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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.

A helper function to convert a data.table or list of equal length to data.frame by reference.


setDF(x, rownames=NULL)



A data.table, data.frame or list of equal length.


A character vector to assign as the row names of x.


All data.table attributes including any keys and indices of the input data.table are stripped off.

When using rownames, recall that the row names of a data.frame must be unique. By default, the assigned set of row names is simply the sequence 1, ..., nrow(x) (or length(x) for lists).


The input data.table is modified by reference to a data.frame and returned (invisibly). If you require a copy, take a copy first (using DT2 = copy(DT)). See ?copy.


X = data.table(x=1:5, y=6:10)
## convert 'X' to data.frame, without any copy.

X = data.table(x=1:5, y=6:10)
## idem, assigning row names
setDF(X, rownames = LETTERS[1:5])

X = list(x=1:5, y=6:10)
# X is converted to a data.frame without any copy.