Get the right number sequence

i want to create a column where it looks like this:

1.1

1.2

1.3

1.4

1.5

1.5

and so on, as long as there is no more value in the other column

and the same i want to do for another table with

2.1

2.2

2.3

2.4

and so on, what is the easiest method? stringmanipulation node?