title: R: Aggregate dynamically based on the data before and id
body: <p>I have a very large dataset that looks simplified like this:</p>
  
  <pre><code>row.    member_id   entry_id    comment_count   timestamp
  1       1            a              4           2008-06-09 12:41:00
  2       1            b              1           2008-07-14 18:41:00
  3       1            c              3           2008-07-17 15:40:00
  4       2            d              12          2008-06-09 12:41:00
  5       2            e              50          2008-09-18 10:22:00
  6       3            f              0           2008-10-03 13:36:00
  </code></pre>
  
  <p>I now want to create new a column, in which I sum up all the "commen_count" of previous ideas ("ID") from the same member. So I only want to sum up the comment_counts from entried that occured before the current entry. I can order my dataset by the member id and the timestamp. </p>
  
  <p>The result should look something like this:</p>
  
  <pre><code>row.    member_id   entry_id    comments_count  timestamp             aggregated_count
  1       1            a              4           2008-06-09 12:41:00        4
  2       1            b              1           2008-07-14 18:41:00        5
  3       1            c              3           2008-07-17 15:40:00        8
  4       2            d              12          2008-06-09 12:41:00        12
  5       2            e              50          2008-09-18 10:22:00        62
  6       3            f              0           2008-10-03 13:36:00        0
  </code></pre>
  
  <p>Some idea how I can do this in R (or Stata)? I tried aggregate, but I don't understand how to sum only the comment_counts before the curren timestamp and only those with the current member_id.</p>