title: Conditional Cumulative Sum in R
body: <p>I have a time series data frame and want to compute cumulative returns for stock symbols intra-day for a range of dates.  When the symbol and/or date changes the cumulative return should reset.  Any help would be appreciated.  A small sample of my data frame is below including what the cumulative sum column should return. Thanks.  </p>
  
  <pre><code>       Date Symbol  Time   Last Return Cumulative.Sum
  1  1/2/2013     AA  9:30  42.00    n/a            n/a
  2  1/2/2013     AA 12:00  42.50  1.19%          1.19%
  3  1/2/2013     AA 16:00  42.88  0.89%          2.08%
  4  1/2/2013   AAPL  9:30 387.00    n/a            n/a
  5  1/2/2013   AAPL 12:00 387.87  0.22%          0.22%
  6  1/2/2013   AAPL 16:00 388.69  0.21%          0.44%
  7  1/3/2013     AA  9:30  42.88    n/a            n/a
  8  1/3/2013     AA 12:00  42.11 -1.80%         -1.80%
  9  1/3/2013     AA 16:00  41.89 -0.52%         -2.32%
  </code></pre>