title: pandas current row * previous row +previous row
body: <p>index_pd:</p>
  
  <pre><code>tradedate  |  percent  |  day_index
  ------ | ------| ------
  2015-06-02 |    0      |     1000
  2015-06-03 |    0.5    |     0
  2015-06-04 |    0.6    |     0
  .....
  </code></pre>
  
  <p>want result:</p>
  
  <pre><code>tradedate  |  percent  |  day_index
  ------ | ------| ------
  2015-06-02 |    0      |     1000
  2015-06-03 |    0.5    |     1500 = 1000 + 1000 * 0.5
  2015-06-04 |    0.6    |     2400 = 1500 + 1500 * 0.6
  .....
  </code></pre>
  
  <p>i try </p>
  
  <pre><code>index_pd['day_index'] =index_pd['day_index'].shift(1) * index_pd['percent'].shift(0) + index_pd['day_index'].shift(1)
  </code></pre>
  
  <p>but it effect second row.
  there are thousand rows in index_pd, how to batch replace,thanks</p>