So I think my question is best shown with a set of pictures.

This is what my current data looks like:
id date month # growth value
1 June 1975 1 12
2 June 1975 1 .
3 June 1975 1 11
1 July 1975 2 .
2 July 1975 2 .
3 July 1975 2 .
1 Aug 1975 3 .
2 Aug 1975 3 .
3 Aug 1975 3 .
... ................. ............ ...................
1 May 1976 12 .
2 May 1976 12 .
3 May 1976 12 .
1 June 1976 13 9
2 June 1976 13 8
3 June 1976 13 11
1 July 1976 14 .
2 July 1976 14 .
3 July 1976 14 .


























So what you will notice is that For months 1 and 13, my column "growth value" has data, and everything inbetween or after is empty.

What I would like is the that months 2-12 use the growth value corresponding to month 1,
and months 14-26 use the growth value corresponding to month 13, and so on.

So my final data should look like this:
id date month # growth value
1 June 1975 1 12
2 June 1975 1 .
3 June 1975 1 11
1 July 1975 2 12
2 July 1975 2 .
3 July 1975 2 11
1 Aug 1975 3 12
2 Aug 1975 3 .
3 Aug 1975 3 11
... ................. ............ ...................
1 May 1976 12 12
2 May 1976 12 .
3 May 1976 12 11
1 June 1976 13 9
2 June 1976 13 8
3 June 1976 13 11
1 July 1976 14 9
2 July 1976 14 8
3 July 1976 14 11



























What would be the best way of achieving this? In my mind, I have something like this planned:

If (growth value) is not empty, then (growth value + 1) = growth value
--> repeat 12 times until growth value + 11 is reached, and do it by id and month#.

I just wasn't sure how to code this. I would appreciate any help.