Merge Multiple Associative Arrays and Add Missing Columns with a Default Value

Combining associative arrays with different sets of keys to create a unified array can be challenging. This question explores a method to achieve this, and the desired output is an array where keys are merged and missing columns are filled with a default value.

To accomplish this, it was suggested to employ the array_merge function in conjunction with a carefully crafted array of keys:

$keys = array();
foreach(new RecursiveIteratorIterator(new RecursiveArrayIterator($d)) as $key => $val) {
    $keys[$key] = '';
}

This loop iterates over all the elements in the input arrays, extracting the unique keys and assigning them empty values. The resulting $keys array contains all the possible keys that can exist in the final merged array.

Next, each input array is merged with the $keys array:

$data = array();
foreach($d as $values) {
    $data[] = array_merge($keys, $values);
}

This step ensures that every row in the final array has all the possible keys, with any missing values being filled with an empty string. The resulting $data array is the desired merged and completed array.

Alternatively, a key-pair array can be created and merged with each input array:

$keys = array_keys(call_user_func_array('array_merge', $d));
$key_pair = array_combine($keys, array_fill(0, count($keys), null));
$values = array_map(function($e) use ($key_pair) {
    return array_merge($key_pair, $e);
}, $d);

This method essentially creates a map of all possible keys to null values. Each input array is then merged with the $key_pair array, achieving the same result as the previous approach.