PHP: 'rotate' an array?

Question

is it possible to easily \'rotate\' an array in PHP ?

Like this: 1, 2, 3, 4 -> 2, 3 ,4 ,1

Is there some kind of built-in PHP function for this?

Answer 1

Looping through the array, and shift-ing and push-ing, may be a common way to rotate an array, however it can often mess up your keys. A more robust method is using a combination of array_merge and array_splice.

/**
 * Rotates an array.
 * 
 * Numerical indexes will be renumbered automatically.
 * Associations will be kept for keys which are strings.
 * 
 * Rotations will always occur similar to shift and push,
 * where the number of items denoted by the distance are
 * removed from the start of the array and are appended.
 * 
 * Negative distances work in reverse, and are similar to
 * pop and unshift instead.
 * 
 * Distance magnitudes greater than the length of the array
 * can be interpreted as rotating an array more than a full
 * rotation. This will be reduced to calculate the remaining
 * rotation after all full rotations.
 * 
 * @param array $array The original array to rotate.
 * Passing a reference may cause the original array to be truncated.
 * @param int $distance The number of elements to move to the end.
 * Distance is automatically interpreted as an integer.
 * @return array The modified array.
 */
function array_rotate($array, $distance = 1) {
    settype($array, 'array');
    $distance %= count($array);
    return array_merge(
        array_splice($array, $distance), // Last elements  - moved to the start
        $array                          //  First elements - appended to the end
    );
}
// Example rotating an array 180°.
$rotated_180 = array_rotate($array, count($array) / 2);

Alternatively, if you also find the need to rotate keys so that they match with different values, you can combine array_keys, array_combine, array_rotate, and array_values.

/**
 * Rotates the keys of an array while keeping values in the same order.
 * 
 * @see array_rotate(); for function arguments and output.
 */
function array_rotate_key($array, $distance = 1) {
    $keys = array_keys((array)$array);
    return array_combine(
        array_rotate($keys, $distance), // Rotated keys
        array_values((array)$array)    //  Values
    );
}

Or alternatively rotating the values while keeping the keys in the same order (equivalent to calling the negative distance on the matching array_rotate_key function call).

/**
 * Rotates the values of an array while keeping keys in the same order.
 * 
 * @see array_rotate(); for function arguments and output.
 */
function array_rotate_value($array, $distance = 1) {
    $values = array_values((array)$array);
    return array_combine(
        array_keys((array)$array),        // Keys
        array_rotate($values, $distance) //  Rotated values
    );
}

And finally, if you want to prevent renumbering of numerical indexes.

/**
 * Rotates an array while keeping all key and value association.
 * 
 * @see array_rotate(); for function arguments and output.
 */
function array_rotate_assoc($array, $distance = 1) {
    $keys = array_keys((array)$array);
    $values = array_values((array)$array);
    return array_combine(
        array_rotate($keys, $distance),   // Rotated keys
        array_rotate($values, $distance) //  Rotated values
    );
}

It could be beneficial to perform some benchmark tests, however, I expect that a small handful of rotations per request wouldn't affect performance noticeably regardless of which method is used.

It should also be possible to rotate an array by using a custom sorting function, but it would most likely be overly complicated. i.e. usort.

Answer 2

Yes it is, here is a function I did myself, where $A is the array and $K the number of times you want to rotate the array:

function solution($A, $K) {

  for($i = 0; $i < $K; $i++): //we cycle $K
    $arrayTemp = $A;
    for($j = 0; $j < count($arrayTemp); $j++): // we cycle the array
       if($j == count($arrayTemp) - 1) $A[0] = $arrayTemp[$j]; // we check for the last position
       else $A[$j + 1] = $arrayTemp[$j]; // all but last position
    endfor;
  endfor;
 return $A;

}

Answer 3

It's very simple and could be done in many ways. Example:

$array   = array( 'a', 'b', 'c' );
$array[] = array_shift( $array );

Answer 4

Here's a function to rotate an array (zero-indexed array) to any position you want:

function rotateArray($inputArray, $rotateIndex) {
  if(isset($inputArray[$rotateIndex])) {
    $startSlice = array_slice($inputArray, 0, $rotateIndex);
    $endSlice = array_slice($inputArray, $rotateIndex);
    return array_merge($endSlice, $startSlice);
  }
  return $inputArray;
}

$testArray = [1,2,3,4,5,6];
$testRotates = [3, 5, 0, 101, -5];

foreach($testRotates as $rotateIndex) {
  print_r(rotateArray($testArray, $rotateIndex));
}

Answer 5

A method to maintain keys and rotate. using the same concept as array_push(array, array_shift(array)), instead we will use array_merge of 2 array_slices

$x = array("a" => 1, "b" => 2, "c" => 3, 'd' => 4);

To move the First element to the end

array_merge(array_slice($x, 1, NULL, true), array_slice($x, 0, 1, true) //'b'=>2, 'c'=>3, 'd'=>4, 'a'=>1

To move the last element to the front

array_merge(array_slice($x, count($x) -1, 1, true), array_slice($x, 0, //'d'=>4, 'a'=>1, 'b'=>2, 'c'=>3

Answer 6

$daynamesArray = array("Monday","Tuesday","Wednesday","Thursday","Friday","Saturday","Sunday");
array_push($daynamesArray, array_shift($daynamesArray)); //shift by one
array_push($daynamesArray, array_shift($daynamesArray)); //shift by two
print_r($daynamesArray);

The output starts at "Wednesday":

Array ( [0] => Wednesday [1] => Thursday [2] => Friday [3] => Saturday [4] => Sunday [5] => Monday [6] => Tuesday