recursive tree traversal - How to keep track of the recursion level?

Question

I'm basically trying to build an html ul/li nested list from a multidimensional array representing a tree structure.

The following code works fine but I want to improve it:

I need a way to keep track of the recursion level so I can apply different classes to different levels, add indenting to the generated output, etc.

function buildTree($tree_array, $display_field, $children_field, $class='', $id='') {

  echo "<ul>\n";

  foreach ($tree_array as $row) {

    echo "<li>\n";
    echo $row[$display_field] . "\n";

    if (isset($row[$children_field])) {

      $this->buildTree($row[$children_field]);
    }
    echo "</li>\n";
  }
  echo "</ul>\n";
} 

The $tree_array looks like this:

Array
(
    [0] => Array
        (
            [category_id] => 1
            [category_name] => calculatoare
            [parent_id] => 0
            [children] => Array
                (
                    [0] => Array
                        (
                            [category_id] => 4
                            [category_name] => placi de baza
                            [parent_id] => 1
                        )

                    [1] => Array
                        (
                            [category_id] => 5
                            [category_name] => carcase
                            [parent_id] => 1
                            [children] => Array
                                (
                                    [0] => Array
                                        (
                                            [category_id] => 6
                                            [category_name] => midi-tower
                                            [parent_id] => 5
                                        )

                                )

                        )

                )

        )

    [1] => Array
        (
            [category_id] => 2
            [category_name] => electronice
            [parent_id] => 0
        )

    [2] => Array
        (
            [category_id] => 3
            [category_name] => carti
            [parent_id] => 0
        )

)

I've tagged this as homework because I'd like to use this as an opportunity to improve on my (poor) understanding of recursion so, I'd appreciate answers that would guide me to the solution rather than provide a complete working example :)

Answer 1

Quick'n'dirty approach (see "spoiler" block below for the implementation):

Add an additional variable $recursionDepth to your function declaration, make it 0 by default.

On each subsequent recursion, call your function with $recursionDepth + 1.

Since the function variables are only "visible" (scoped) for the respective instance of the function, you'll end up with an indicator of the current iteration depth.

Also, line 12 of your function

$this->buildTree();

doesn't look to me as if it would work – the reason being that you're not passing your variables on to the next instance of buildTree.

It probably should look like this:

$this->buildTree($row[$children_field], $display_field, $children_field, $class, $id)

Here's the changes I'd make to your code to achieve what you want:

function buildTree($tree_array, $display_field, $children_field, $class='', $id='', $recursionDepth = 0, $maxDepth = false)
{
    if ($maxDepth && ($recursionDepth == $maxDepth)) return;

    echo "<ul>\n";

    foreach ($tree_array as $row)
    {
        echo "<li>\n";
        echo $row[$display_field] . "\n";

        if (isset($row[$children_field]))
            $this->buildTree($row[$children_field], $display_field, $children_field, $class, $id, $recursionDepth + 1, $maxDepth);

        echo "</li>\n";
    }
    echo "</ul>\n";
}