How to efficiently insert elements after another known (by key or pointer) element in a PHP array?

Question

Given an array:

$a = array(
    \'abc\',
    123,
    \'k1\'=>\'v1\',
    \'k2\'=>\'v2\',
    78,
    \'tt\',
    \'k3\'=>\'v3\'
);

Wi

Answer 1

Generally speaking doubly linked list would be ideal for this task.

There is a built-in implementation of that since PHP 5.3, called SplDoublyLinkedList and since PHP 5.5 it also has add method, which allows adding/inserting values in the middle.

Answer 2

There's a nice function that would help you here: https://gist.github.com/scribu/588429

Answer 3

You can't use internal array pointer to insert elements.

There's array_splice which can insert/remove/replace elements and subarrays, but it's intended for integer-indexed arrays.

I'm afraid you'll have to rebuild the array to insert element (except cases where you want to insert first/last element) or use separate integer-indexed array for holding keys in the order you want.

Answer 4

You could do it by splitting your array using array_keys and array_values, then splice them both, then combine them again.

$insertKey = 'k1';

$keys = array_keys($arr);
$vals = array_values($arr);

$insertAfter = array_search($insertKey, $keys) + 1;

$keys2 = array_splice($keys, $insertAfter);
$vals2 = array_splice($vals, $insertAfter);

$keys[] = "myNewKey";
$vals[] = "myNewValue";

$newArray = array_merge(array_combine($keys, $vals), array_combine($keys2, $vals2));

Answer 5

I found a great answer here that works really well. I want to document it, so others on SO can find it easily:

/*
 * Inserts a new key/value before the key in the array.
 *
 * @param $key
 *   The key to insert before.
 * @param $array
 *   An array to insert in to.
 * @param $new_key
 *   The key to insert.
 * @param $new_value
 *   An value to insert.
 *
 * @return
 *   The new array if the key exists, FALSE otherwise.
 *
 * @see array_insert_after()
 */
function array_insert_before($key, array &$array, $new_key, $new_value) {
  if (array_key_exists($key, $array)) {
    $new = array();
    foreach ($array as $k => $value) {
      if ($k === $key) {
        $new[$new_key] = $new_value;
      }
      $new[$k] = $value;
    }
    return $new;
  }
  return FALSE;
}

/*
 * Inserts a new key/value after the key in the array.
 *
 * @param $key
 *   The key to insert after.
 * @param $array
 *   An array to insert in to.
 * @param $new_key
 *   The key to insert.
 * @param $new_value
 *   An value to insert.
 *
 * @return
 *   The new array if the key exists, FALSE otherwise.
 *
 * @see array_insert_before()
 */
function array_insert_after($key, array &$array, $new_key, $new_value) {
  if (array_key_exists ($key, $array)) {
    $new = array();
    foreach ($array as $k => $value) {
      $new[$k] = $value;
      if ($k === $key) {
        $new[$new_key] = $new_value;
      }
    }
    return $new;
  }
  return FALSE;
}

Answer 6

This way is fine for new values without keys. You can not insert value with a key, and numeric indexes will be 'reset' as 0 to N-1.

$keys = array_keys($a);
$index = array_flip($keys);

$key = key($a); //current element
//or 
$key = 'k1';

array_splice($a, $index[$key] + 1, 0, array('value'));