Combining chaining and assignment by reference in a data.table

Question

Is it possible to combine chaining and assignment by reference in a data.table?

For example, I would like to do this:

DT[a == 1][b == 0, c := 2]

However, this leaves the original table unchanged, as a temporary table seems to be created after DT[a == 1] which is subsequently changed and returned.

I would rather not do

DT[a == 1 & b == 0, c := 2]

as this is very slow and I would also rather avoid

 DT <- DT[a == 1][b == 0, c := 2]

as I would prefer to do the assignment by reference. This question is part of the question [1], where it is left unanswered.

[1] Conditional binary join and update by reference using the data.table package

Answer 1

I'm not sure why you think that even if DT[a == 1][b == 0, c := 2] worked in theory it would be more efficient than DT[a == 1 & b == 0, c := 2]

Either way, the most efficient solution in your case would be to key by both a and b and conduct the assignment by reference while performing a binary join on both

DT <- data.table(a = c(1, 1, 1, 2, 2), b = c(0, 2, 0, 1, 1)) ## mock data
setkey(DT, a, b) ## keying by both `a` and `b`
DT[J(1, 0), c := 2] ## Update `c` by reference
DT
#    a b  c
# 1: 1 0  2
# 2: 1 0  2
# 3: 1 2 NA
# 4: 2 1 NA
# 5: 2 1 NA