backward fill and forward fill

[Aphreven]

Hello I would like to do something very simple but that I struggle with : 

I have a table with several columns and 1 specifically that have missing values that i want to fill with a backward then a forward fill stragegy based on a ID column. 

have : 

ID   colA  colB    Coltofill                       

1      ...        ...                     

1                                                                  

1                               5    

2                              12    

2                              

want : 

ID   colA  colB    Coltofill                       

1      ...        ...           5        

1                               5                                   

1                               5    

2                              12    

2                              12

More precisely i want to only fill the col "Coltofill". I found a good reponse in this post : https://communities.sas.com/t5/SAS-Programming/Backward-and-Forward-Filling-Information/m-p/699624  But the problem is that it fills every columns. 

Thank you !

[Kurt_Bremser · Accepted solution]

proc sql;
create table want as
  select
    id, cola, colb,
    max(coltofill) as coltofill
  from have
  group by id
;
quit;