Match in lagged group in data.table

Question

I'm trying to create a new column that indicates if an ID was present in a previous group. Here's my data:

data <- data.table(ID = c(1:3, c(9,2,3,4),c(5,1)),
                   groups = c(rep(c("a", "b", "c"), c(3, 4,2))))
   ID groups
1:  1      a
2:  2      a
3:  3      a
4:  9      b
5:  2      b
6:  3      b
7:  4      b
8:  5      c
9:  1      c

I'm not sure how to specify lagged groups. I tried to use shift, but it's not working:

data[,.(ID=ID,match_lagged=ID %in% shift(ID)),by=groups]

Here's my desired result.

The first 3 lines are not matched because there is no previous group. FALSE would also work for these three rows. ID=4 (in group b) is not matched in group a. ID=5 (in group c) is not matched in group b.

Note that ID 1 in group c is not matched in group b so it should be false even though it exists in group a. This is why duplicated(data$ID) does not work. Data from a group has to be matched from the previous group.

groups ID match_lagged
1:      a  1         NA
2:      a  2         NA
3:      a  3         NA
4:      b  9         FALSE
5:      b  2         TRUE
6:      b  3         TRUE
7:      b  4         FALSE
8:      c  5         FALSE
9:      c  1         FALSE

A dplyr solution would also work.

Answer 1

Number the groups, and then check if the diff is equal to one for each ID.

data[, grp.id := .GRP, by = groups]
data[, match_lagged := c(FALSE, diff(grp.id) == 1), by = ID][
     grp.id == 1, match_lagged := NA][]
#   ID groups grp.id match_lagged
#1:  1      a      1           NA
#2:  2      a      1           NA
#3:  3      a      1           NA
#4:  9      b      2        FALSE
#5:  2      b      2         TRUE
#6:  3      b      2         TRUE
#7:  4      b      2        FALSE
#8:  5      c      3        FALSE
#9:  1      c      3        FALSE

This assumes that you find each ID only once in each group. If that's not the case you can unique, do the above, and then merge in.