SAS: Coding a dummy variable for a value of a variable by group within group - group-by

Question

I have a dataset of CASE_ID (x y and z), a set of multiple dates (including duplicate dates) for each CASE_ID, and a variable VAR. I would like to create a dummy variable DUMMYVAR by group within a group whereby if VAR="C" for CASE_ID x on some specific date, then DUMMYVAR=1 for all observations corresponding to CASE_ID x on with that date.
I believe that a Classic 2XDOW would be the key here but this is my third week using SAS and having difficulty getting this by two BY groups here.
I have referenced and attempted to write a variation of Haikuo's code here:
PROC SORT have;
by CASE_ID DATE;
RUN;
data want;
do until (last.DATE);
set HAVE;
by date notsorted;
if var='c' then DUMMYVAR=1;
do until (last.DATE);
set HAVE;
by DATE notsorted;
if DATE=1 then ????????
end;
run;

Answer 1

Change your BY statements to match the grouping you are doing. And in the second loop add a simple OUTPUT; statement. Then your new dataset will have all the rows in your original dataset and the new variable DUMMYVAR.
data want;
do until (last.DATE);
set HAVE;
by case_id date;
if var='c' then DUMMYVAR=1;
end;
do until (last.DATE);
set HAVE;
by case_id date;
output;
end;
run;
This will create the variable DUMMYVAR with values of either 1 or missing. If you want the values to be 1 or 0 then you could either set it to 0 before the first DO loop. Or add if first.date then dummyvar=0; statement before the existing IF statement.