PHP-Laravel foreach only loops once [duplicate]

Question

I have a categories table with parent_id column, referring to parent category's ID. I need to retrieve all of the ID's that are under a certain category (all of its children & grandchildren) I wrote this recursive function but for some reason it only returns the direct children of the given category id.

What am I missing here?

function subCategoriesList($id, $data=[]){
    $children = Category::whereParentId($id)->get();
    foreach ($children as $child){
        $data[] = $child->id;
        subCategoriesList($child->id, $data);
    }
    return $data;
}

Also covered by "Recursive function in PHP function : how to prevent return value?", "struggling to walk through php array element in recursive function", "rebuilding a tree as flat array", "Algorithm to find amount of levels in an array of arrays" — outis
– outis, Commented Oct 9, 2022 at 21:21

Sammitch · Accepted Answer · 2022-10-07 23:35:19Z

1

You're not passing $data by reference, but you're making the recursive call as if you are.

Try:

function subCategoriesList($id){
    $data=[]
    $children = Category::whereParentId($id)->get();
    foreach ($children as $child){
        $data[] = $child->id;
        $data = array_merge($data, subCategoriesList($child->id));
    }
    return $data;
}

Conversely, and for completeness, with references:

function subCategoriesList($id, &$data){
    $children = Category::whereParentId($id)->get();
    foreach ($children as $child){
        $data[] = $child->id;
        subCategoriesList($child->id, $data);
    }
}

// You must call it as follows, though
$my_data = [];
subCategoriesList($some_id, $my_data);

Personally, though, I tend to recommend that references be avoided unless there is a clear reason for them. IMO they tend to result in confusing code and side-effects, while rarely delivering the benefits that the writer thinks they are getting.

edited Oct 7, 2022 at 23:35

answered Oct 7, 2022 at 23:24

Sammitch

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3 Comments

Kemal Özvarol Over a year ago

I didn't know I had to do that! Thanks so much! It's returning same child category 6 times but nothing I can't handle 😂 there was an array_unique function for that I believe.

Sammitch Over a year ago

Well unless there's a subcategory that has 6 parents, there's probably a logic issue in there to be resolved. I didn't have any usable test data to actually run while I wrote this.

Sammitch Over a year ago

Oh I think we need to get rid of $data from the parameter list. There's no need for it to be there in the non-reference version.

Rick Muller · Accepted Answer · 2022-10-07 23:28:58Z

1

You're not doing anything with the returndata when you call subCategoriesList within the function.

subCategoriesList($child->id, $data);

should be

$data = array_merge($data, subCategoriesList($child->id, $data));

edited Oct 7, 2022 at 23:28

answered Oct 7, 2022 at 23:25

Rick Muller

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