cannot use mutating member on immutable value: 'XXX' is a 'let' constant
<p>i am new to swift .i am  trying  with several way but i failed when try as suggest error then anther error occur . what is going on exactly here i don't know </p>

<pre><code>  public struct ThermometerStruct {
          var temperature: Double = 0.0
      public mutating func registerTemperature(temperature: Double) {
        self.temperature = temperature
      }
    }

    let thermometerStruct = ThermometerStruct()
    thermometerStruct.registerTemperature(temperature : 56.0)
</code></pre>

<blockquote>
  <p>ERROR at line 14, col 5: cannot use mutating member on immutable value: 'thermometerStruct' is a 'let' constant
      thermometerStruct.registerTemperature(temperature : 56.0)
      ^~~~~~~~~~~~~~~~~
  <br>
  <br>
  INFO at line 13, col 5: change 'let' to 'var' to make it mutable
      let thermometerStruct = ThermometerStruct()
      ^~~
      var</p>
</blockquote>