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 'key_' that contains the row-by-row concatenated values of a and b separated by '_':

   a   b  key_
1: 1 foo 1_foo
2: 2 bar 2_bar

For the hard-coded case, there is a simple solution:

x[, key_ := paste0(a, '_', b)]

How could I achieve this in the more generic case when the columns to be concatenated are given as an arbitrarily long character vector?

Thanks for your help!

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.