Count rows after value 1 where rest of rows are NAs in r

I have this data:

df <- tibble(a = c(NA, NA, 1, NA, NA, NA, 1, NA, NA))

I want to fill the NAs after the occurence of 1 with counts. Like this:

df2 <- tibble(a = c(NA, NA, 1, 2, 3, 4, 1, 2, 3))

I’ve no solution so far. Any ideas?

>Solution :

one possible way to solve this within the tidyverse:

library(dplyr)

df %>% 
    # return TRUE or FALSE (1 or 0) if is not NA and run cummulative sum to identify groups
    dplyr::mutate(grp = cumsum(!is.na(a))) %>% 
    # build grouping
    dplyr::group_by(grp) %>% 
    # give rownumber per group if group != 0 (first rows until a = 1 for the frist time
    dplyr::transmute(a = ifelse(grp != 0, dplyr::row_number(), NA)) %>%  
    # release groupings to prevent unwanted behaviour down stream
    dplyr::ungroup() %>% 
    # unselect grp if you do not need it further on in your calculations
    dplyr::select(-grp)

# A tibble: 9 x 1
      a
  <int>
1    NA
2    NA
3     1
4     2
5     3
6     4
7     1
8     2
9     3