title: how to make an integer index corresponding to a string value?
body: <p>I'm currently building a recommender system using Goodreads data.</p>
  <p>I want to change string user ids into integers.
  Current user ids are like this: <code>'0d688fe079530ee1fe6fa85eab10ec5c'</code></p>
  <p>I want to change it into integers(e.g. <code>1</code>, <code>2</code>, <code>3</code>, ...), to have the same integer ids which share the same string ids. I've considered using function <code>df.groupby('user_id')</code>, but I couldn't figure out how to do this.</p>
  <p>I would be very thankful if anybody let me know how to change.</p>
  <p><img src="https://i.stack.imgur.com/Es7Pg.png" alt="screenshot" /></p>