title: How do I select an element in array column of a data frame?
body: <p>I have the following data frame:</p>
  
  <pre><code>pa=pd.DataFrame({'a':np.array([[1.,4.],[2.],[3.,4.,5.]])})
  </code></pre>
  
  <p>I want to select the column 'a' and then only a particular element (i.e. first: 1., 2., 3.)</p>
  
  <p>What do I need to add to:</p>
  
  <pre><code>pa.loc[:,['a']]
  </code></pre>
  
  <p>?</p>