PHP Non-Recursive Unlimited Level Category Tree

Question

I've searched about how to make category tree and always found with using recursive. But I want to know how to make a list of category tree without using recursive, Is it possible?

Data:

$arrItems = array(
    array('id' => 1, 'parent_id' => 0),
    array('id' => 2, 'parent_id' => 1),
    array('id' => 3,  'parent_id' => 2),
    ...
);

Output:

<ul>
    <li> 1 
        <ul>
            <li> 3 </li>
            <li> 5 
                <ul>
                    <li> 7 </li>
                    ...
                </ul>
            </li>
        </ul>
    </li>
    <li> 2 
        ...
    </li>
</ul>

Thanks in advance.

Yes it is possible.. You can create your own stack and achieve that. — mighty_dev
– mighty_dev, Commented Nov 29, 2018 at 20:24

trincot · Accepted Answer · 2018-11-30 23:35:12Z

2

You can do it by converting the parent relationships to child relationships. For that you need one iteration. You could use this function for it:

function toTree($arrItems) {
    $children = [];
    foreach($arrItems as $item) $children[$item["parent_id"]][] = $item["id"];
    return $children;
}

Then, once you have that data structure, you can convert it to your HTML structure with the following function:

function toHTML(&$children, $parent=0, $indent="") {
    if (!isset($children[$parent])) return ""; 
    return "$indent<ul>\n" . implode("", array_map(function($id) use ($children, $indent) {
        return "$indent  <li>$id\n" . toHTML($children, $id, "$indent    ") . "$indent  </li>\n";
    }, $children[$parent])) . "$indent</ul>\n";
}

Example use:

$arrItems = [
    ['id' => 1, 'parent_id' => 0],
    ['id' => 2, 'parent_id' => 1],
    ['id' => 3,  'parent_id' => 2],
    ['id' => 4,  'parent_id' => 1],
];

echo toHTML(toTree($arrItems));

The above outputs:

<ul>
  <li>1
    <ul>
      <li>2
        <ul>
          <li>3
          </li>
        </ul>
      </li>
      <li>4
      </li>
    </ul>
  </li>
</ul>

answered Nov 30, 2018 at 23:35

trincot

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3 Comments

Vintage Beef Over a year ago

Thank you so much. But can you explain how's "&$variable" works?

trincot Over a year ago

That is to make it a reference parameter. In this case it would also work without &, but then PHP copies the array that is passed as argument into the (new) function parameter variable. While with & the parameter variable really is a reference to the array that is passed as argument -- so without making a copy. See Passing by Reference

Vintage Beef Over a year ago

One more question, Is it possible to make it without using a reference parameter?

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