concatenate values across columns in data.table, row by row

Question

I have data.table like this:

x <- data.table(
       a = c( 1,     2),     
       b = c(\'foo\', \'bar\'))

I want to add a new column \

Answer 1

You can use do.call(), using .SDcols to supply the columns.

x[, key_ := do.call(paste, c(.SD, sep = "_")), .SDcols = names(x)]

.SDcols = names(x) supplies all the columns of x. You can supply any vector of names or column numbers there.