Filling up and down missing values with non-missing values

[pp2014]

I have following data:

data have;
input id $2. tr1 $4. tr2 $4.;
cards;

12 AB4D
12 AB4D MN2X
13 AB4D MN2X
13 MN2X
14 AB4D MN2X
14 AB4D MN2X
run;

I want the output as follows:

ID    t r1         tr2

12   AB4D   MN2X
12   AB4D   MN2X
13   AB4D   MN2X
13   AB4D   MN2X
14   AB4D   MN2X
14   AB4D   MN2X

Can anybody help me in getting the missing values filled using SAS?

Thanks

[Reeza]

What are the rules?

[pp2014]

Sometimes particular ID may have missing tr2 Value and have only tr1 value. In that case keep only tr1 value and leave tr2 blank

for example

15  AB4D

15  AB4D

[Reeza]

I'm sorry, that's not clear to me. Hopefully someone else understands your question.

[Ksharp]

data have;
input id  tr1  $ tr2 $; 
cards;
12 AB4D .
12 AB4D MN2X
13 AB4D MN2X
13 .    MN2X
14 AB4D MN2X
14 AB4D MN2X 
;
run;
data temp1;
 set have;
 by id;
 if first.id then n=0;
 n+1;
run;
data temp2;
 update temp1(obs=0) temp1;
 by id;
 output;
run;
proc sort data=temp2;
by id descending n;
run;
data want;
 update temp2(obs=0) temp2;
 by id;
 output;
run;

[pp2014]

Thanks a lot.  It helped...