Remove all duplicates except last instance

Question

So I have a dataset in R with the following layout as an example:

ID Date Tally
1 2/1/2011 1
2 2/1/2011 2
3 2/1/2011 3
1 2/1/2011 4
2 2/1/2011 5
1 2/1/2011 6
3 2/1/2011 7
4 2/1/2011 8
2 2/1/2011 9

I want to remove all instances except the LAST instance of the post id. Right now everything I can find online, and functions I am using is removing everything except the FIRST instance.

So my new data frame would look like:

ID Date Tally
1 2/1/2011 6
3 2/1/2011 7
4 2/1/2011 8
2 2/1/2011 9

How do I do this? Right now I am only able to keep the first instance. I want it to do the opposite? Any help?

Bear with me I am new to R :)

Answer 1

Use !rev(duplicated(rev(ID))) to filter out all but the last unique occurrences.

To get the dataset filtered, use dataset[!rev(duplicated(rev(dataset$ID))),]

Answer 2

Wouldn't this just be the standard case for using the 'fromLast' parameter to duplicated?

 dat[ !duplicated(dat[, c("ID", "Date")], fromLast=T),]
#---------
  ID     Date Tally
6  1 2/1/2011     6
7  3 2/1/2011     7
8  4 2/1/2011     8
9  2 2/1/2011     9

Your example was not rich enough to tell whether you needed the "Date" column in the test fro duplication, so perhaps you could simplify. I'm leaving it in to illustrate that duplicated has a data.frame method. I prefer !duplicated to unique because it allows easy access to the set complement if you are comparing groups.

Answer 3

Using a data.table join, you can set mult = 'last'

For example

library(data.table)
DT <- data.table(DF, key = 'id')

# join with the unique ID values
DT[unique(DT[,list(ID)]), mult= 'last']

   ID     Date Tally
1:  1 2/1/2011     6
2:  2 2/1/2011     9
3:  3 2/1/2011     7
4:  4 2/1/2011     8

If you knew the unique IDs you could also any of the following

DT[.(1:4), mult='last']
DT[list(1:4), mult = 'last']

Answer 4

Use dplyr:

data <- data %>%
  group_by(ID) %>%
  slice(which.max(Tally))