Store value of a variable to reuse it anywhere else in data

[augustcrez]

Hi all, 

I have a data as following 

Subj tpt grpid

1 24 bd

1 36 bid

1 72

And many more. I want when my tpt is 72 and irrespective of grpid missing or non missing should take grpid of tpt=24

So my data should look

Subj tpt grpid

1 24 qd

1 36 bid

1 72 qd

Please help! 

[PeterClemmensen]

data have;
input Subj tpt grpid $;
infile datalines missover;
datalines;
1 24 bd
1 36 bid
1 72
;

data want(drop=temp);
   set have;
   if tpt=24 then temp=grpid;
   if tpt=72 then grpid=temp;
   retain temp;
run;

[Astounding]

Some questions that might determine whether a solution works or not:

Will each SUBJ always have a TPT=24 record? 

If so, will the TPT=24 record always come before the TPT=72 record?

If not, what should happen on the TPT=72 record?