title: Create empty dictionary
body: <p>I have a dictionary initialized</p>
  
  <pre><code>var dictionary = [String: [Double]]()
  </code></pre>
  
  <p>And I want to append</p>
  
  <pre><code>dictionary["Hello"].append(0.0)
  </code></pre>
  
  <p>but this gives me error "nil".</p>
  
  <p>I tried to solve this by</p>
  
  <pre><code>extension Dictionary { 
  
      func appendish(key: String, value: Double) {
  
          if self[key] == nil {
  
              this give me error "Ambiguous reference to member 'subscript'"
          }
      } 
  } 
  </code></pre>
  
  <p>How do I solve this? Been stuck at this for hours.</p>