title: Adding a DataFrame column with len() of another column's values
body: <p>I'm having a problem trying to get a character count column of the string values in another column, and haven't figured out how to do it efficiently.</p>
  
  <pre><code>for index in range(len(df)):
      df['char_length'][index] = len(df['string'][index]))
  </code></pre>
  
  <p>This apparently involves first creating a column of nulls and then rewriting it, and it takes a really long time on my data set. So what's the most effective way of getting something like</p>
  
  <pre><code>'string'     'char_length'
  abcd          4
  abcde         5
  </code></pre>
  
  <p>I've checked around quite a bit, but I haven't been able to figure it out.</p>