Calculating stock's return by a variable interval in data.table

Here is my data.table

structure(list(index = structure(c(17007, 17008, 17009, 17010, 
17011, 17014, 17015, 17016, 17017), tzone = "UTC", tclass = "Date", class = "Date"), 
    close = c(97.339996, 96.669998, 102.949997, 104.339996, 104.209999, 
    106.050003, 104.480003, 105.790001, 105.870003), daily_return = c(-0.688306993560994, 
    6.49632681279251, 1.35016905342893, -0.124589807344833, 1.76566933850562, 
    -1.48043371578217, 1.25382653367649, 0.0756234041438208, 
    1.5207329313101), group = c(0, 0, 3, 3, 3, 3, 3, 3, 3), qq = c(1, 
    0, 6, 5, 4, 3, 2, 1, 0)), class = c("data.table", "data.frame"
), row.names = c(NA, -9L), .internal.selfref = <pointer: 0x558d2914e9a0>, sorted = "index")

and it looks like as follows -

        index  close daily_return group qq
1: 2016-07-25  97.34   -0.6883070     0  1
2: 2016-07-26  96.67    6.4963268     0  0
3: 2016-07-27 102.95    1.3501691     3  6
4: 2016-07-28 104.34   -0.1245898     3  5
5: 2016-07-29 104.21    1.7656693     3  4
6: 2016-08-01 106.05   -1.4804337     3  3
7: 2016-08-02 104.48    1.2538265     3  2
8: 2016-08-03 105.79    0.0756234     3  1
9: 2016-08-04 105.87    1.5207329     3  0

In this data.table, I need to find the return (from column close) by the interval given in qq column for each group. E.g. in row 3, the look forwarding interval is 6 and the return is to be calculated between column 3 and column 9 (3 + 6 lookforward).

When the group changes, the return is the be calculated between the last element of the first group and the first element of the second group. e.g, as the group changes between row 2 and row3, the return is calculated between these two rows.

Following is the desired result -

        index  close daily_return group qq desired_result
1: 2016-07-25  97.34   -0.6883070     0  1          -0.68
2: 2016-07-26  96.67    6.4963268     0  0           6.40
3: 2016-07-27 102.95    1.3501691     3  6           2.76
4: 2016-07-28 104.34   -0.1245898     3  5           1.39
5: 2016-07-29 104.21    1.7656693     3  4           1.52
6: 2016-08-01 106.05   -1.4804337     3  3          -0.23
7: 2016-08-02 104.48    1.2538265     3  2           1.34
8: 2016-08-03 105.79    0.0756234     3  1           0.09
9: 2016-08-04 105.87    1.5207329     3  0             NA

I am sure there must be an easy way to accomplish this using data.table.

I have tried the following and it works only on specific rows as k is constant.

lead(Delt(close, k = 6, type = "arithmetic") * 100, n = 6)

Is there a way to pass the column qq dynamically in the k variable of Delt command? OR, is there any other solution to this problem?