Fast column reordering of a data.table by reference

In data.table parlance, all set* functions change their input by reference. That is, no copy is made at all, other than temporary working memory, which is as large as one column. The only other data.table operator that modifies input by reference is :=. Check out the See Also section below for other set* function data.table provides.

setcolorder reorders the columns of data.table, by reference, to the new order provided.

Usage

setcolorder(x, neworder=key(x), before=NULL, after=NULL)

Arguments

x: A data.table.
neworder: Character vector of the new column name ordering. May also be column numbers. If length(neworder) < length(x), the specified columns are moved in order to the "front" of x. By default, setcolorder without a specified neworder moves the key columns in order to the "front" of x.
before, after: If one of them (not both) was provided with a column name or number, neworder will be inserted before or after that column.

Details

To reorder data.table columns, the idiomatic way is to use setcolorder(x, neworder), instead of doing x <- x[, ..neworder] (or x <- x[, neworder, with=FALSE]). This is because the latter makes an entire copy of the data.table, which maybe unnecessary in most situations. setcolorder also allows column numbers instead of names for neworder argument, although we recommend using names as a good programming practice.

Value

The input is modified by reference, and returned (invisibly) so it can be used in compound statements. If you require a copy, take a copy first (using DT2 = copy(DT)). See ?copy.

Examples


set.seed(45L)
DT = data.table(A=sample(3, 10, TRUE),
         B=sample(letters[1:3], 10, TRUE), C=sample(10))

setcolorder(DT, c("C", "A", "B"))

#incomplete specification
setcolorder(DT, "A")

# insert new column as first column
set(DT, j="D", value=sample(10))
setcolorder(DT, "D", before=1)

# move column to last column place
setcolorder(DT, "A", after=ncol(DT))