Create group number for runs of non-NA values

Question

I have the following dataframe df (dput below):

> df
   id value
1   1     1
2   2     3
3   3     2
4  NA     1
5  NA     3
6   8     4
7   9     2
8  10     1
9  NA     1
10 NA     3
11 15     2
12 16     1
13 NA     3
14 NA     4
15 NA     2
16 20     1
17 21     1
18 22     3
19 NA     2
20 NA     1
21 NA     3
22 66     4
23 67     2
24 68     1

For each consecutive run of non-NA "id", I would like to create a unique group ID (variable "id2" in the example). For the NA in between, the "id2" should be NA.

In the example, the first run of non-NA "id" (1, 2 and 3), should all belong to group 1 (id2 = 1). The NA on row 4-5 should have id2 = NA. The second run of non-NA "id" (8, 9, 10) should have id2 = 2, and so on.

The desired output should look like this:

   id value id2
1   1     1   1
2   2     3   1
3   3     2   1
4  NA     1  NA
5  NA     3  NA
6   8     4   2
7   9     2   2
8  10     1   2
9  NA     1  NA
10 NA     3  NA
11 15     2   3
12 16     1   3
13 NA     3  NA
14 NA     4  NA
15 NA     2  NA
16 20     1   4
17 21     1   4
18 22     3   4
19 NA     2  NA
20 NA     1  NA
21 NA     3  NA
22 66     4   5
23 67     2   5
24 68     1   5

As you can see, id2 shows the unique id I want for the values of id between NA. It may be that there is only a NA after the values, like for id's 1, 2 and 3. So I was wondering if anyone knows how to create a unique ID for values between NA's?

---

dput of df:

df <- structure(list(id = c(1, 2, 3, NA, NA, 8, 9, 10, NA, NA, 15, 
16, NA, NA, NA, 20, 21, 22, NA, NA, NA, 66, 67, 68), value = c(1, 
3, 2, 1, 3, 4, 2, 1, 1, 3, 2, 1, 3, 4, 2, 1, 1, 3, 2, 1, 3, 4, 
2, 1)), class = "data.frame", row.names = c(NA, -24L))

Answer 1

Here's a possibility, maybe not the most efficient:

1. find the transitions between a valid id and an NA. That gives you a 1 each time we have an id which is NA but the previous one is valid.
2. make the cumulative sum
3. add NA where needed

library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union
df <- structure(list(id = c(1, 2, 3, NA, NA, 8, 9, 10, NA, NA, 15, 
                            16, NA, NA, NA, 20, 21, 22, NA, NA, NA, 66, 67, 68), value = c(1, 
                                                                                           3, 2, 1, 3, 4, 2, 1, 1, 3, 2, 1, 3, 4, 2, 1, 1, 3, 2, 1, 3, 4, 
                                                                                           2, 1)), class = "data.frame", row.names = c(NA, -24L))

df |> 
  mutate(
    id2 = as.numeric(is.na(id) & !is.na(lag(id))),
    id2 = cumsum(id2) + 1,
    id2 = ifelse(is.na(id), NA, id2)
  )
#>    id value id2
#> 1   1     1   1
#> 2   2     3   1
#> 3   3     2   1
#> 4  NA     1  NA
#> 5  NA     3  NA
#> 6   8     4   2
#> 7   9     2   2
#> 8  10     1   2
#> 9  NA     1  NA
#> 10 NA     3  NA
#> 11 15     2   3
#> 12 16     1   3
#> 13 NA     3  NA
#> 14 NA     4  NA
#> 15 NA     2  NA
#> 16 20     1   4
#> 17 21     1   4
#> 18 22     3   4
#> 19 NA     2  NA
#> 20 NA     1  NA
#> 21 NA     3  NA
#> 22 66     4   5
#> 23 67     2   5
#> 24 68     1   5

^{Created on 2023-03-21 with reprex v2.0.2}