title: How to find index of item in contract mapping
body: <p>I have a mapping defined in my ethereum contract as follows</p>
  
  <pre><code>mapping (string =&gt; string) client;
  </code></pre>
  
  <p>I have a function that is trying to add clients to this mapping but before i add i wanna check if item already exists. Is there a way i can get index of an item in that either by a out of the box method which is like "get me the index of item in this mapping if value is this" or maybe iterate through the mapping inside the contract</p>
  
  <pre><code>function AddClient(string clientName) {
  
  }
  </code></pre>