April 4, 2012
How to recursively build a <select> with unknown tree depth
Question by simone
I have a MySQL table with a tree data structure. The fields are _id, name and parentId. When the record hasn’t a parent, parentId defaults as 0. This way I can build an array and then recursively print each record.
The builded array looks like this:
Array
(
[1] => Array
(
[parentId] => 0
[name] => Countries
[_id] => 1
[children] => Array
(
[2] => Array
(
[parentId] => 1
[name] => America
[_id] => 2
[children] => Array
(
[3] => Array
(
[parentId] => 2
[name] => Canada
[_id] => 3
[children] => Array
(
[4] => Array
(
[parentId] => 3
[name] => Ottawa
[_id] => 4
)
)
)
)
)
[5] => Array
(
[parentId] => 1
[name] => Asia
[_id] => 5
)
[6] => Array
(
[parentId] => 1
[name] => Europe
[_id] => 6
[children] => Array
(
[7] => Array
(
[parentId] => 6
[name] => Italy
[_id] => 7
)
[11] => Array
(
[parentId] => 6
[name] => Germany
[_id] => 11
)
[12] => Array
(
[parentId] => 6
[name] => France
[_id] => 12
)
)
)
[8] => Array
(
[parentId] => 1
[name] => Oceania
[_id] => 8
)
)
)
)
Printing an unordered list <ul> is very simple with recursion. Here’s the function I use:
function toUL ($arr) {
$html = '<ul>' . PHP_EOL;
foreach ( $arr as $v ) {
$html.= '<li>' . $v['name'] . '</li>' . PHP_EOL;
if ( array_key_exists('children', $v) ) {
$html.= toUL($v['children']);
}
}
$html.= '</ul>' . PHP_EOL;
return $html;
}
But I’m stuck at printing a <select> in a tree-structured way:
Countries
-- America
---- Canada
------ Ottawa
-- Asia
-- Europe
---- Italy
---- Germany
---- France
-- Oceania
I thought to print -- as many times as the element’s depth, but I don’t know how to calculate the depth.
My question is: is it possible to build a <select> without knowing the depth?
Thank you in advance.
Answer by Starx
Pass a parameter to count the iteration like $pass
function toUL ($arr, $pass = 0) {
$html = '<ul>' . PHP_EOL;
foreach ( $arr as $v ) {
$html.= '<li>';
$html .= str_repeat("--", $pass); // use the $pass value to create the --
$html .= $v['name'] . '</li>' . PHP_EOL;
if ( array_key_exists('children', $v) ) {
$html.= toUL($v['children'], $pass+1);
}
}
$html.= '</ul>' . PHP_EOL;
return $html;
}
