Merge data sets by row differening columns [duplicate]

Answer 1

Using data.table::rbindlist with fill = TRUE option:

data.table::rbindlist(
  list(data.frame(x1,x2,x3,x4,x5), 
       data.frame(x1,x3,x4,x5),
       data.frame(x2,x3,x4,x5), 
       data.frame(x1,x2,x3,x4,x5)),
  fill = TRUE)