I have been trying to use a solution provided in this question, however it is not working for some reason.
Basically, i want to find all IDs that have at least 4 consecutive years and only keep rows that belong to those IDs.
Any ideas?
# example data
ID <- c(rep("A", 5), rep("B", 6), rep("C", 2), rep("D", 3), rep("E", 4))
year <- as.numeric(c(rep(c(2012, 2013, 2014, 2015), 4), 2012, 2013, 2015, 2016))
mydata <- cbind(ID, year)
mydata <- as.data.table(mydata)
mydata$year <- as.numeric(mydata$year)
# provided solution
mydata2 <- setDT(mydata)[, grp := cumsum(c(0, diff(year)) > 1), by = ID
][, if (.N > 4) .SD, by = grp][, grp := NULL][]
Here's how you could do that with data.table. Basically, you want to make sure to have the table sorted first. Second, I'm using diff(year)==1. We want consecutive years, so it's important that the diff is exactly equal to 1 to prevent jumps in years. Finally, I'm only keeping IDs that have a max(cumsum) of 3 or more (3 not 4 since we are starting to count from 0).
setorder(mydata)
mydata2 <- mydata[, grp := cumsum(c(0, diff(year)==1)), by = ID]
mydata2[,max_grp := max(grp), by=ID]
mydata2[max_grp>=3]
ID year grp max_grp
1: A 2012 0 3
2: A 2012 0 3
3: A 2013 1 3
4: A 2014 2 3
5: A 2015 3 3
6: B 2012 0 3
7: B 2013 1 3
8: B 2013 1 3
9: B 2014 2 3
10: B 2014 2 3
11: B 2015 3 3