Don’t use the same for the data.frame and variables in it.
library(data.table)a <- data.table(a = c(1,2,3,1,2,3), b = c("a", "b", "a", "b","a", "b"))b <- a[b == "b"]data.table cannot distinguish which b is. b represents data.table b and the variable b in data.table a.
a[a %in% b$a]Error in b$a : $ operator is invalid for atomic vectors