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Is there a way to mutate an array using array_reduce in PHP?

I'm trying to do something like this:

Given some ordered list of ids:

$array = [["id" => 1], ["id" => 13], ["id" => 4]];

And a tree that has a subtree matching the corresponding ids:

$tree = [
  "id" => 2334,
  "children" => [
    [
      "id" => 111,
      "children" => []
    ],
    [
      "id" => 1, // <- this is a match
      "children" => [
        [
          "id" => 13, // <- this is a match
          "children" => [
            [
              "id" => 4, // <- this is a match
              "children" => []
            ],
            [
              "id" => 225893,
              "children" => []
            ],
            [
              "id" => 225902,
              "children" => []
            ]
          ]
        ]
      ]
    ]
  ]
];

How can I mutate the arrays in that subtree?

I'm currently trying to use array_reduce to walk down the tree and mutate it. However, the mutation isn't being applied to the originally passed in $tree.

array_reduce($array, function (&$acc, $item) {
  $index = array_search($item['id'], array_column($acc['children'], 'id'));
  $acc['children'][$index]['mutated'] = true; // mutation here
  return $acc['children'][$index];
}, $tree);

echo "<pre>";
var_dump($tree); // $tree is unchanged here
echo "</pre>";

Why is $tree not mutated after the running above array_reduce?

Is there a way to use foreach in this case?

Improve this question
4
  • 1
    No, my bad, I misunderstood what you were doing with the return. Commented May 24, 2019 at 0:19
  • @Nick, yeah. My confusion is that I am using &$acc in the callback and then am mutating $acc, but those mutations aren't visible in $tree after array_reduce runs. Commented May 24, 2019 at 0:22
  • The reason $tree isn't being changed is that the $initial value isn't passed by reference (it can't be, since it is allowed to be a literal value e.g. 0 and defaults to NULL) Commented May 24, 2019 at 0:27
  • @Nick Ok that makes sense. Would you recommend another strategy? foreach? Perhaps another array function? Commented May 24, 2019 at 0:30

1 Answer 1

Reset to default
1

I think this function will do what you want. It recurses down $tree, looking for id values that are in $array and setting the mutation flag for those children:

function mutate(&$tree, $array) {
    if (in_array($tree['id'], array_column($array, 'id'))) {
        $tree['mutated'] = true;
    }
    foreach ($tree['children'] as &$child) {
        mutate($child, $array);
    }
}
mutate($tree, $array);
var_export($tree);

Output:

array (
  'id' => 2334,
  'children' => array (
    0 => array (
      'id' => 111,
      'children' => array ( ),
    ),
    1 => array (
      'id' => 1,
      'children' => array (
        0 => array (
          'id' => 13,
          'children' => array (
            0 => array (
              'id' => 4,
              'children' => array ( ),
              'mutated' => true,
            ),
            1 => array (
              'id' => 225893,
              'children' => array ( ),
            ),
            2 => array (
              'id' => 225902,
              'children' => array ( ),
            ),
          ),
          'mutated' => true,
        ),
      ),
      'mutated' => true,
    ), 
  ), 
)

Demo on 3v4l.org

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