title: Fastest way to do cumulative totals in Pandas dataframe
body: <p>I've got a pandas dataframe of golfers' round scores going back to 2003 (approx 300000 rows). It looks something like this:</p>
  
  <p>Date----Golfer---Tournament-----Score---Player Total Rounds Played</p>
  
  <p>2008-01-01---Tiger Woods----Invented Tournament R1---72---50</p>
  
  <p>2008-01-01---Phil Mickelson----Invented Tournament R1---73---108</p>
  
  <p>I want the 'Player Total Rounds Played' column to be a running total of the number of rounds (i.e. instance in the dataframe) that a player has played up to that date. Is there a quick way of doing it? My current solution (basically using iterrows and then a one-line function) works fine but will take approx 11hrs to run.</p>
  
  <p>Thanks,</p>
  
  <p>Tom</p>