title: how to do math operation in data frame and make a new column of its results?
body: <p>I want to use some basic math to know how much stock we need per day. i would like to divide stock by difference of start date and end date, so i could get stock per day.
  I currently have data like this</p>
  
  <pre><code> promo  start_date  end_date  item   discount   stock
      a     30/08/19   02/09/19   ax      15%       200      
      a     30/08/19   02/09/19   bx      15%       200
      a     27/08/19   27/08/19   cx      25%       200
      a     27/08/19   27/08/19   dx      15%       200
      b     03/09/19   10/09/19   ex      15%       200
      b     04/09/19   04/09/19   fx      15%       200
  </code></pre>
  
  <p>to be like this</p>
  
  <pre><code>promo   date    item   discount   stock  stock/day
     a    30/08/19   ax      15%       200     50
     a    31/08/19   ax      15%       200     50
     a    01/09/19   ax      15%       200     50
     a    02/09/19   ax      15%       200     50
     a    30/08/19   bx      15%       200     50
     a    31/08/19   bx      15%       200     50
     a    01/09/19   bx      15%       200     50
     a    02/09/19   bx      15%       200     50
     a    27/08/19   cx      25%       200    200 
     a    27/08/19   dx      15%       200    200
     b    03/09/19   ex      15%       200     25
     b    04/09/19   ex      15%       200     25
          ... continue as above
     b    09/09/19   ex      15%       200     25
     b    10/09/19   ex      15%       200     25
     b    04/09/19   fx      15%       200    200
  </code></pre>
  
  <p>Does anyone know how to do this?</p>