title: Cannot assign value of type 'Double' to type 'String?'
body: <p>I am very new to swift. Whenever i try to run my code I get this error. </p>
  
  <blockquote>
    <p>Cannot assign value of type 'Double' to type 'String?'</p>
  </blockquote>
  
  <p>How do I go about fixing it? Thanks in advance</p>
  
  <pre><code>func getWaterVolumeLabelText() -&gt; Double {
     let dnv: Double = dNTPsVolumeLabel.text?.doubleValue() ?? 0
          let fp: Double = forwardPrimerLabel.text?.doubleValue() ?? 0
          let rp: Double = reversePrimerLabel.text?.doubleValue() ?? 0
          let dp: Double = dnaPolymeraseLabel.text?.doubleValue() ?? 0
        //  let votd: Double = volumeOfTemplateDna.text.doubleValue() ?? 0
          let bv: Double = bufferVolumeLabel.text?.doubleValue() ?? 0
  
  
          waterVolumeLabel.text = (numberOfSamples.doubleValue() ) * (50.0 - dnv )// This is where the error shows.
          }
  </code></pre>