How to Merge Associative Arrays with Missing Columns and Provide Default Values?

Merging Associative Arrays with Missing Columns and Default Values

Given an array of associative arrays, the task is to combine them, merging the array keys and filling the missing columns with a default value.

<code class="php">$a = array('a' => 'some value', 'b' => 'some value', 'c' => 'some value');
$b = array('a' => 'another value', 'd' => 'another value', 'e' => 'another value', 'f' => 'another value');
$c = array('b' => 'some more value', 'x' => 'some more value', 'y' => 'some more value', 'z' => 'some more value');

$d = array($a, $b, $c);</code>

var_export($d)

will output:

array (
  0 =>
  array (
    'a' => 'some value',
    'b' => 'some value',
    'c' => 'some value',
  ),
  1 =>
  array (
    'a' => 'another value',
    'd' => 'another value',
    'e' => 'another value',
    'f' => 'another value',
  ),
  2 =>
  array (
    'b' => 'some more value',
    'x' => 'some more value',
    'y' => 'some more value',
    'z' => 'some more value',
  ),
)

The desired output is:

Array
(
    [0] => Array
        (
            [a] => some value
            [b] => some value
            [c] => some value
            [d] =>
            [e] =>
            [f] =>
            [x] =>
            [y] =>
            [z] =>
        )

    [1] => Array
        (
            [a] => another value
            [b] =>
            [c] =>
            [d] => another value
            [e] => another value
            [f] => another value
            [x] =>
            [y] =>
            [z] =>
        )

    [2] => Array
        (
            [a] =>
            [b] => some more value
            [c] =>
            [d] =>
            [e] =>
            [f] =>
            [x] => some more value
            [y] => some more value
            [z] => some more value
        )
)

To achieve this, array_merge() can be used.

<code class="php">$keys = array();
foreach(new RecursiveIteratorIterator(new RecursiveArrayIterator($d)) as $key => $val) $keys[$key] = '';
$data = array();
foreach($d as $values) {
    $data[] = array_merge($keys, $values);
}
echo '<pre class="brush:php;toolbar:false">';
print_r($data);</code>

An alternative method is to create key pair values and then map each $d and merge.

<code class="php">$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);</code>

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