title: Cannot use mutating member on immutable value of type [String]
body: <p>I failed to append String by using following code:</p>
  
  <pre><code>(labGroups[labGroupIndex!].labs[labIndex!].tstDspGps[tstDspGpsIndex!].tests[testsIndex!]["specialReqValues"] as![String]).append("sdfsdf")
  </code></pre>
  
  <p>Error:</p>
  
  <blockquote>
    <p>Cannot use mutating member on immutable value of type
    '[String]'</p>
  </blockquote>
  
  <p>And I can append value by using following code:</p>
  
  <pre><code>var arr = labGroups[labGroupIndex!].labs[labIndex!].tstDspGps[tstDspGpsIndex!].tests[testsIndex!]["specialReqValues"] as![String]    
  arr.append("testValue")
  </code></pre>
  
  <p>However, it only affect the variable <code>arr</code>. </p>
  
  <p>That is not what I want, when I print:</p>
  
  <pre><code>((labGroups[labGroupIndex!].labs[labIndex!].tstDspGps[tstDspGpsIndex!].tests[testsIndex!]["specialReqValues"] as![String])) 
  </code></pre>
  
  <p>it still has nil value.</p>
  
  <p>Can anyone help to solve this problem?</p>