Retain with multiple first statements

[Tpham]

This is the data I have and want.

data want;
input ID visit question response v1_response;
cards;
1 1 1 4 4
1 2 1 3 4
1 1 2 6 6
1 2 2 9 6
;

Basically, I want to create variable v1_response, where it shows the visit 1 response by ID and question in order to calculate the change from visit 1 score for each question. I know how to do this when it is only 1 question in the dataset. But my approach is keeping only the value of the first question across all questions. 

This is my code so far

data have;
input ID visit question response;
cards;
1	1	1	4
1	2	1	3
1	1	2	6
1	2	2	9
;

proc sort data=have;
by ID visit question;
run;

data want;
set have;
by ID visit question;

retain v1_response;

if first.ID  AND first.question and visit=1 then do;
	v1_response=response;
end;
else if visit GT 1 do;
	change=response-v1_response;
end;

run;

[novinosrin · Accepted solution]

data want;
input ID visit question response v1_response;
cards;
1 1 1 4 4
1 2 1 3 4
1 1 2 6 6
1 2 2 9 6
;

data want2;
 set want;
 by id question;
 retain want;
 if first.question then want=response;
run;

[Tpham]

THanks, this did work for me. So basically, i don't sort it by visit also it looks like.

However, how does SAS know, which is the first visit?

[Kurt_Bremser]

Because of the by statement, SAS keeps track of the by variables, and sets the first. and last. variables at every group change.

[Kurt_Bremser]

Keep the order as is, run the data step with

by id question;

and set v1_response at first.question.