Question

So far, if I have to loop through a multidimensional array, I use a foreach loop for each dimension.

e.g for two dimensions

foreach($array as $key=>$value)
{
    foreach($value as $k2=>$v2)
    {
         echo
    }
}

What do I do when I don’t know the depth of the array? ie the depth is variable.

The only thing I can think of is to code a whole stack of loops and to break the loop if the next value is not an array.This seems a little silly.

Is there a better way?

Answer 1

Yes, you can use recursion. Here’s an example where you output all the elements in an array:

function printAll($a) {
  if (!is_array($a)) {
    echo $a, ' ';
    return;
  }

  foreach($a as $v) {
    printAll($v);
  }
}

$array = array('hello',
               array('world',
                     '!',
                     array('whats'),
                     'up'),
               array('?'));
printAll($array);

What you should always remember when doing recursion is that you need a base case where you won’t go any deeper.

I like to check for the base case before continuing the function. That’s a common idiom, but is not strictly necessary. You can just as well check in the foreach loop if you should output or do a recursive call, but I often find the code to be harder to maintain that way.

The “distance” between your current input and the base case is called a variant and is an integer. The variant should be strictly decreasing in every recursive call. The variant in the previous example is the depth of $a. If you don’t think about the variant you risk ending up with infinite recursions and eventually the script will die due to a stack overflow. It’s not uncommon to document exactly what the variant is in a comment before recursive functions.