Replace missing values with non-missing value

[Chen Samulsion]

Dear Stata users,

Suppose I have a variable x composed of missing values and non-missing values, and the non-missing values all are equal. I want to replace missing values with (this?) non-missing value, how can I achieve it? Thank you.
Take an example, I want to replace variable x with 12 if x are missing.

* Example generated by -dataex-. To install: ssc install dataex
clear
input float x
.
12
 .
12
 .
12
 .
 .
12
 .
end

[Ashish Bandhu]

Code
recode x (.=12)

[William Lisowski]

. egen newx = max(x)

. list, clean

        x   newx  
  1.    .     12  
  2.   12     12  
  3.    .     12  
  4.   12     12  
  5.    .     12  
  6.   12     12  
  7.    .     12  
  8.    .     12  
  9.   12     12  
 10.    .     12

More generally, suppose this is panel data with a panel identifier variable named id and a time variable named year. Then you may want

by id year, sort: egen newx = max(x)

which will give the nonmissing value separately for each combination of id and year.

[Chen Samulsion]

Thank you very much William Lisowski. I used summarize to get r(max) and used it in replace expression here. From time to time, I forget that egen function can do the same.