Creates a Join data table
Creates a data.table to be passed in as the i to a [.data.table join.
# DT[J(...)] # J() only for use inside DT[...]. SJ(...) # DT[SJ(...)] CJ(...) # DT[CJ(...)]
- Each argument is a vector. Generally each vector is the same length but if they are not then usual silent repitition is applied.
CJ are convenience functions for creating a data.table in the context of a data.table 'query' on
x[data.table(id)] is the same as
x[J(id)] but the latter is more readable. Identical alternatives are
x must have a key when passing in a join table as the
- J : the same result as calling list. J is a direct alias for list but results in clearer more readable code.
SJ : (S)orted (J)oin. The same value as J() but additionally setkey() is called on all the columns in the order they were passed in to SJ. For efficiency, to invoke a binary merge rather than a repeated binary full search for each row of
i. CJ : (C)ross (J)oin. A data.table is formed from the cross product of the vectors. For example, 10 ids, and 100 dates, CJ returns a 1000 row table containing all the dates for all the ids.
DT = data.table(A=5:1,B=letters[5:1]) setkey(DT,B) # re-orders table and marks it sorted. DT[J("b")] # returns the 2nd row DT[.("b")] # same. Style of package plyr. DT[list("b")] # same