J
Creates a Join data table
Creates a data.table to be passed in as the i to a [.data.table join.
- Keywords
- data
Usage
# DT[J(...)] # J() only for use inside DT[...].
SJ(...) # DT[SJ(...)]
CJ(..., sorted = TRUE, unique = FALSE) # DT[CJ(...)]Arguments
- …
Each argument is a vector. Generally each vector is the same length but if they are not then the usual silent repetition is applied.
- sorted
logical. Should the input be sorted (ascending order)? If
FALSE, the input order is retained.- unique
logical. When
TRUE, only unique values of each vectors are used (automatically).
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
Value
J: the same result as calling list.Jis a direct alias for list but results in clearer more readable code.SJ: (S)orted (J)oin. The same value asJ()but additionallysetkey()is called on all the columns in the order they were passed in toSJ. For efficiency, to invoke a binary merge rather than a repeated binary full search for each row ofi.CJ: (C)ross (J)oin. Adata.tableis formed from the cross product of the vectors. For example, 10 ids, and 100 dates,CJreturns a 1000 row table containing all the dates for all the ids. It gainssorted, which by default isTRUEfor backwards compatibility.FALSEretains input order.
See Also
Examples
# NOT RUN {
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
# CJ usage examples
CJ(c(5,NA,1), c(1,3,2)) # sorted and keyed data.table
do.call(CJ, list(c(5,NA,1), c(1,3,2))) # same as above
CJ(c(5,NA,1), c(1,3,2), sorted=FALSE) # same order as input, unkeyed
# use for 'unique=' argument
x = c(1,1,2)
y = c(4,6,4)
CJ(x, y) # output columns are automatically named 'x' and 'y'
CJ(x, y, unique=TRUE) # unique(x) and unique(y) are computed automatically
# }