data.table paste selected columns by index

Question

I have a data.table as follows

DT <- structure(list(V1 = structure(1:3, .Label = c("S01", "S02", "S03"), class = "factor"), V2 = structure(c(1L, 3L, 2L), .Label = c("Alan", "Bruce", "Jay"), class = "factor"), V3 = structure(c(3L, 1L, 2L), .Label = c("Barry", "Dick", "Hal"), class = "factor"), V4 = structure(c(1L, 3L, 2L), .Label = c("Guy", "Jean-Paul", "Wally"), class = "factor"), V5 = structure(c(3L, 1L, 2L), .Label = c("Bart", "Damien", "John"), class = "factor")), .Names = c("V1", "V2", "V3", "V4", "V5"), class = "data.frame", row.names = c(NA, -3L))
setDT(DT)
setkey(DT, V1)

I am trying to paste together selected columns (selcol) in DT to a new column.

selcol = c("V3", "V4")

I know that DT[, "NEW" := paste0(V3, V4), with = FALSE] does the trick. But I would like to use selcol in the code.

I have tried the following

DT[, "NEW" := paste0(which(colnames(DT) %in% selcol)), with = TRUE]
DT[, "NEW" := paste0(which(colnames(DT) %in% selcol)), with = TRUE]
DT[, "NEW" := paste0(3, 4), with = TRUE]
DT[, "NEW" := paste(3, 4), with = TRUE]

How to do this with data.table.

Answer 1

DT[, NEW := do.call(paste0, .SD), .SDcols = selcol] 
#    V1    V2    V3        V4     V5           NEW
#1: S01  Alan   Hal       Guy   John        HalGuy
#2: S02   Jay Barry     Wally   Bart    BarryWally
#3: S03 Bruce  Dick Jean-Paul Damien DickJean-Paul