Rank values in order of occurence within groups

Question

I am currently trying to rank column values within groups of IDs in the order that they occur. My dataset currently looks like this:

  ID Value      Date
1  1     a  1/1/2019
2  1     b  2/5/2018
3  1     a  3/3/2019
4  2     a 6/12/1975
5  2     b  5/4/2017
6  2     b 12/3/2016
7  3     c  1/3/2015
8  3     a  2/1/2015
9  4     a  1/1/1991

and I would like to add another column so that it looks like this:

  ID    Value   Date    Occurence
1  1     a  1/1/2019         1
2  1     b  2/5/2018         1
3  1     a  3/3/2019         2
4  2     a 6/12/1975         1
5  2     b  5/4/2017         2
6  2     b 12/3/2016         1
7  3     c  1/3/2015         1
8  3     a  2/1/2015         1
9  4     a  1/1/1991         1

So we can see, where ID is equal to 1 , a occurs twice and is ranked in order from oldest to newest and b is only ranked once because it only occurs once for ID 1 .

I have come across code to rank things in order of occurrence for the whole data set, but I would like to rank things in order of occurrence within IDs. There is also code to help me rank all dates within an ID...but I want to be able to look at individual values.

How would I do this? Thank you!

Answer 1

For the updated question with sorting by date:

library(lubridate)
df %>% 
mutate_at("Date",dmy) %>%  #convert to date-object
arrange(Date) %>%  #sort by date
group_by(ID,Value) %>%  #group by ID-Value pairs
mutate(Occurence = row_number()) #mutate new column with occurence

df
     ID Value Date       Occurence
  <int> <chr> <date>         <int>
1     2 a     1975-12-06         1
2     4 a     1991-01-01         1
3     3 a     2015-01-02         1
4     3 c     2015-03-01         1
5     2 b     2016-03-12         1
6     2 b     2017-04-05         2
7     1 b     2018-05-02         1
8     1 a     2019-01-01         1
9     1 a     2019-03-03         2