Cartesian Product using data.table package

Question

Using the data.table package in R, I am trying to create a cartesian product of two data.tables using the merge method as one would do in base R.

In base the following w

Answer 1

merge.data.table(x, y) is a convenience function that wraps a call to x[y], so the merge needs to be based on columns that are in both data.tables. (That's what that error message is trying to tell you).

One work-around is to add a dummy column to both data.tables, whose only purpose is to make the merge possible:

## Add a column "k", and append it to each data.table's vector of keyed columns.
setkeyv(cust.dt[,k:=1], c(key(cust.dt), "k"))
setkeyv(dates.dt[,k:=1], c(key(dates.dt), "k"))

## Merge and then remove the dummy column
res <- merge(dates.dt, cust.dt, by="k")
head(res[,k:=NULL])
#          date first.name last.name
# 1: 2012-08-28     George     Smith
# 2: 2012-08-28      Henry     Smith
# 3: 2012-08-28       John       Doe
# 4: 2012-08-29     George     Smith
# 5: 2012-08-29      Henry     Smith
# 6: 2012-08-29       John       Doe

## Maybe also clean up cust.dt and dates.dt    
# cust.dt[,k:=NULL]
# dates.dt[,k=NULL]