Sum the values of two associative multi-dimensional arrays in PHP

Question

I am trying to sum the values of two associative arrays. This is the first array:

Array
(
[Jan 01, 2013] => Array
    (
        [COM] => 100
        [         


        

Answer 1

Most directly/simply, iterate the levels of your second array and determine if the values should be appended to the first array, or merge-summed into it.

Code: (Demo)

$arr1 = [
    "Jan 01, 2013" => ["COM" => 100, "RES" => 200, "FOO" => 5],
    "Oct 28, 2014" => ["COM" => 300, "RES" => 400]
];
$arr2 = [
    "Jan 01, 2013" => ["COM" => 10, "RES" => 20],
    "Oct 28, 2014" => ["COM" => 30, "RES" => 40]
];

foreach ($arr2 as $date => $set) {
    foreach ($set as $key => $val) {
        if (!isset($arr1[$date][$key])) {            // val is unique to 1st array, set it
            $arr1[$date][$key] = $val;
        } else {                // val already exists in 1st array, add 2nd val to 1st val
            $arr1[$date][$key] += $val;
        }
    }
}
var_export($arr1);

---

Because you don't know the subarray keys for your incoming data and you have associative keys, you can use array_merge_recursive() to merge your two arrays in a way that sets up very simple summing.

After merging based on associative values, you can loop the new multi-dimensional array. While iterating, signify the subarray and the subarray's values as "modifiable by reference" -- this means that the foreach loops will process the actual array rather than a copy of the array. Effectively, this will make the changes affect the new array and deliver the desired result via $assoc_merged.

Code: (Demo)

$arr1 = [
    "Jan 01, 2013" => ["COM" => 100, "RES" => 200, "FOO" => 5],
    "Oct 28, 2014" => ["COM" => 300, "RES" => 400]
];
$arr2 = [
    "Jan 01, 2013" => ["COM" => 10, "RES" => 20],
    "Oct 28, 2014" => ["COM" => 30, "RES" => 40]
];

$assoc_merged =array_merge_recursive($arr1, $arr2);
// var_export($assoc_merged);  // see what this makes

foreach ($assoc_merged as $date => &$set) {
    foreach ($set as $key => &$val) {
        $val = array_sum((array)$val);
        // (array) forces a lone value to be an array containing one element; avoids generating Warnings
    }
}
var_export($assoc_merged);

---

Both approaches produce the same output:

array (
  'Jan 01, 2013' => 
  array (
    'COM' => 110,
    'RES' => 220,
    'FOO' => 5,
  ),
  'Oct 28, 2014' => 
  array (
    'COM' => 330,
    'RES' => 440,
  ),
)