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

Since version v1.9.6 (on CRAN 19 Sep 2015), the shift() function is available in data.table:

DT[, difference := shift(numviews, type = "lead") - numviews, by = Userid][
  xeventinweek != 0L]

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

Answer 2

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