Dear Statalists,

I have a dataset in the following shape for several counties and years:

County Year Variable
1 1977 .
1 1978 .
1 1979 .
1 1980 3000
1 1981 3000
...
3 1977 .
3 1978 .
3 1979 .
3 1980 4000
3 1981 4000
...

For all counties, the values for the years 1977 to 1979 are missing for "variable" and I want to replace those missings with the value from the year 1980 of the respective county. I could find comments on how to fill in values from the top to the bottom but not vice versa. Does anyone have a suggestion for me? I suppose I will need a loop as I want to replace the values for several counties.
Thank you in advance!