title: Convert float to int and leave nulls
body: <p>I have the following dataframe, I want to convert values in column 'b' to integer</p>
  
  <pre><code>    a   b       c
  0   1   NaN     3
  1   5   7200.0  20
  2   5   580.0   20
  </code></pre>
  
  <p>The following code is throwing exception 
  "ValueError: Cannot convert NA to integer"</p>
  
  <pre><code>df['b'] = df['b'].astype(int)
  </code></pre>
  
  <p>How do i convert only floats to int and leave the nulls as is?</p>