title: Stata: Concatenate string variable on by condition
body: <p>I'm creating a variable called result in the following sample data which shows a 'W' if a bet was a win and 'L' if it was a loss. </p>
  
  <p>How can I concatenate this variable with itself on a row by row basis in strict order by timestamp for each username?</p>
  
  <pre><code>clear
  input str16 username str40 betdate winnings
  player1 "12NOV2008:19:04:01" -10
  player1 "12NOV2008:12:03:44" 50
  player2 "07NOV2008:14:03:33" -50
  player2 "05NOV2008:09:00:00" -100
  end
  
  generate double timestamp=clock(betdate,"DMY hms") 
  format timestamp %tc
  
  cap drop result
  generate result = "L"
  replace result = "W" if (winnings &gt;0)
  
  cap drop resulthistory
  generate resulthistory = ""
  replace resulthistory = concat(resulthisory + result), by(USERNAME timestamp) 
  </code></pre>