Merge two 2d arrays grouping on one column value and summing another column value within each group

Question

Basically I need to take two arrays, merge them with unique values and sum one of columns. It makes more sense when written out below:

$a = [
    ['ID' => 1, 'Count' => 2],
];

$b = [
    ['ID' => 1, 'Count' => 4],
    ['ID' => 2, 'Count' => 3]
];

and I need the final product to be:

$a_plus_b = [
    ['ID' => 1, 'Count' => 6],
    ['ID' => 2, 'Count' => 3]
];

I have been playing with different variations of array_merge() and array_unique(), but I can't find an efficient way to do what I need. I know I can always do nested loops, but I was hoping for something easier. Any ideas?

Answer 1

This should do the trick

Note: This solution requires PHP >= 5.3. There is a PHP < 5.3 solution below.

$input = array($a, $b);
// add as many result arrays to $input as you want; e.g.,
// $input = array($a, $b, $c, $d);

$output = array_count_values(
  call_user_func_array(
    'array_merge',
     array_map(
       function($arr) {
         return array_fill(0, $arr['Count'], $arr['ID']);
       },
       call_user_func_array(
         'array_merge',
         $input
       )
     )
  )
);

print_r($output);

Output

Array
(
    [1] => 6
    [2] => 3
)

Note the array keys above are ID values. The array values are Count values.

---

If you're running PHP < 5.2 you won't be able to use the inline closure with array_fill. You have to define it as a separate function.

$input = array($a, $b);

function _fill($arr) {
  return array_fill(0, $arr['Count'], $arr['ID']);
}

$output = array_count_values(
  call_user_func_array(
    'array_merge',
    array_map(
      '_fill',
      call_user_func_array(
        'array_merge',
        $input
      )
    )
  )
);

print_r($output);

---

From here, converting the output to your desired format is a trivial task.