; ++ $slot) {
            for($j = 0, $n = count($partition[$slot]); $j < $n; ++ $j) {
                $array[$i ++] = &$partition[$slot][$j];
            }
        }
        return $array;
    }
    

    6

    • 4

      @deceze you covered all the basics .. i had to look for another way to be relevant 🙂

      – Baba

      Jun 28, 2013 at 12:18

    • 5

      I don’t see anything wrong with the more academic sorting methods 🙂 alot less useful for most applications but occasionally they may be asked for / required is handy to have a reference especially since I’d forgotton about most of these over time

      – Dave

      Jun 28, 2013 at 12:18

    • Actually, for quick sort it is recommended to select pivot as a median of three values: first, middle and last elements. This is my example for pivot seletion. That allows to avoid worst-case reverse-sorted array (which causes O(n^2) comparisons if we’ll use just first element as pivot)

      – Alma Do

      May 3, 2014 at 21:10


    • I have heard that spl work faster than normal array sorting .Is it right?

      – jewelhuq

      Jan 13, 2016 at 10:18

    • I agree with Dave, nowadays, almost fw have included that why I rarely remember or use it.

      Mar 10, 2016 at 4:03

    43

    Stable sort

    Let’s say you have an array like this:

    ['Kale', 'Kaleidoscope', 'Aardvark', 'Apple', 'Leicester', 'Lovely']
    

    And now you want to sort on the first letter only:

    usort($array, function($a, $b) {
        return strcmp($a[0], $b[0]);
    });
    

    The outcome is this:

    ['Apple', 'Aardvark', 'Kale', 'Kaleidoscope', 'Lovely', 'Leicester']
    

    The sort wasn’t stable!

    The keen observer may have noticed that the array sorting algorithm (QuickSort) didn’t produce a stable outcome and that the original order between words of the same first letter wasn’t preserved. This case is trivial and we should have compared the whole string, but let’s assume your use-case is more complicated, such as two consecutive sorts on different fields that shouldn’t cancel out each other’s work.

    The Schwartzian transform

    The Schwartzian transform, also referred to as the decorate-sort-undecorate idiom, effects a stable sort with an inherently unstable sorting algorithm.

    First, you decorate each array element with another array comprising a primary key (the value) and a secondary key (its index or position):

    array_walk($array, function(&$element, $index) {
        $element = array($element, $index); // decorate
    });
    

    This transforms the array into this:

    [
        ['Kale', 0], ['Kaleidoscope', 1], 
        ['Aardvark', 2], ['Apple', 3], 
        ['Leicester', 4], ['Lovely', 5]
    ]
    

    Now, we adjust the comparison step; we compare the first letter again, but if they’re the same, the secondary key is used to retain the original ordering:

    usort($array, function($a, $b) {
        // $a[0] and $b[0] contain the primary sort key
        // $a[1] and $b[1] contain the secondary sort key
        $tmp = strcmp($a[0][0], $b[0][0]);
    
        if ($tmp != 0) {
            return $tmp; // use primary key comparison results
        }
    
        return $a[1] - $b[1]; // use secondary key
    });
    

    Afterwards, we undecorate:

    array_walk($array, function(&$element) {
        $element = $element[0];
    });
    

    The final result:

    ['Aardvark', 'Apple', 'Kale', 'Kaleidoscope', 'Leicester', 'Lovely']
    

    What about reuse?

    You had to rewrite your comparison function to work with the transformed array elements; you may not want to edit your delicate comparison functions, so here’s a wrapper for the comparison function:

    function stablecmp($fn)
    {
        return function($a, $b) use ($fn) {
            if (($tmp = call_user_func($fn, $a[0], $b[0])) != 0) {
                return $tmp;
            } else {
                return $a[1] - $b[1];
            }
        };
    }
    

    Let’s write the sort step using this function:

    usort($array, stablecmp(function($a, $b) {
        return strcmp($a[0], $b[0]);
    }));
    

    Voila! Your pristine comparison code is back.

    2

    • Your phrase “effects a stable sort with an inherently unstable sorting algorithm” was the ah-ha moment for me. The wikipedia page has no mention of the word stable, which seems to me to be the beauty of the transform. Shame.

      Mar 2, 2015 at 5:44

    • 1

      @TylerCollier Yeah, you need to read between the lines of that Wikipedia reference … I saved you the trouble of doing that 😉

      – Ja͢ck

      Mar 2, 2015 at 5:49