title: how to set the float_format in dataframe.to_csv(...) only to specific columns, but not others?
body: <p>I have a frame with 6 columns with <code>float64</code> data.</p>
  
  <pre><code>A B C D E F
  </code></pre>
  
  <ol>
  <li>How do I change the dataframe in place to make <code>E</code> and <code>F</code> <code>uint64</code> instead?, or</li>
  <li>How do I tell <code>to_csv()</code> to print the integer part of the float for <code>E</code> and <code>F</code> only?</li>
  </ol>