J: Creates a Join data table

Description

Creates a data.table to be passed in as the i to a [.data.table join.

Usage

# DT[J(...)]    # J() only for use inside DT[...].
SJ(...)         # DT[SJ(...)]
CJ(...)         # DT[CJ(...)]

Arguments

...

Each argument is a vector. Generally each vector is the same length but if they are not then usual silent repitition is applied.

Value

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.

Details

SJ and CJ are convenience functions for creating a data.table in the context of a data.table 'query' on x. x[data.table(id)] is the same as x[J(id)] but the latter is more readable. Identical alternatives are x[list(id)] and x[.(id)]. x must have a key when passing in a join table as the i. See [.data.table

Examples

Run this code

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