Copy a row if there is only one record for that group

I have some pre-post data (in R) for which some individuals only have a value for time 1, like episode "2" here:

episode <- c('1','1','2','3','3')
score <- c('44','12','37','40','9')

df <- data.frame(episode,score)

For all the records where there is only "pre" data (1 score per episode), I would like to use R (dplyr preferred) to copy that record and then indicate for all records which is the pre and which is the post. So the final should look something like:

Thanks!

>Solution :

Here is one option – create a column of frequency count by ‘episode’ and if the value is 1, then add 1 on the logical (n == 1) and replicate the rows with uncount

library(dplyr)
library(tidyr)
df %>% 
 add_count(episode) %>% 
 mutate(n = (n == 1) + 1) %>%
 uncount(n) %>%
 group_by(episode) %>%
 mutate(time = row_number()) %>%
 ungroup

-output

# A tibble: 6 × 3
  episode score  time
  <chr>   <chr> <int>
1 1       44        1
2 1       12        2
3 2       37        1
4 2       37        2
5 3       40        1
6 3       9         2

Or create the ‘time’ column first and then use complete with fill

df %>% 
  group_by(episode) %>%
  mutate(time = row_number()) %>% 
  ungroup %>% 
  complete(episode, time) %>%
  fill(score)