I have a dataframe (or datatable, if that’s easier) with incomplete rows:

ID Var1 Var2 Var3
1     2    5    1
2    12    3
3     8
4     4

The empty cells are always at the end of a row.

I would like to fill the empty cells in each row with the value in the last non-empty cell in that row, e.g.:

ID Var1 Var2 Var3
1     2    5    1
2    12    3 -> 3
3     8 -> 8 -> 8
4     4 -> 4 -> 4

How do I do that?