Is it possible to modify a data.frame in-place (destructively)?

Question

It appears that one can add/delete a column to a data.table in-place, i.e., without copying all the other columns over to a new table.

Is it possible to do that with a vanilla data.frame?

PS. I know how to add/delete columns "functionally", i.e., creating a new frame without modifying the original one.

Answer 1

You can delete or modify an existing column from a data.frame by reference with data.table::set. I doubt you can add a column without making a copy. The reason that you can add a column to a data.table without making a copy is that data.table over allocates memory. See ?alloc.col for more.

R> library(data.table) R> data(mtcars) R> tracemem(mtcars) [1] "<0x59fef68>" R> set(mtcars, j="mpg", value=NULL)        # remove a column R> set(mtcars, j="cyl", value=rep(42, 32)) # modify a column R> untracemem(mtcars) R> str(mtcars) 'data.frame':   32 obs. of  10 variables:  $ cyl : num  42 42 42 42 42 42 42 42 42 42 ...  $ disp: num  160 160 108 258 360 ...  $ hp  : num  110 110 93 110 175 105 245 62 95 123 ...  $ drat: num  3.9 3.9 3.85 3.08 3.15 2.76 3.21 3.69 3.92 3.92 ...  $ wt  : num  2.62 2.88 2.32 3.21 3.44 ...  $ qsec: num  16.5 17 18.6 19.4 17 ...  $ vs  : num  0 0 1 1 0 1 0 1 1 1 ...  $ am  : num  1 1 1 0 0 0 0 0 0 0 ...  $ gear: num  4 4 4 3 3 3 3 4 4 4 ...  $ carb: num  4 4 1 1 2 1 4 2 2 4 ... 

Compare that with normal data.frame operations

R> data(mtcars) R> tracemem(mtcars) [1] "<0x6b3ec30>" R> mtcars[, "mpg"] <- NULL tracemem[0x6b3ec30 -> 0x84de0c8]:  tracemem[0x84de0c8 -> 0x84de410]: [<-.data.frame [<-  tracemem[0x84de410 -> 0x84de6b0]: [<-.data.frame [<-  R> tracemem(mtcars) [1] "<0x84dca30>" R> mtcars[, "cyl"] <- rep(42, 32) tracemem[0x84dca30 -> 0x84dcc28]:  tracemem[0x84dcc28 -> 0x84dd018]: [<-.data.frame [<-  tracemem[0x84dd018 -> 0x84dff70]: [<-.data.frame [<-  R> untracemem(mtcars) R> data(mtcars)