Cannot use mutating member on immutable value of type [String]
<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>