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.
setDT converts lists (both named and unnamed) and data.frames to data.tables by reference. This feature was requested on Stackoverflow.
setDT(x, keep.rownames=FALSE, key=NULL, check.names=FALSE)The input is modified by reference, and returned (invisibly) so it can be used in compound statements; e.g., setDT(X)[, sum(B), by=A]. If you require a copy, take a copy first (using DT2 = copy(DT)). See ?copy.
A named or unnamed list, data.frame or data.table.
For data.frames, TRUE retains the data.frame's row names under a new column rn. keep.rownames = "id" names the column "id" instead.
Character vector of one or more column names which is passed to setkeyv.
Just as check.names in data.frame.
When working on large lists or data.frames, it might be both time- and memory-consuming to convert them to a data.table using as.data.table(.), which will make a complete copy of the input object before converting it to a data.table. setDT takes care of this issue by converting any list (named or unnamed, data.frame or not) by reference instead. That is, the input object is modified in place with no copy.
This should come with low overhead, but note that setDT does check that the input is valid by looking for inconsistent input lengths and inadmissible column types (e.g. matrix).
data.table, as.data.table, setDF, copy, setkey, setcolorder, setattr, setnames, set, :=, setorder
set.seed(45L)
X = data.frame(
A=sample(3, 10, TRUE),
B=sample(letters[1:3], 10, TRUE),
C=sample(10))
# Convert X to data.table by reference and
# get the frequency of each "A,B" combination
setDT(X)[, .N, by=.(A,B)]
# convert list to data.table
# autofill names
X = list(1:4, letters[1:4])
setDT(X)
# don't provide names
X = list(a=1:4, letters[1:4])
setDT(X, FALSE)
# setkey directly
X = list(a = 4:1, b=runif(4))
setDT(X, key="a")[]
# check.names argument
X = list(a=1:5, a=6:10)
setDT(X, check.names=TRUE)[]
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