Perform a semi-join with data.table

Question

How do I perform a semi-join with data.table? A semi-join is like an inner join except that it only returns the columns of X (not also those of Y), and does not repeat the r

Answer 1

The package dplyr supports the following four join types:

inner_join, left_join, semi_join, anti_join

So for the semi-join try the following code

library("dplyr")

table1 <- data.table(x = 1:2, y = c("a", "b"))
table2 <- data.table(x = c(1, 1), z = 10:11)

semi_join(table1, table2)

The output is as expected:

# Joining by: "x"
# Source: local data table [1 x 2]
# 
#       x     y
#   (int) (chr)
# 1     1     a