title: ValueError in computing precision using scikit-learn
body: <pre><code>from sklearn.metrics import precision_score
  
  precision_score(expected, predicted)
  </code></pre>
  
  <p>where expected is <code>array([ 4.,  3.])</code></p>
  
  <p>and predicted is <code>array([ 2.,  4.])</code></p>
  
  <p>I get the foll. error: <code>*** ValueError: pos_label=1 is not a valid label: array([ 2.,  3.,  4.])</code></p>
  
  <p>How can this be fixed?</p>