cartesian product with dplyr R

Use crossing from the tidyr package:

x <- data.frame(x=c("a","b","c"))
y <- data.frame(y=c(1,2,3))

crossing(x, y)

Result:

   x y
 1 a 1
 2 a 2
 3 a 3
 4 b 1
 5 b 2
 6 b 3
 7 c 1
 8 c 2
 9 c 3

Apologies to all: the below example does not appear to work with data.frames or data.tables.

When x and y are database tbls (tbl_dbi / tbl_sql) you can now also do:

full_join(x, y, by = character())

Added to dplyr at the end of 2017, and also gets translated to a CROSS JOIN in the DB world. Saves the nastiness of having to introduce the fake variables.