title: how to add new column based on the above row's value
body: <p>I have one dataframe as below. At first,they have three columns('date','time','flag'). I want to add one column which based on the flag and date which means when I get flag=1 ,then the rest of this day the target is 1, otherwise the target is zero. </p>
  
  <pre><code>  date        time      flag  target
  0 2017/4/10   10:00:00  0     0
  1 2017/4/10   11:00:00  1     1
  2 2017/4/10   12:00:00  0     1
  3 2017/4/10   13:00:00  0     1
  4 2017/4/10   14:00:00  0     1
  5 2017/4/11   10:00:00  1     1
  6 2017/4/11   11:00:00  0     1
  7 2017/4/11   12:00:00  1     1
  8 2017/4/11   13:00:00  1     1
  9 2017/4/11   14:00:00  0     1
  10 2017/4/12  10:00:00  0     0
  11 2017/4/12  11:00:00  0     0
  12 2017/4/12  12:00:00  0     0
  13 2017/4/12  13:00:00  0     0
  14 2017/4/12  14:00:00  0     0
  15 2017/4/13  10:00:00  0     0
  16 2017/4/13  11:00:00  1     1
  17 2017/4/13  12:00:00  0     1
  18 2017/4/13  13:00:00  1     1
  19 2017/4/13  14:00:00  0     1
  </code></pre>