title: Scala find missing values in a range
body: <p>For a given range, for instance</p>
  
  <pre><code>val range = (1 to 5).toArray
  val ready = Array(2,4)
  </code></pre>
  
  <p>the missing values (not <em>ready</em>) are</p>
  
  <pre><code>val missing = range.toSet diff ready.toSet
  Set(5, 1, 3)
  </code></pre>
  
  <p>The real use case includes thousands of range instances with (possibly) thousands of <em>missing</em> or not <em>ready</em> values. Is there a more time-efficient approach in Scala?</p>