Compare to next row, grouped, data.table

Question

I have a data frame containing number of page views per user, per week. I want to determine, for each user, whether their views increased, decreased, or stayed the same after a

Answer 1

Using match:

dat[, numnextweek := numviews[match(week + 1, week)] , by=Userid]
dat[, difference := numviews - numnextweek , by=Userid]
dat[xeventinweek != 0]

#   Userid week xeventinweek numviews numnextweek difference
#1:  Alice    1            2        5           3          2
#2:  Alice    4            1        6          NA         NA
#3:    Bob    2            2        3           5         -2