How to Replace the Missing Values with the Preceding Value within Group in Stata?

[smith Jason]

I have a dataset like this,
clear
input byte (id state)
1 0
1 .
1 1
2 0
2 .
2 1
3 0
3 .
3 1
3 .
3 .
3 .
3 0
3 .
4 0
4 .
4 .
4 .
4 .
4 .
4 .
4 1
end

I want to replace the missing values with the preceding value within id.
For example, when id==3, the filled data looks like as follows,
3 0
3 0
3 1
3 1
3 1
3 1
3 0
3 0
Can someone help me with Stata code?
Thank you!

[Carlo Lazzaro]

Jason:
being aware of the methodological risks that the LOCF approach comes with, you may want to try:

. bysort id: replace state=state[_n-1] if state[_n-1]!=. & state[_n]==.
(13 real changes made)

. list

     +------------+
     | id   state |
     |------------|
  1. |  1       0 |
  2. |  1       0 |
  3. |  1       1 |
  4. |  2       0 |
  5. |  2       0 |
     |------------|
  6. |  2       1 |
  7. |  3       0 |
  8. |  3       0 |
  9. |  3       1 |
 10. |  3       1 |
     |------------|
 11. |  3       1 |
 12. |  3       1 |
 13. |  3       0 |
 14. |  3       0 |
 15. |  4       0 |
     |------------|
 16. |  4       0 |
 17. |  4       0 |
 18. |  4       0 |
 19. |  4       0 |
 20. |  4       0 |
     |------------|
 21. |  4       0 |
 22. |  4       1 |
     +------------+

.

[smith Jason]

Thank you!

[Nick Cox]

This is also an FAQ: https://www.stata.com/support/faqs/d...issing-values/