Recursion and passing by reference

Question

I have a tree of categories of the following structure:

[6] => Array
    (
        [id] => 6
        [name] => computers
        [productCount] => 0
        [children] => Array
            (
                [91] => Array
                    (
                        [id] => 91
                        [name] => notebook
                        [productCount] => 5
                        [children] => Array
                            (
                            )
                    )

                [86] => Array
                    (
                        [id] => 86
                        [name] => desktop
                        [productCount] => 0
                        [children] => Array
                            (
                            )
                    )
            )
    )

Beside a subcategory, each category may contain products (like a folder may contain subfolders and just files).

I'm trying to write a recursive function which I want to take this array as reference and strip both leaf categories with [productCount] = 0 and all parent categories that contain such empty nodes. In other words, after processing I want to have only those categories that hold products on any sublevels.

I've wrote some code, now debugging it and it doesn't strip empty nodes. May be I'm not using references properly. Please, help me fix it, if possible.

    function pruneTree( & $node) {
    if ( ! $node['children'] && ! $node['productCount']) {
        unset($node);
    }
    if ( ! empty($node['children'])) {
        foreach ($node['children'] as $key => $child) {
            pruneTree($node['children'][$key]);
        }
    }
    return;
}

Answer 1

You could also change the parameter in the function to take an array of nodes instead of a single node. This changes the recursion slightly, and prevents the need to pass along a key:

function pruneTree(&$nodes) {
    foreach ($nodes as $key => $node) {
        if (!$node['children'] && !$node['productCount']) {
            unset($nodes[$key]);
        } elseif (!empty($node['children'])) {
            pruneTree($nodes[$key]['children']);
            // This line checks if all the children have been pruned away:
            if (empty($nodes[$key]['children'])) {
                unset($nodes[$key]);
            }
        }
    }
}

Also, added a check that ensures that if all child nodes are pruned, the parent (now, leaf) node also gets pruned.

Hope this helps!

---

Test data:

$data = array(
    6 => array(
        'id' => 6,
        'name' => 'computers',
        'productCount' => 0,
        'children' => array(
            91 => array(
                'id' => 91,
                'name' => 'notebook',
                'productCount' => 5,
                'children' => array()
            ),
            86 => array(
                'id' => 86,
                'name' => 'desktop',
                'productCount' => 0,
                'children' => array()
            )
        )
    )
);

The Call:

pruneTree($data);
echo '<pre>';
print_r($data);
echo '</pre>';

Answer 2

unset deletes only the reference but not the referenced variable:

If a variable that is PASSED BY REFERENCE is unset() inside of a function, only the local variable is destroyed. The variable in the calling environment will retain the same value as before unset() was called.

So you need to pass the parent array and the key to delete that variable:

function pruneTree(&$parent, $key) {
    $node = &$parent[$key];
    if (!$node['children'] && !$node['productCount']) {
        unset($parent[$key]);
    }
    if (!empty($node['children'])) {
        foreach ($node['children'] as $key => &$child) {
            pruneTree($node['children'], $key);
        }
    }
}