title: Format decimals as percentages in a column
body: <p>Let's say I have the following pandas DataFrame:</p>
  
  <pre><code>df = pd.DataFrame({'name': ['Johnny', 'Brad'], 'rating': [1.0, 0.9]})
  </code></pre>
  
  <p>I want to convert the <code>rating</code> column from a decimal to a percentage as a string (e.g. <code>1.0</code> to <code>'100%'</code>). The following works okay:</p>
  
  <pre><code>def decimal_to_percent_string(row):
      return '{}%'.format(row['rating'] * 100)
  
  df['rating'] = df.apply(func=decimal_to_percent_string, axis=1)
  </code></pre>
  
  <p>This seems very inefficient to me as it applies the function to the entire DataFrame which isn't ideal because my DataFrame is very large. Is there a better way to do this?</p>