title: pandas: set row values to letter of the alphabet corresponding to index number?
body: <p>I have a dataframe: </p>
  
  <pre><code>   a    b   c    country
  0  5    7   11   Morocco
  1  5    9   9    Nigeria
  2  6    2   13   Spain
  </code></pre>
  
  <p>I'd like to add a column <code>e</code> that is the letter of the alphabet corresponding to the index number, for example:</p>
  
  <pre><code>   a    b   c    country    e
  0  5    7   11   Morocco    A
  1  5    9   9    Nigeria    B
  2  6    2   13   Spain      C
  </code></pre>
  
  <p>How can I do this? I've tried:</p>
  
  <pre><code> df['e'] = chr(ord('a') + df.index.astype(int))
  </code></pre>
  
  <p>But I get:</p>
  
  <pre><code>TypeError: int() argument must be a string or a number, not 'Int64Index'
  </code></pre>