Remove the first N rows from each factor level in an r data.frame

Question

With the dat below. How can I make a new dataframe subset that includes all values except the first five rows for each IndID? Said differently I want new data frame with the first 5 rows for each IndID excluded.

set.seed(123)
dat <- data.frame(IndID = rep(c("AAA", "BBB", "CCC", "DDD"), each  = 10),
                  Number = sample(1:100,40))

I have seen a number of SO posts that select data, but I am not sure how to remove as mentioned above.

Answer 1

We can use dplyr's slice() functionality:

dat %>% 
    group_by(IndID) %>% 
    slice(6:n())

Answer 2

In base R, tapply() is handy when used on a sequence of row numbers with tail().

idx <- unlist(tapply(1:nrow(dat), dat$IndID, tail, -5))
dat[idx, ]

Note that this will be more efficient with use.names=FALSE in unlist().

With data.table, you can do the following with tail().

library(data.table)

setDT(dat)[dat[, tail(.I, -5), by=IndID]$V1]