Assigning a Count Conditionally

[DBrunson]

Hello all,

    I want to look at the rows in my table and assign a counting variable to increase by 1 if a certain set of conditions are met. 

In the following table, for each study ID WANT needs to start counting at the LEGACY value. PLT lists the initial donation number for each participant. Then it needs to increase by 1 if the DonationType=1. If the donation type is ^=1 then the cell needs to be missing. Right now, I can get it to fill in the correct value for the first and instance of each study ID, but it will not fill down. 

STUDYID	DonationNumb	DONTYPE	LEGACY	PLT	WANT	HAVE
12	1	1	9	10	10	10
12	2	1	.		11	11
12	3	2	.		.
12	4	1	.		12
12	5	1			13
12	6	1	.		14
13	1	2	0	0	.	0
13	2	1	.		1	1
13	3	1	.		2

Data want;
	set have;

retain HAVE;
HAVE=lag(HAVE);
if DONATIONNUMB=1 then HAVE=PLT;
if (DONATIONNUMB>1 AND DONATIONTYPE=1) then HAVE=(HAVE)+1;
else if(DONATIONNUMB>1 AND DONATIONTYPE>1) then HAVE=HAVE+0;
run;

Any help is greatly appreciated!

[Kurt_Bremser · Accepted solution]

data want;
set have;
by studyid;
if first.studyid
then do;
  _have = plt;
  if dontype = 1
  then have = _have;
  else have = .;
end;
else do;
  if dontype = 1
  then do;
    _have + 1; /* sum statement causes automatic retain */
    have = _have;
  end;
  else have = .;
end;
drop _have;
run;

The LAG() can't be reasonably used with a newly created variable; it fills its queue every time it is called, and at that moment the variable is still missing, or has the previous value anyway, because of the RETAIN.