Repeat a sequential numbering of duplicated values starting from a given value in R

Question

In a new variable row2, how to repeat a sequential numbering (here a sequence from 3 to 6) by group of duplicated row1 values, which would start from a given value (here from row1 = 3), even if the last sequence is incomplete (here 3 to 5 for example)?

Thanks for help

Desired output:

> dat1
   row1 row2
1     1    1
2     1    1
3     2    2
4     3    3 # start the sequence
5     4    4
6     4    4
7     4    4
8     5    5
9     5    5
10    6    6
11    6    6
12    6    6
13    7    3 # repeat the sequence
14    7    3
15    8    4
16    8    4
17    9    5
18    9    5
19    9    5
20   10    6
21   11    3 # and repeat again...
22   11    3
23   11    3
24   12    4
25   13    5
26   13    5 # ...even if incomplete 

Initial data:

row1 <- c(1,1,2,
          3,4,4,4,5,5,6,6,6,
          7,7,8,8,9,9,9,10,
          11,11,11,12,13,13)
dat1 <- data.frame(row1)

Answer 1

You could use if_else to apply modulo to val >=3

1. (row1 - 3) %% 4 cycles through 1,2,3, effectively mapping row1 values into 3,4,5,6 repeatedly.

2. +3 shifts the sequence to start at 3.

3. Values of row1 < 3 are kept untouched

---

dat1$row2 <- if_else(dat1$row1 >= 3, (dat1$row1 - 3) %% 4 + 3, dat1$row1)

   row1 row2
1     1    1
2     1    1
3     2    2
4     3    3
5     4    4
6     4    4
7     4    4
8     5    5
9     5    5
10    6    6
11    6    6
12    6    6
13    7    3
14    7    3
15    8    4
16    8    4
17    9    5
18    9    5
19    9    5
20   10    6
21   11    3
22   11    3
23   11    3
24   12    4
25   13    5
26   13    5