Convert flat array to the multi-dimensional

Question

I have an array with tree data (by parent id). I want to convert it to multidimensional array. What is the best way to achieve that? Is there any short function for that?

Answer 1

I was looking for an example of how to do this, with categories. This example assumes that parents will always have a parent id of '0'. The example is using ZF2.

No references, or recursion. The trick is in the output, you look for the [0] index, and for the children, you specify the parent_id as the index.

$categoryLookup = $this->getCategoryLookup($associateById=true);

if ($assignedCategories) {          
    $categoryHeirarchy = array();
    foreach($assignedCategories as $assignedCategory) {
        $child = $categoryLookup[$assignedCategory->category_id];
        $parent = $categoryLookup[$child->parent_id];               
        $categoryHeirarchy[$child->parent_id][] = $categoryLookup[$child->category_id];
        $categoryHeirarchy[$parent->parent_id][$parent->category_id] = $categoryLookup[$parent->category_id];
    }           

    return $categoryHeirarchy;  
}


<h3>Categories</h3>
<dl class="dl-horizontal">
    <?php foreach($this->categoryHeirarchy[0] as $parent): ?>
        <dt><?php echo $this->escapeHtml($parent->name); ?></dt>
        <?php foreach($this->categoryHeirarchy[$parent->category_id] as $child): ?>
            <dd><?php echo $this->escapeHtml($child->name); ?></dd>
        <?php endforeach; ?>
    <?php endforeach; ?>                    
</dl>

Answer 2

I wrote this variant considering root parent_id is 0 or missing. No matter children after parents in DB ($source) or not.

$source_by_id = array();
foreach ($source as &$row){
  $source_by_id[$row['id']] = &$row;
}
foreach ($source_by_id as $id => &$row){
  $source_by_id[ intval($row['parent_id']) ]['children'][$id] = &$row;
}
// remove cycling itself
unset($source_by_id[0]['children'][0]);

$result = $source_by_id[0]['children'];

Result array keys are appropriate ids. Enjoy!

Answer 3

I don't think there is a built-in function in PHP that does this.

I tried the following code, and it seems to work to prepare the nested array the way you describe:

$nodes = array();
$tree = array();
foreach ($source as &$node) {
  $node["Children"] = array();
  $id = $node["Menu"]["id"];
  $parent_id = $node["Menu"]["parent_id"];
  $nodes[$id] =& $node;
  if (array_key_exists($parent_id, $nodes)) {
    $nodes[$parent_id]["Children"][] =& $node;
  } else {
    $tree[] =& $node;
  }
}

var_dump($tree);

I wrote a similar algorithm in a PHP class I wrote for my presentation Hierarchical Models in SQL and PHP, but I was using objects instead of plain arrays.