How can I calculate col2?

col2 should increment each time a value in col1 appears after being interrupted by a different value, e.g. creating a grouping variable that changes every time a new sequence starts.

structure(list(col1 = c("A", "A", "A", "A", "B", "B", "B", "C", 
"C", "C", "C", "A", "A", "E", "E", "E", "F", "F", "F", "G", "G", 
"G", "C", "C", "C", "A", "A", "A"), col2 = c(1L, 1L, 1L, 1L, 
1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 
1L, 1L, 2L, 2L, 2L, 3L, 3L, 3L)), class = "data.frame", row.names = c(NA, 
-28L))

Desired output:

   col1 col2
1     A    1
2     A    1
3     A    1
4     A    1
5     B    1
6     B    1
7     B    1
8     C    1
9     C    1
10    C    1
11    C    1
12    A    2
13    A    2
14    E    1
15    E    1
16    E    1
17    F    1
18    F    1
19    F    1
20    G    1
21    G    1
22    G    1
23    C    2
24    C    2
25    C    2
26    A    3
27    A    3
28    A    3

>Solution :

One option would be to use consecutive_id two times like so:

library(dplyr, warn = FALSE)

df |>
  select(-col2) |>
  mutate(col2 = consecutive_id(col1)) |>
  mutate(col2 = consecutive_id(col2), .by = col1)
#>    col1 col2
#> 1     A    1
#> 2     A    1
#> 3     A    1
#> 4     A    1
#> 5     B    1
#> 6     B    1
#> 7     B    1
#> 8     C    1
#> 9     C    1
#> 10    C    1
#> 11    C    1
#> 12    A    2
#> 13    A    2
#> 14    E    1
#> 15    E    1
#> 16    E    1
#> 17    F    1
#> 18    F    1
#> 19    F    1
#> 20    G    1
#> 21    G    1
#> 22    G    1
#> 23    C    2
#> 24    C    2
#> 25    C    2
#> 26    A    3
#> 27    A    3
#> 28    A    3