title: How to remove all string elements from a numpy object array
body: <p>origin array is like:</p>
  <pre><code>array([nan, nan, 'hello', ..., nan, 'N', 61.0], dtype=object)
  </code></pre>
  <p>How can I remove all string from this array and get a new array with dtype float?</p>
  <p>I know I can do this using python list:</p>
  <pre><code>[i for i in x if type(i) == float]
  </code></pre>
  <p>but this way will change <code>numpy.ndarray</code> to <code>list</code>, is there a way to do this in numpy?</p>