Create nested list from Multidimensional Array

Question

I have an array in PHP, which looks like this:

array (
    [0] => array (
        [id] => 1
        [title] => "Title 1"
        [parent_id] => NULL
        [depth] => 0
    )
    [1] => array (
        [id] => 2
        [title] => "Title 2"
        [parent_id] => NULL
        [depth] => 0
    )
    [2] => array (
        [id] => 3
        [title] => "Title 3"
        [parent_id] => 2
        [depth] => 1
    )
    [3] => array (
        [id] => 4
        [title] => "Title 4"
        [parent_id] => 2
        [depth] => 1
    )
    [4] => array (
        [id] => 5
        [title] => "Title 5"
        [parent_id] => NULL
        [depth] => 0
    )
    [5] => array (
        [id] => 6
        [title] => "Title 6"
        [parent_id] => 4
        [depth] => 2
    )
)

What i want to do is iterate over this array and create a nested <ol> list from it. So the result should look like this:

<ol>
    <li>Title 1</li> // id = 1
    <li>Title 2</li> // id = 2
    <ol>
        <li>Title 3</li> // id = 3 -> parent_id = 2
        <li>Title 4</li> // id = 4 -> parent_id = 2
        <ol>
            <li>Title 6</li> // id = 6 -> parent_id = 4
        </ol>
    </ol>
    <li>Title 5</li> // id = 5
</ol>

I've been trying to think of a way how i could get this done. But so far every attempt failed...

Anyone any idea how i can create such a nested <ol> list from an array like that?

Please note that i do not have any control on the given data. I simply make a call to an API and it returns json data, which i convert to an array. And the array looks exactly like the one i described.

On the surface it looks simple, but I think you're still lacking information in order to get the best answer for your situation. First, how did you get the array? Is it from a database? Would it be possible to store it differently or are you stuck in that way? Is the depth guaranteed to not (now or in the future) be more than 1 or 2 or will you ever plan on doing an indeterminate number of sub-levels? How often will the categories be added or deleted? I would suggest you check out http://mikehillyer.com/articles/managing-hierarchical-data-in-mysql/ as a nested model may be more what you need. — Mike, Oct 07 '12 at 19:04

dani herrera · Accepted Answer · 2012-10-07T20:50:29.140

You should use recursion:

First the array in 'php' syntax:

<?php
$a=array (
    '0' => array (
        'id' => 1,
        'title' => "Title 1",
        'parent_id' => 'NULL',
        'depth' => 0
    ),
    '1' => array (
        'id' => 2,
        'title' => "Title 2",
        'parent_id' => 'NULL',
        'depth' => 0
    ),
    '2' => array (
        'id' => 3,
        'title' => "Title 3",
        'parent_id' => 2,
        'depth' => 1
    ),
    '3' => array (
        'id' => 4,
        'title' => "Title 4",
        'parent_id' => 2,
        'depth' => 1
    ),
    '4' => array (
        'id' => 5,
        'title' => "Title 5",
        'parent_id' => 'NULL',
        'depth' => 0
    ),
    '5' => array (
        'id' => 6,
        'title' => "Title 6",
        'parent_id' => 4,
        'depth' => 0
    )
);

Here the code:

$level = 'NULL';

function r( $a, $level) {
   $r = "<ol>";
   foreach ( $a as $i ) {
       if ($i['parent_id'] == $level ) {
          $r = $r . "<li>" . $i['title'] . r( $a, $i['id'] ) . "</li>";
       }
   }
   $r = $r . "</ol>";
   return $r;
}

print r( $a, $level );

?>

The results:

<ol><li>Title 1<ol></ol></li><li>Title 2<ol><li>Title 3<ol>
</ol></li><li>Title 4<ol><li>Title 6<ol></ol></li></ol></li></ol></li><li>Title 5
<ol></ol></li></ol>

Title 1\n
Title 2\n
1. Title 3\n
2. Title 4\n
  1. Title 6\n
Title 5\n

EDITED AFTER CHECK AS SOLUTION

To avoid empty leafs:

function r( $a, $level) {
   $r = '' ;
   foreach ( $a as $i ) {
       if ($i['parent_id'] == $level ) {
          $r = $r . "<li>" . $i['title'] . r( $a, $i['id'] ) . "</li>";
       }
   }
   return ($r==''?'':"<ol>". $r . "</ol>");
}

Baba · Answer 2 · 2012-10-07T19:26:06.747

You can try the following

$array = array (
    "0" => array (
        "id" => 1,
        "title" => "Title 1",
        "parent_id" => NULL,
        "depth" => 0
    ),
    "1" => array (
        "id" => 2,
        "title" => "Title 2",
        "parent_id" => NULL,
        "depth" => 0
    ),
    "2" => array (
        "id" => 3,
        "title" => "Title 3",
        "parent_id" => 2,
        "depth" => 1
    ),
    "3" => array (
        "id" => 4,
        "title" => "Title 4",
        "parent_id" => 2,
        "depth" => 1
    ),
    "4" => array (
        "id" => 5,
        "title" => "Title 5",
        "parent_id" => NULL,
        "depth" => 0
    ),
    "5" => array (
        "id" => 6,
        "title" => "Title 6",
        "parent_id" => 4,
        "depth" => 0
    )
);

echo(make($array));

Output

<ol>
    <li>Title 1</li>
    <li>Title 2</li>
    <ol>
        <li>Title 3</li>
        <li>Title 4</li>
        <ol>
            <li>Title 6</li>
        </ol>
    </ol>
    <li>Title 5</li>
</ol>

Function Used

function make(array $array, $no = 0) {
    $child = hasChildren($array, $no);
    if (empty($child))
        return "";
    $content = "<ol>\n";
    foreach ( $child as $value ) {
        $content .= sprintf("\t<li>%s</li>\n", $value['title']);
        $content .= make($array, $value['id']);
    }
    $content .= "</ol>\n";
    return $content;
}

function hasChildren($array, $id) {
    return array_filter($array, function ($var) use($id) {
        return $var['parent_id'] == $id;
    });
}

See Live Demo

great answer thank you. Can you tell me how we can set max depth assuming that array elements has no `depth` key (as I haven't got).. We need a `$level = 0;` and `$max_level = 3` I suppose. But how to place them in your function? — , Oct 08 '12 at 17:25
@Baba, He makes a nice spot. Hope you'd have to time to review this. — , Oct 08 '12 at 17:28

Carlos Arturo Alaniz · Answer 3 · 2014-07-21T20:56:26.073

The following array:

Array
(
    [0] => Content
    [1] => Array
        (
            [0] => International
            [1] => Array
                (
                    [0] => Mexico
                    [1] => Array
                        (
                            [0] => Tamaulipas
                        )

                    [2] => USA
                )

        )

)

With this function:

function r($element) {
    foreach ($element as $value) {
        if (!is_array($value)) {
            echo "<li>";
            echo $value;
        } else {
            echo "<ul>";
            r($value);
            echo "</li>";
            echo "</ul>";
        }
    }
}

PHP code:

echo "<ul>";
r($array);
echo "</ul>";

Returns:

<ul>
    <li>Public</li>
    <li>User</li>
    <li>Content
        <ul>
            <li>International
                <ul>
                    <li>Mexico
                        <ul>
                            <li>Tamaulipas</li>
                        </ul>  
                    </li>
                    <li>USA</li>
                </ul>
            </li>
        </ul>
    </li>
</ul>

Create nested list from Multidimensional Array

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