How to group and ungroup in R? - r

Question

I have a dataframe like as shown below
test_df <- data.frame("SN" = c("ABC123","ABC123","ABC123","MNO098","MNO098","MNO098"),
"code" = c("ABC1111","DEF222","GHI133","","MNO1123","MNO567"),
"d_time" = c("2220-08-27","2220-05-27","2220-02-27","2220-11-27","2220-02-27",""))
I am trying to do 2 things
1) create 2 new columns (p_id,v_id) by stripping alphabets from columns SN and code and retain only 9 digits
2) create a lag column (p_vid) based on v_id for each person sorted based on his/her d_time
t_df <- test_df %>% group_by(SN)
t_df %>% arrange((d_time), .by_group = TRUE) ->> sorted_df #sorted based on d_time
transform_ids = function(DF){ # this function is to create person and visit_occurrence ids
DF %>%
mutate(p_id = as.integer(str_remove_all(.$SN,"[a-z]|[A-Z]") %>% #retaining only the numeric part
str_sub(1,9))) %>%
mutate(v_id = as.integer(str_remove_all(.$code,"[a-z]|[A-Z]") %>%
str_sub(1,9))) %>%
group_by(p_id) %>%
mutate(pre_vid = lag(v_id)) %>%
ungroup
}
transform_ids(sorted_df)
But when I do this I encounter the below error
Error in View : Column p_id must be length 3 (the group size) or one, not 6
Error: Column p_id must be length 3 (the group size) or one, not 6
In addition: Warning message:
In view(transform_ids(t_df)) :
Show Traceback
Rerun with Debug
Error: Column p_id must be length 3 (the group size) or one, not 6
I expect my output to be like as shown below. Basically I am trying to link each v_id of a person to his previous visit which is p_vid

Answer 1

To generate the p_id and v_id columns, just use sub:
t_df$p_id <- gsub("[A-Z]+", "", t_df$SN)
t_df$v_id <- gsub("[A-Z]+", "", t_df$code)
For the p_vid column, use lag() from the dplyr package:
t_df %>%
group_by(p_id) %>%
mutate(p_vid = lag(v_id, order_by=d_time, default=0))
The output from the above actually gives you a tibble. If you want a data frame, just use:
t_df <- as.data.frame(t_df)
Output:
SN code d_time p_id v_id p_vid
<fct> <fct> <fct> <chr> <chr> <chr>
1 ABC123 ABC1111 2220-08-27 123 1111 222
2 ABC123 DEF222 2220-05-27 123 222 133
3 ABC123 GHI133 2220-02-27 123 133 0
4 MNO098 "" 2220-11-27 098 "" 1123
5 MNO098 MNO1123 2220-02-27 098 1123 567
6 MNO098 MNO567 "" 098 567 0