r - Create new rows and put a flag to differentiate between existing row

Question

I have a dataset like this:

df_have <- data.frame(id = rep("a",3), time = c(1,3,5), flag = c(0,1,1))

The data has one row per time per id but I need to have the second row duplicated and put into the data.frame like this:

df_want <- data.frame(id = rep("a",4), time = c(1,3,3,5), flag = c(0,0,1,1))

The flag variables should become 0 with the new row added and all other information the same. Any help would be appreciated.

Edit: The comments below are helpful but I would also need to do this in groups by id and some ids have more rows than other ids. After reading this and seeing the comments below I see the logic isn't clear. My original data does not have a count variable (what I call flag) but it needs it in the final output. What I would need is that every row besides for the first and last timepoint (within each id) to be duplicated and every time there is a duplicate make a counter to demonstrate when a row was created until the next new row is created.

df_have2 <- data.frame(id = c(rep("a",3),rep("b",4))  , 
                      time = c(1,3,5,1,3,5,7))


df_want2 <- data.frame(id = c(rep("a",4),rep("b",6)),
                       time = c(1,3,3,5,1,3,3,5,5,7),
                       flag = c(1,1,2,2,1,1,2,2,3,3))

Answer 1

We could expand the data with slice and then create the 'flag' by matching the 'time' with unique values of 'time' and take the lag of it

library(dplyr)
df_have2 %>% 
     group_by(id) %>% 
     slice(rep(row_number(), c(1, rep(2, n() - 2), 1))) %>%
     mutate(flag = lag(match(time, unique(time)),  default = 1)) %>% 
     ungroup
# A tibble: 10 x 3
#   id     time  flag
#   <chr> <dbl> <dbl>
# 1 a         1     1
# 2 a         3     1
# 3 a         3     2
# 4 a         5     2
# 5 b         1     1
# 6 b         3     1
# 7 b         3     2
# 8 b         5     2
# 9 b         5     3
#10 b         7     3