Get category id from child category id. where is mistake?

Hi everyone, i want to get category id, i got child category id from post id already. My code is not working, please help find my mistake.

if (is_single()){
    $categories = get_the_category($post->ID);
    foreach($categories as $category) {
    $cat_id = $category->cat_ID;
    $parent = $cat_id->category_parent;
    echo $cat_id;
    echo $parent;