title: Swift - How to convert String to Double
body: <p>I'm trying to write a BMI program in swift language.
  And I got this problem: how to convert a String to a Double?</p>
  
  <p>In Objective-C, I can do like this:</p>
  
  <p><code>double myDouble = [myString doubleValue];</code></p>
  
  <p>But how can I achieve this in Swift language?</p>