johnmackintosh

library(data.table)

?`[.data.table`

DT <- data.table(x=rep(c("b","a","c"),each=3), y=c(1,3,6), v=1:9)
X  <- data.table(x=c("c","b"), v=8:7, foo=c(4,2))

colnames(DT)
# [1] "x" "y" "v"