How do I impute a missing value with a value I have for a given year (1999) within a panel dataset?

[Nico Ochmann]

Dear Stata Forum,

I have an unbalanced panel dataset for which I have a variable x whose value I only observe in the year 1999.
Within a given individual, I would like to replace the missings for this value before 1999 and after 1999 with the value I observe in 1999 for this individual.
I would highly appreciate your assistance with this matter.
Have a great day,
Best,
Nico

* Example generated by -dataex-. For more info, type help dataex
clear
input float(id year x)
  687 1991  .
 1367 1991  .
 1367 1992  .
 2051 1991  .
 2051 1992  .
 2051 1993  .
 2727 1991  .
 2727 1992  .
 2727 1993  .
 2727 1998  .
 2727 1999 -8
 2727 2000  .
 2727 2001  .
 3407 1991  .
 3407 1992  .
 3407 1993  .
 3407 1994  .
 3407 1998  .
 3407 1999  .
 3407 2001  .
 3407 2002  .
 3407 2005  .
 4091 1991  .
 4091 1992  .
 4091 1993  .
 4091 1994  .
 4091 1995  .
 4767 1991  .
 4767 1992  .
 4767 1993  .
 5451 1991  .
 5451 1992  .
 5451 1993  .
 5451 1995  .
 6135 1991  .
 6135 1992  .
 6135 1993  .
 6807 1991  .
 6807 1992  .
 6807 1993  .
 6882 1997  .
 6882 1998  .
 7487 1991  .
 7487 1992  .
 7487 1993  .
 7487 1994  .
 7487 1996  .
 8167 1991  .
 8167 1992  .
 8167 1993  .
 8167 1995  .
 8167 1996  .
 8847 1991  .
 9527 1991  .
 9527 1992  .
 9527 1993  .
 9527 1994  .
 9527 1995  .
 9527 1996  .
 9527 1997  .
 9527 1998  .
 9527 1999 20
 9527 2000  .
 9527 2002  .
 9527 2003  .
10211 1991  .
12251 1991  .
12251 1992  .
12251 1993  .
12251 1994  .
12251 1995  .
12251 1997  .
12251 1998  .
12251 2000  .
12251 2001  .
12251 2002  .
12251 2003  .
12251 2004  .
12935 1991  .
12935 1992  .
12935 1993  .
12935 1995  .
12935 1997  .
12935 1998  .
12935 2000  .
13607 1991  .
14287 1991  .
14287 1992  .
14287 1993  .
14287 1994  .
14287 1995  .
14287 1996  .
14287 1997  .
14287 1998  .
14287 1999 16
14287 2000  .
14971 1991  .
14971 1992  .
14971 1993  .
14971 1994  .
end

[Andrew Musau]

for which I have a variable x whose value I only observe in the year 1999

The year is irrelevant here then. Just sort and replace. Nonmissing numerical values are sorted before missing.

bys id (x): replace x= x[1]

[Nico Ochmann]

Dear Andrew,
This is exactly what I needed.
Thanks so much for your help.
Have a great day!
Best,
Nico