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

More possibilities :

w = unique(x[y,which=TRUE])  # the row numbers in x which have a match from y
x[w]

If there are duplicate key values in x, then that needs :

w = unique(x[y,which=TRUE,allow.cartesian=TRUE])
x[w]

Or, the other way around :

setkey(y,x)
w = !is.na(y[x,which=TRUE,mult="first"])
x[w]

If nrow(x) << nrow(y) then the y[x] approach should be faster.
If nrow(x) >> nrow(y) then the x[y] approach should be faster.

But the anti anti join appeals too :-)

Answer 2

Try the following:

 w <- y[,unique(x)]
 x[x %in% w]

Output will be:

   x y
1: 1 a