Is there way to expand rows in a data frame by conditioning on two columns?

Question

My data structure is as follows:

rad_1	division_num	interval
1	2	4
1	3	2
2	3	3

I want to expand the rows by division and interval. If the interval is 4 and the division is 2 then expand the row eight times as 4 x 2 is 8. Similarly, if the interval is 2 and the division is 3 then expand the row 6 times as 2 x 3 is 6.

Code I tried:

dt1[, row_index := 1:.N]

dt1 <- dt1[rep(seq_len(.N), dt1$division), ]

dt1[, new_row := seq_len(division) - 1 + interval * (row_index - 1), by = row_index]

Expected output:

rad_1	division_num	interval
1	2	4
1	2	4
1	2	4
1	2	4
1	2	4
1	2	4
1	2	4
1	2	4
1	3	2
1	3	2
1	3	2
1	3	2
1	3	2
1	3	2
2	3	3
2	3	3
2	3	3
2	3	3
2	3	3
2	3	3
2	3	3
2	3	3
2	3	3

Answer 1

You could use tidyr::uncount

 tidyr::uncount(df, division_num * interval)

   rad_1 division_num interval
1      1            2        4
2      1            2        4
3      1            2        4
4      1            2        4
5      1            2        4
6      1            2        4
7      1            2        4
8      1            2        4
9      1            3        2
10     1            3        2
11     1            3        2
12     1            3        2
13     1            3        2
14     1            3        2
15     2            3        3
16     2            3        3
17     2            3        3
18     2            3        3
19     2            3        3
20     2            3        3
21     2            3        3
22     2            3        3
23     2            3        3