title: how to use Array class "partition" method
body: <p>I'm currently studying Array class in swift. I have encountered the following method </p>
  
  <pre><code>mutating func partition(by belongsInSecondPartition: (Element) throws -&gt; Bool) rethrows -&gt; Int
  </code></pre>
  
  <p>But the <a href="https://developer.apple.com/reference/swift/array/2294611-partition" rel="nofollow noreferrer">apple official documentation</a> does not have any explanation on what is method is and how it is used.</p>
  
  <p>Why no explanation is provided for this method? Is this a exception or a lot of other functions doesn't have explanation as well? When encountering situations like this, what is the best way to deal with it (i.e. learning how to use the method)?</p>