title: Pandas dataframe to 1-d array
body: <p>I have a dataframe with lots of columns.</p>
  
  <p>I first choose only one column from the dataframe by
  <code>r_i = df.iloc[:, i: i + 1]</code></p>
  
  <p>Then I want to turn this <code>r_i</code> into array simply by 
  <code>np.array(r_i)</code>.</p>
  
  <p>the result I want is like:
  <code>array([-1, -2, -3])</code>. In other words, it should be array of one list.</p>
  
  <p>However, it gives me array of one list which consists of sublists:
  <code>array([[-1], [-2], [-3]])</code>.</p>
  
  <p>How do I prevent this from happening?</p>
  
  <p>Thank you.</p>