Two Arrays, One Output (How to ForEach?)

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 n开发者_如何学Pythonot sure what to ask for precisely. Any help??

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.

I would urge you to redesign your array when you fill them with data to instead look something like this.

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

Then you would have all the relational data connected and would just have to loop through your array of categories and print the images associated with each one in turn. The numbered indexes could even be changed to associative names if the id of the catagory weren't important for example. Then the array could be indexed with the name of the category and just contain the images of that category.

If the images are to be used in other places where you initial layout of those fits better you could still use this layout for your main data graph. Just replace the actual data of the images in the images array under each category with a reference to the actual image object.