Copy values based on certain criteria

[lpy0521]

Hi, there

I am curious how to achieve this in SAS. My data is sorted by id and date. for id "a", if the days between date in row 3 and row 4 is equal to 3, then I want to copy var1 to var2. Otherwise, in case id"b", if the days between date in row 3 and row 4 is not equal to 3, then leave it blank. 

Thanks in advance!!!

data have;
infile cards dlm=',';			
Input ID$ date :ddmmyy10. var1 var2;
format date YYMMDDn8.; 
cards;
a,	01/01/2018,	.	,111,
a,	02/01/2018,	.	,111,
a,	03/01/2018,	.	,111,
a,	06/01/2018,	222	,.,
a,	07/01/2018,	222	,.,
b,	08/01/2018,	.	,111,
b,	09/01/2018,	.	,111,
b,	10/01/2018,	.	,111,
b,	11/01/2018,	222	,.,
b,	12/01/2018,	222	,.,
;			
run;

data want;
infile cards dlm=',';			
Input ID$ date :ddmmyy10. var1 var2;
format date YYMMDDn8.; 
cards;
a,	01/01/2018,	.	,111,
a,	02/01/2018,	.	,111,
a,	03/01/2018,	.	,111,
a,	06/01/2018,	222	,222,
a,	07/01/2018,	222	,.,
b,	08/01/2018,	.	,111,
b,	09/01/2018,	.	,111,
b,	10/01/2018,	.	,111,
b,	11/01/2018,	222	,.,
b,	12/01/2018,	222	,.,
;			
run;

[PaigeMiller]

data want;
	set have;
    by id;
	prev_date=lag(date);
	if date-prev_date=3 and not first.id then var2=var1;
    drop prev_date;
run;

[novinosrin]

data have;
infile cards dlm=',';			
Input ID$ date :ddmmyy10. var1 var2;
format date YYMMDDn8.; 
cards;
a,	01/01/2018,	.	,111,
a,	02/01/2018,	.	,111,
a,	03/01/2018,	.	,111,
a,	06/01/2018,	222	,.,
a,	07/01/2018,	222	,.,
b,	08/01/2018,	.	,111,
b,	09/01/2018,	.	,111,
b,	10/01/2018,	.	,111,
b,	11/01/2018,	222	,.,
b,	12/01/2018,	222	,.,
;			
run;

data want;
do _n_=1 by 1 until(last.id);
set have;
by id;
if _n_=4 and date-lag(date)=3 then var2=var1;
output;
end;
run;