Finding the children of the sub-sub-category from a sub-category

Question

I currently have a code snippet where for each category, it would find the sub-categories:

   $categories = array_map(
        function($child)
        {
            $child['children'] =
                $this->getChildren(
                    $child['id'],
                    !empty($this->request->get['language_id']) ?
                        $this->request->get['language_id'] : 1
                );
            return $child;
        }, $categories);

getChildren() would recursively get the children of one category:

private function getChildren($parent_id, $language_id) {
    $this->load->model('official/category');

    $children = 
        $this->model_official_category->getCategoriesByParentId(
            $parent_id,
            $language_id
        );

    // For each child, find the children.
    foreach ($children as $child) {
        $child['children'] = $this->getChildren(
            $child['id'],
            $language_id
        );
    }

    return $children;
}

Currently, using my lambda function within the array_map(), only the sub-category's children would be retrieve, so if each sub-category has its own sub-sub-category, it would not be saved into its children.

How could I show the sub-sub-category given the sub-category we have?

What I wanted to do with my code was to take a parent, get its children, and then treat each of those children as a parent and get its children recursively, however my JSON output does not reflect that. Only the parent has children - the children has no children (despite my database having them).

score 1 · Accepted Answer · edited May 23 '17 at 12:30

The problem is that your recursion foreach loop assigns the children that it retrieves to a copy of the child data, rather than the child data itself.

To resolve this you could use foreach loop that references the child data, like so:

foreach ($children as &$child) {

However, due to a number of reasons related to how foreach is implemented internally in PHP (more info if you're interested), it would be considerably more memory efficient to use a for loop instead, as this will avoid quite a few copy-on-write copies of the child data:

for ($i = 0; isset($children[$i]); $i++) {
    $children[$i]['children'] = $this->getChildren(
        $children[$i]['id'],
        $language_id
    );
}

This is one place where using objects instead of arrays to represent the child data might be a good idea, because objects are always passed by reference (kind of) and the behaviour would be more like what you were expecting initially.

Perfect answer. Thank you. Do you suggest using for-loops instead of foreach in all situations? Coming from Java, for-loops seem automatically more intuitive and readable for me. — theGreenCabbage, Nov 03 '14 at 09:40
@theGreenCabbage in all situations where you are looping over an array (not an object) and where you are modifying the array in the loop, yes. — DaveRandom, Nov 03 '14 at 09:42

Finding the children of the sub-sub-category from a sub-category

1 Answers1