title: How can I find how many useful digits are in any given a number N
body: <p>A digit in the number is useful if the number is divisible by that digit.</p>
  
  <p>I have been working on this for 2 days now.</p>
  
  <p>Here is what I have:</p>
  
  <pre><code>func selfDivide(integer: Int) -&gt; Int {
      var numString = String(integer)
      for character in numString.characters {
          if character % numString == 0 {
              return character
          }
      }
  }
  </code></pre>
  
  <p>I'm thinking I have to find a way to use % between that string and character.</p>
  
  <p>The error that I get is:</p>
  
  <p>Binary operator '%' cannot be applied to characters of type 'Character' and 'String'</p>