title: Format multiple dataframe columns with the thousand separator
body: <p>I was able to put the thousand separator to one column slice. How would I do that to multiple columns?</p>
  
  <p>This works:</p>
  
  <pre><code>summary.iloc[:,6]=summary.iloc[:,6].map('{:,}'.format)
  </code></pre>
  
  <p>But this does not:</p>
  
  <pre><code>summary.iloc[:,6]=summary.iloc[:,6:].map('{:,}'.format)
  </code></pre>