比较两个多维数组并根据匹配键替换值

Question

I have 2 arrays of different length:

$array1 = Array
(
    [0] => Array
        (
            ['_id'] => "Group1"
            ['M'] => 0
            ['F'] => 0
        )
    [1] => Array
        (
            ['_id'] => "Group2"
            ['M'] => 0
            ['F'] => 0
        )
    [2] => Array
        (
            ['_id'] => "Group3"
            ['M'] => 0
            ['F'] => 0
        )
    [3] => Array
        (
            ['_id'] => "Group4"
            ['M'] => 0
            ['F'] => 0
        )
)


$array2 = Array
(
    [0] => Array
        (
            ['_id'] => "Group2"
            ['M'] => 180
            ['F'] => 200
        )
    [1] => Array
        (
            ['_id'] => "Group4"
            ['M'] => 360
            ['F'] => 500
        )
)

I want to compare the values of ['_id'] in both array and if they match, I will replace the values of ['M'] and ['F'] in array1 with those from array2 based on the corresponding ['_id].

So my desired ouptput would be:

$array1 = Array
(
    [0] => Array
        (
            ['_id'] => "Group1"
            ['M'] => 0
            ['F'] => 0
        )
    [1] => Array
        (
            ['_id'] => "Group2"
            ['M'] => 180
            ['F'] => 200
        )
    [2] => Array
        (
            ['_id'] => "Group3"
            ['M'] => 360
            ['F'] => 500
        )
    [3] => Array
        (
            ['_id'] => "Group4"
            ['M'] => 0
            ['F'] => 0
        )
)

This is my code but I can't seem to get the values replaced with the new values. The values are still the same as before.

foreach ($array1 as $defArr)
{
  foreach ($array2 as $dayArr)
  {
    if($dayArr['_id'] == $defArr['_id'])
    {
      $defArr['M'] = $dayArr['M'];
      $defArr['F'] = $dayArr['F'];
    }
  }
}

Answer 1

This can be a one-character change:

foreach ($array1 as $defArr)

goes to

foreach ($array1 as &$defArr)
#                   ^

The & reference operator points to the original sub array in the foreach loop context rather than a temporary variable.

However, it's a bit safer to use the index explicitly:

foreach ($array1 as $i => $defArr) {
    foreach ($array2 as $j => $dayArr) {
        if ($dayArr['_id'] == $defArr['_id']) {
            $array1[$i]['M'] = $array2[$j]['M'];
            $array1[$i]['F'] = $array2[$j]['F'];
        }
    }
}

If speed is important or $array2 is large, the time complexity of your algorithm is O(n * m). I recommend hashing $array2 for fast lookups as follows (O(n)):

$lookup = array_reduce($array2, function ($a, $e) {
    $a[$e['_id']] = $e;
    return $a;
});

foreach ($array1 as $i => $e) {
    if (array_key_exists($e['_id'], $lookup)) {
        $array1[$i]['M'] = $lookup[$e['_id']]['M'];
        $array1[$i]['F'] = $lookup[$e['_id']]['F'];
    }
}

Try it!