Delete duplicates between groups in R

Asked By: Anonymous

Thanks in advance for any help.
my Data looks like this:

|year|class|
|---|----|
|2007|a|
|2007|b|
|2007|c|
|2007|d|
|2008|a|
|2008|b|
|2008|e|
|2008|f|
|2009|c|
|2009|d|
|2009|e|
|2009|g|

The goal would be to delete any classes which occure in the previous year, so the final data looks like this:

|year|class|
|---|----|
|2007|a|
|2007|b|
|2007|c|
|2007|d|
|2008|e|
|2008|f|
|2009|c|
|2009|d|
|2009|g|

I tried this code, I intendet to group the data and then delete all within group duplicates but it did not remove everything just a few rows.
Instead of duplicates() I also tried unique() which did not work.

d %>% group_by(class, Group = c(0, cumsum(diff(year) != 1))) %>%
  filter(!(duplicated(class, fromLast = TRUE)| duplicated(class))) %>%
  ungroup() %>%
  select(-Group)

Is there maybe another R function which can look at group differences?
Thanks for any help

Edit: Thanks too all for your very helpfull answers!

Solution

Answered By: Anonymous

Left join DF to itself on class and a year difference of 1 and retain only those rows for which there is no such match.

library(sqldf)

sqldf("select a.*
  from DF a
  left join DF b on b.class = a.class and b.year = a.year - 1
  where  b.year is null")

giving:

  year class
1 2007     a
2 2007     b
3 2007     c
4 2007     d
5 2008     e
6 2008     f
7 2009     c
8 2009     d
9 2009     g

Note

Lines <- "|year|class|
|2007|a|
|2007|b|
|2007|c|
|2007|d|
|2008|a|
|2008|b|
|2008|e|
|2008|f|
|2009|c|
|2009|d|
|2009|e|
|2009|g|"
DF <- read.table(text = Lines, sep = "|", header = TRUE)[2:3]