Retain a variable value till condition is met - date

Question

I have dataset which has 3 variables which includes ID, date and enrol, and it is sorted by ID and date. My goal is to create a new dataset which has another variable say enrol_flag which should be 1 once the enrol is 1 but it should become 0 once enrol is 0 and retains that 0 value.
I am using SAS to program.
My dataset looks like:
ID Date enrol
1 01JAN21 0
1 O1FEB21 1
1 01MAR21 1
1 01APR21 1
1 01MAY21 0
1 01JUN21 0
1 01JUL21 1
1 01AUG21 1
2 01JAN21 1
2 O1FEB21 1
2 01MAR21 1
2 01APR21 1
2 01MAY21 1
2 01JUN21 0
2 01JUL21 0
2 01AUG21 0
3 O1FEB21 0
3 01MAR21 0
3 01APR21 0
3 01MAY21 0
3 01JUN21 0
3 01JUL21 1
Output I want
ID Date enrol enrol_flag
1 01JAN21 0 0
1 O1FEB21 1 1
1 01MAR21 1 1
1 01APR21 1 1
1 01MAY21 0 0
1 01JUN21 0 0
1 01JUL21 1 0
1 01AUG21 1 0
2 01JAN21 1 1
2 O1FEB21 1 1
2 01MAR21 1 1
2 01APR21 1 1
2 01MAY21 1 1
2 01JUN21 0 0
2 01JUL21 0 0
2 01AUG21 0 0
3 O1FEB21 0 0
3 01MAR21 0 0
3 01APR21 0 0
3 01MAY21 0 0
3 01JUN21 0 0
3 01JUL21 1 1
Thank you in advance for your help.

Answer 1

Try this
data have;
input ID Date :date7. enrol;
format Date date7.;
datalines;
1 01JAN21 0
1 01FEB21 1
1 01MAR21 1
1 01APR21 1
1 01MAY21 0
1 01JUN21 0
1 01JUL21 1
1 01AUG21 1
2 01JAN21 1
2 01FEB21 1
2 01MAR21 1
2 01APR21 1
2 01MAY21 1
2 01JUN21 0
2 01JUL21 0
2 01AUG21 0
3 01FEB21 0
3 01MAR21 0
3 01APR21 0
3 01MAY21 0
3 01JUN21 0
3 01JUL21 1
;
data want;
set have;
by ID Date;
if first.ID then do;
enrol_flag = enrol;
d = 0;
end;
if enrol = 1 & d = 0 then do;
enrol_flag = 1;
d = 1;
end;
if enrol = 0 and d = 1 then enrol_flag = 0;
retain enrol_flag d;
run;

Answer 2

The sequences of enrol with in group id can form bands of 0's and 1's. You want to set flag to zero for every point after enrol transitions from 1 to 0, thus you need a secondary variable to track that case for enforcing the 0 rule.
Example (DOW loop):
data have;
input id date enrol;
attrib date informat=date7. format=date7.;
datalines;
1 01JAN21 0
1 01FEB21 1
1 01MAR21 1
1 01APR21 1
1 01MAY21 0
1 01JUN21 0
1 01JUL21 1
1 01AUG21 1
2 01JAN21 1
2 01FEB21 1
2 01MAR21 1
2 01APR21 1
2 01MAY21 1
2 01JUN21 0
2 01JUL21 0
2 01AUG21 1
3 01FEB21 0
3 01MAR21 0
3 01APR21 0
3 01MAY21 0
3 01JUN21 0
3 01JUL21 1
;
data want (keep=id date enrol flag);
do until (last.id);
set have;
by id;
_d = dif(enrol); /* unconditional place ensures consistent dif */
if first.id then flag=enrol;
else
if _d = -1 then _z = 1; /* track transition 1 -> 0 */
if _z
then flag = 0; /* enforce rule */
else flag = enrol;
output;
end;
run;