Adding values in two data.tables

Question

I have two data.tables, and one has a subset of rows/columns of another. I\'d like to add values of the smaller data.table to the values of the larger one:

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Answer 1

I prefer Richard's way, but here's an alternative that looks more like the OP's initial idea:

vs = setdiff(names(DT1),"rn")
DT2[DT1, (vs) := {
  x.SD = mget(vs) 
  i.SD = mget(paste0("i.",vs)) 
  Map("+", x.SD, i.SD)
}, on="rn", by=.EACHI]
#    rn a b c
# 1:  a 0 4 1
# 2:  b 1 5 0
# 3:  c 1 1 3

Answer 2

You can use rbindlist() to bring the two together, then sum the values based on rn

rbindlist(list(DT1, DT2), fill=TRUE)[, lapply(.SD, sum, na.rm = TRUE), by = rn]
#    rn a b c
# 1:  a 0 4 1
# 2:  b 1 5 0
# 3:  c 1 1 3