How to set multiple columns in a data table to values from different columns in the same data table?

data.table, r

Solution

You can use the `:=` operator and name column numbers in `.SD`:

dt[, 1:3 := .SD, .SDcols = 4:6]

> dt
   X1 X2 X3 X4 X5 X6
1: 43 75 29 43 75 29
2: 88 17  1 88 17  1
3: 83 51 28 83 51 28
4: 32 10 81 32 10 81
5: 63 21 25 63 21 25

Problem

Suppose I have a data frame with 6 columns, and I want to set col 1:3 to the values in col 4:6 (this comes up a lot when merging). With data frames it's easy: ``` set.seed(1) df <- data.frame(matrix(sample(1:100,30),ncol=6)) df # X1 X2 X3 X4 X5 X6 # 1 27 86 19 43 75 29 # 2 37 97 16 88 17 1 # 3 57 62 61 83 51 28 # 4 89 58 34 32 10 81 # 5 20 6 67 63 21 25 df[,1:3] <- df[,4:6] # very, very straightforward... df # X1 X2 X3 X4 X5 X6 # 1 43 75 29 43 75 29 # 2 88 17 1 88 17 1 # 3 83 51 28 83 51 28 # 4 32 10 81 32 10 81 # 5 63 21 25 63 21 25 ``` With data.tables, not so much: ``` library(data.table) dt <- data.table(df) dt[,1:3,with=F] <- dt[,4:6,with=F] ## Error in `[<-.data.table`(`*tmp*`, , 1:3, with = F, value = list(X4 = c(43L, : unused argument (with = F) ``` This works, but seems extremely complicated for such a simple transformation: ``` dt[, names(dt)[1:3]:=dt[,4:6,with=F]] # very, very complicated... dt # X1 X2 X3 X4 X5 X6 # 1: 43 75 29 43 75 29 # 2: 88 17 1 88 17 1 # 3: 83 51 28 83 51 28 # 4: 32 10 81 32 10 81 # 5: 63 21 25 63 21 25 ``` The question is: is there a simpler way to assign one set of columns in a data table to the values from another set of columns in the same data table?

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