Question

Is there a way to foreach() through one array based on a matching value with a different key in another array? In this example, I have a category array ($cat_data) with cat_id as a key and an image array ($img_data) with category_id as a key.

Array (
   [0] => Array (
        [cat_id] => 1
        [cat_name] => Category 1
   )
   [1] => Array (
        [cat_id] => 2
        [cat_name] => Category 2
   )
)
Array (
   [0] => Array (
        [img_id] => 2
        [img_name] => demo1.jpg
        [img_label] => Demo 1
        [category_id] => 2
        [img_order] => 1
   )
   [1] => Array (
        [img_id] => 3
        [img_name] => demo2.jpg
        [img_label] => Demo 2
        [category_id] => 2
        [img_order] => 2
   )
   [2] => Array (
        [img_id] => 4
        [img_name] => demo3.jpg
        [img_label] => Demo 3
        [category_id] => 1
        [img_order] => 1
   )
)

What I want is to output my display so it looks like the following:

Category 1
  demo3.jpg

Category 2
  demo1.jpg
  demo2.jpg

Since I’m really not great at fully grasping arrays, I thought I’d try Stack, and I haven’t been able to find an answer to my question, partially because I’m not sure what to ask for precisely. Any help??

Answer 1

The naïve way:

foreach ($cat_data as $cat) {
    echo $cat['cat_name'];

    foreach ($img_data as $img) {
        if ($img['category_id'] != $cat['cat_id']) {
            continue;
        }

        echo $img['img_name'];
    }
}

This is rather inefficient, since it loops through the $imgs array several times, but easy and works.

More efficient:

$images = array();
foreach ($img_data as $img) {
    $images[$img['category_id']][] = $img;
}

foreach ($cat_data as $cat) {
    echo $cat['cat_name'];

    if (isset($images[$cat['cat_id']])) {
        foreach ($images[$cat['cat_id']] as $img) {
            echo $img['img_name'];
        }
    }
}

This first groups all images by category into a new array, which you can then loop over directly once.