title: How do you add a simple counter column that increases by one in each row to a Pandas DataFrame?
body: <p>I have been running into this problem a lot. If you have an existing DataFrame in Pandas, and you want to add a row that is simply an increasing count, ie. 0, 1, 2..., what is the most efficient way to do it?</p>
  
  <p>Thanks!</p>
  
  <p>Sam</p>