Use a value from the previous row in an R data.table calculation

Using dplyr you could do:

mutate(DT, D = lag(B) + C)

Which gives:

#   A  B   C   D
#1: 1 10 100  NA
#2: 2 20 200 210
#3: 3 30 300 320
#4: 4 40 400 430
#5: 5 50 500 540

With shift() implemented in v1.9.6, this is quite straightforward.

DT[ , D := C + shift(B, 1L, type="lag")]
# or equivalently, in this case,
DT[ , D := C + shift(B)]

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From NEWS:

5. New function shift() implements fast lead/lag of vector, list, data.frames or data.tables. It takes a type argument which can be either "lag" (default) or "lead". It enables very convenient usage along with := or set(). For example: DT[, (cols) := shift(.SD, 1L), by=id]. Please have a look at ?shift for more info.

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See history for previous answers.