Coerce lists and data.frames to 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.

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)

Arguments

x	A named or unnamed list, data.frame or data.table.
keep.rownames	For data.frames, TRUE retains the data.frame's row names under a new column rn. keep.rownames = "id" names the column "id" instead.
key	Character vector of one or more column names which is passed to setkeyv. It may be a single comma separated string such as key="x,y,z", or a vector of names such as key=c("x","y","z").
check.names	Just as check.names in data.frame.

Details

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(.), as this will make a complete copy of the input object before to convert it to a data.table. The setDT function takes care of this issue by allowing to convert lists - both named and unnamed lists and data.frames by reference instead. That is, the input object is modified in place, no copy is being made.

Value

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.

See also

data.table, as.data.table, setDF, copy, setkey, setcolorder, setattr, setnames, set, :=, setorder

Examples

set.seed(45L)
X = data.frame(A=sample(3, 10, TRUE),
         B=sample(letters[1:3], 10, TRUE),
         C=sample(10), stringsAsFactors=FALSE)

# Convert X to data.table by reference and
# get the frequency of each "A,B" combination
setDT(X)[, .N, by=.(A,B)]
#>    A B N
#> 1: 1 b 1
#> 2: 3 a 3
#> 3: 2 a 1
#> 4: 3 c 1
#> 5: 2 c 1
#> 6: 3 b 2
#> 7: 1 c 1

# 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")[]
#>    a         b
#> 1: 1 0.3396133
#> 2: 2 0.5762478
#> 3: 3 0.4272740
#> 4: 4 0.5409554

# check.names argument
X = list(a=1:5, a=6:10)
setDT(X, check.names=TRUE)[]
#>    a a.1
#> 1: 1   6
#> 2: 2   7
#> 3: 3   8
#> 4: 4   9
#> 5: 5  10