This article details how to merge elements of one flat array to the end of each sub-array of another multidimensional array. By using `array_walk` to traverse the main array, combined with `array_merge` to append the corresponding elements to the sub-array, this tutorial provides an efficient and practical PHP solution, while explaining the behavior of `array_merge` when dealing with numeric keys.

Array merging: Append flattened array elements to multidimensional array children

In PHP programming, we often encounter situations where we need to integrate data from arrays of different structures. This tutorial will focus on a specific scenario: appending each element of a flat array, according to its key, to the end of a subarray of the corresponding key in another multidimensional array. This operation is useful when working with related data collections, for example, adding additional related attributes (elements of a flat array) to a main record (a child of a multidimensional array).

scene description

Suppose we have two arrays, $a is a multi-dimensional array with top-level keys corresponding to several sub-arrays. $b is a flat array whose keys match the top-level keys of $a and whose values ??are the elements that need to be appended to the corresponding subarray of $a.

The initial array structure is as follows:

 $a = [
    1 => [
        1 => "a",
        2 => "b",
        3 => "c"
    ],
    2 => [
        1 => "d",
        2 => "e",
        3 => "f"
    ],
    3 => [
        1 => "g",
        2 => "h",
        3 => "i"
    ]
];

$b = [
    1 => "1",
    2 => "2",
    3 => "3"
];

Our goal is to append the element in $b that corresponds to the top-level key of $a to the end of each subarray in $a. For example, the value "1" of $b[1] should be added to the $a[1] array.

Solution: use array_walk and array_merge

PHP provides the array_walk function, which can traverse each element of the array and execute a user-defined callback function for each element. Combined with the array_merge function, we can efficiently implement the required array merging operations.

Here is the code to implement this functionality:

  "a", 2 => "b", 3 => "c"])
    // $key is the current top-level key in $a (e.g. 1)
    // $b_array is the entire $b array passed in through use // Convert the elements of the corresponding key in $b_array into an array, and then merge them with $item // array_merge will re-index the numeric key $result[$key] = array_merge($item, (array) $b_array[$key]);
}, $b); // Pass the $b array as user data to the callback function // Output the merged result var_dump($result);

?>

Code analysis

1. $result = []; : We initialize an empty array $result to store the final merged data. This is because array_walk does not modify the original array by default, but performs operations on each element. By use (&$result), we can modify the $result array in the callback function.
2. array_walk($a, function($item, $key, $b_array) use (&$result) { ... }, $b); :
   • $a is the main array we want to iterate over.
   • The anonymous function is a callback executed for each element in $a.
     • $item: The subarray in the $a array currently traversed (for example [1 => "a", 2 => "b", 3 => "c"]).
     • $key: The key of the current subarray in $a (for example, 1).
     • $b_array: This is the entire flat array passed in through the third parameter $b of array_walk.
   • use (&$result): allows the callback function to access and modify the $result variable.
   • $b: As the third parameter of array_walk, it will be passed to the third parameter $b_array of the callback function.
3. $result[$key] = array_merge($item, (array) $b_array[$key]); :
   • $b_array[$key] Gets the element in the $b array corresponding to the current $a subarray key (for example, "1").
   • (array) $b_array[$key] Casts this scalar value to an array. For example, "1" becomes [0 => "1"]. This is a format that array_merge can handle.
   • array_merge($item, ...) Merges the current subarray $item with the converted array of $b elements.

Output result analysis

After executing the above code, var_dump($result) will display the following output:

 array(3) {
  [1]=>
  array(4) {
    [0]=>
    string(1) "a"
    [1]=>
    string(1) "b"
    [2]=>
    string(1) "c"
    [3]=>
    string(1) "1"
  }
  [2]=>
  array(4) {
    [0]=>
    string(1) "d"
    [1]=>
    string(1) "e"
    [2]=>
    string(1) "f"
    [3]=>
    string(1) "2"
  }
  [3]=>
  array(4) {
    [0]=>
    string(1) "g"
    [1]=>
    string(1) "h"
    [2]=>
    string(1) "i"
    [3]=>
    string(1) "3"
  }
}

Note: array_merge’s handling of numeric keys

It is important to note that when array_merge is used to merge arrays with numeric keys, it will re-index those numeric keys, starting from 0. In our example, after the original subarray [1 => "a", 2 => "b", 3 => "c"] is merged with [0 => "1"], the result is [0 => "a", 1 => "b", 2 => "c", 3 => "1"]. If your requirement is to retain the original numeric key and add a specific new numeric key (for example, 4), the behavior of array_merge may not be as expected, and you need to use direct assignment: $result[$key][4] = $b_array[$key];. However, for the scenario described in this tutorial, where elements are simply appended to the end, the behavior of array_merge is acceptable.

Summarize

By cleverly combining the traversal capabilities of array_walk and the merging capabilities of array_merge, we can efficiently implement the specific task of appending the elements of a flat array to the children of another multidimensional array. It is crucial to understand the behavior of array_merge when dealing with numeric keys to ensure that the end result is as expected. This mode is very practical when processing and converting complex data structures, and is an important skill in PHP array operations.

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