How to change the name of an observation according to its occurrence

Question

I have patients who have diagnoses and each patient is represented by an ID (I have three one here). Some diagnoses occur recurrently, if this is the case I would like to add the number of the recurrence in front of the diagnosis. For example for the patient ID=2, the second occurrence of "SLE" will be rename in "SLE2" and the third occurrence would be renamed in "SLE3"

ID<-c(rep(1,10),rep(2,10),rep(3,5))
time<-c(1:10,1:10,1:5)
diag<-c("ANN",NA,NA,NA,"SLE","ANN",NA,NA,NA,NA,
        "SLE",NA,NA,NA,"SLE","BPG","SLE",NA,NA,NA,"SLE",NA,NA,"ANN",NA)

mydata<-data.frame(ID,time,diag)

My new variable must be like this:

   ID time diag diag2
1   1    1  ANN   ANN
2   1    2 <NA>  <NA>
3   1    3 <NA>  <NA>
4   1    4 <NA>  <NA>
5   1    5  SLE   SLE
6   1    6  ANN  ANN2
7   1    7 <NA>  <NA>
8   1    8 <NA>  <NA>
9   1    9 <NA>  <NA>
10  1   10 <NA>  <NA>
11  2    1  SLE   SLE
12  2    2 <NA>  <NA>
13  2    3 <NA>  <NA>
14  2    4 <NA>  <NA>
15  2    5  SLE  SLE2
16  2    6  BPG   BPG
17  2    7  SLE  SLE3
18  2    8 <NA>  <NA>
19  2    9 <NA>  <NA>
20  2   10 <NA>  <NA>
21  3    1  SLE   SLE
22  3    2 <NA>  <NA>
23  3    3 <NA>  <NA>
24  3    4  ANN   ANN
25  3    5 <NA>  <NA>

Answer 1

One way would be to make use of dense_rank from dplyr package:

library(dplyr)

mydata %>%
  group_by(ID, diag) %>%
  mutate(rank = dense_rank(time)) %>%
  mutate(diag2 = case_when(is.na(diag) ~ diag,
                           rank == 1 ~ diag,                               
                           TRUE ~ paste0(diag, rank))) %>%
  dplyr::select(-rank)