title: How to add calculated column to Dataframe counting frequency in column in pandas
body: <p>I have dataframe like this:</p>
  
  <pre><code>   county
  1     N
  2     N
  3     C
  4     N
  5     S
  6     N
  7     N
  </code></pre>
  
  <p>and what I'd like to reach is:</p>
  
  <pre><code>    county  frequency
  1   N       5
  2   N       5
  3   C       1
  4   N       5
  5   S       1
  6   N       5
  7   N       5
  </code></pre>
  
  <p>Is there any possibility to add such column directly without any intermediate df. I know that I could create another df with group and size function and merge this two dataframes. Howewer, I wonder if there is any function which enable such solution without any intermediate df but maybe with usage of 'apply' and some function or lamba?</p>