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I have the following array. It has a parent id that correspond with the id. I manage to create a function to sort and turn this array into a tree array. My problem stands that sometimes it does not work properly if the parent is after the child.

So how would I turn an array like the one below to a tree that doesn't need to be sorted first?

    [0] => Array
        (
            [menu] => 
            [parent] => 0
            [id] => 1
        )
    ,
    [1] => Array
        (
            [menu] => 
            [parent] => 
            [id] => 2
        )
    ,
    [2] => Array
        (
            [menu] => 
            [parent] => 1
            [id] => 3
        )
    ,
    [3] => Array
        (
            [menu] => 
            [parent] => 1
            [id] => 4
        )
    ,
    [4] => Array
        (
            [menu] => 
            [parent] => 4
            [id] => 5
        )

I have this function which does not work properly:

function page_tree($rows) {
    if(!is_array($rows) || empty($rows) ){
        return false;
    }
    // $rows = array();  //stores all the database rows that are to be converted into a tree
    $tree = array();  //stores the tree
    $tree_index = array();  //an array used to quickly find nodes in the tree
    $id_column = "id";  //The column that contains the id of each node
    $parent_column = "parent";  //The column that contains the id of each node's parent
    $text_column = "title";  //The column to display when printing the tree to html
    //build the tree - this will complete in a single pass if no parents are defined after children
    // vp(count($rows) );die();
    // while(count($rows) > 0){
    foreach($rows as $row_id => $row){
        $row_id = $row['id'];
        if($row[$parent_column]){
            if((!array_key_exists($row[$parent_column], $rows)) and (!array_key_exists($row[$parent_column], $tree_index))){
               unset($rows[$row_id]);
            }
            else{
              if(array_key_exists($row[$parent_column], $tree_index)){
                $parent = & $tree_index[$row[$parent_column]];
                $parent['children'][$row_id] =$row;
                $parent['children'][$row_id]["children"] = array();
                $tree_index[$row_id] = & $parent['children'][$row_id];
                unset($rows[$row_id]);
              }
            }
        }
        else{
            $tree[$row_id] = $row;
            $tree[$row_id]["children"] = array();
            $tree_index[$row_id] = & $tree[$row_id];
            unset($rows[$row_id]);
        }
    }
    // }
    return $tree;
}

Please note: where parent is (empty) (='';) it means it's the root.

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100% accept rate
Quick question: why in your example is $rows[0][parent] == 0? – Zecc Nov 26 '10 at 11:09
because there is a db row that has an id as 0 – Val Nov 26 '10 at 11:16
And a quick suggestion: ignoring the fact you may want your code to be more robust, you can assume your flat hierarchy will come in order if in your database you use a powerful technique called the Nested Set Model. See dev.mysql.com/tech-resources/articles/hierarchical-data.html (scroll down until you see Nested Set Model) – Zecc Nov 26 '10 at 11:32
Never mind. I was assuming the indexes on the array corresponded to the ids of the nodes. – Zecc Nov 26 '10 at 11:33
@Zecc Nested Set Model adds complexity and may be overkill if you are always retrieving the full set. – John Nov 26 '10 at 11:52
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1 Answer

up vote 3 down vote accepted

The trick is to keep a kind of index (named $all below) with references to all nodes in the tree. The example below will add nodes that still need to be processed to an array named $dangling and add the final output to the $output array.

<?
// Test input
$input = array(
array( 'menu' => 'A', 'parent' => 2, 'id' => 4),
    array( 'menu' => 'B', 'parent' => 1, 'id' => 3),
    array( 'menu' => 'C', 'parent' => 2, 'id' => 1),
    array( 'menu' => 'D', 'parent' => '', 'id' => 2)
);

$output = array();
$all = array();
$dangling = array();

// Initialize arrays
foreach ($input as $entry) {
    $entry['children'] = array();
    $id = $entry['id'];

    // If this is a top-level node, add it to the output immediately
    if ($entry['parent'] == '') {
        $all[$id] = $entry;
        $output[] =& $all[$id];

    // If this isn't a top-level node, we have to process it later
    } else {
        $dangling[$id] = $entry; 
    }
}

// Process all 'dangling' nodes
while (count($dangling) > 0) {
    foreach($dangling as $entry) {
        $id = $entry['id'];
        $pid = $entry['parent'];

        // If the parent has already been added to the output, it's
        // safe to add this node too
        if (isset($all[$pid])) {
            $all[$id] = $entry;
            $all[$pid]['children'][] =& $all[$id]; 
            unset($dangling[$entry['id']]);
        }
    }
}

print_r($output);

Note that this will go horribly wrong if your input data is incorrect (e.g. an item with an invalid value for parent will cause an infinite loop).

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thanks john.. . this sounds good however I have hit that infinite loop thing :( – Val Nov 26 '10 at 11:59
found the problem it seems that parent id 0 (zero) should should be (null) so it's top level(root) node. – Val Nov 26 '10 at 12:09
I have also tested it re sorting in different ways and it still works nicely so thnx again :) – Val Nov 26 '10 at 12:09
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