R data table: update join

Question

Suppose I have two data tables:

X <- data.table(id = 1:5, L = letters[1:5])

   id L
1:  1 a
2:  2 b
3:  3 c
4:  4 d
5:  5 e

Y <- data.table(id = 3:5,         


        

Answer 1

It is an update join:

library(data.table)
X <- data.table(id = 1:5, L = letters[1:5])
Y <- data.table(id = 3:5, L = c(NA, "g", "h"), N = c(10, NA, 12))
X[Y, on=.(id), c("L", "N"):=.(i.L, i.N)][]
#    id  L  N
# 1:  1  a NA
# 2:  2  b NA
# 3:  3 NA 10
# 4:  4  g NA
# 5:  5  h 12

gives you the desired result.
Here I found a solution for multiple columns:

library(data.table)
X <- data.table(id = 1:5, L = letters[1:5])
Y <- data.table(id = 3:5, L = c(NA, "g", "h"), N = c(10, NA, 12))

X[Y, on=.(id), names(Y)[-1]:=mget(paste0("i.", names(Y)[-1]))]

Another variant:

n <- names(Y)
X[Y, on=.(id), (n):=mget(paste0("i.", n))]