Do loop to carry variable across IDs not working

[EpiNovice]

Hi Everyone,

I'm trying to carry a visit date across all rows of each participant ID, to create a start date column. I've used the below code to carry values across IDs in other coding, but now it just keeps telling me that _id is uninitialized. Is there something I'm missing? Is there a better way to do this?

Code:

data TOSStart(drop=_:);
do until (last.id);
set toscount;
by id;
if id > _id then do;
_Start = Start;
end;
end;
do until (last.id);
set toscount;
by id;
Start = _Start;
output;
end;
run;

Have this:

ID	Start
1	1/1/2022
1
1
1
4	3/5/2022
4
4
6	2/8/2022
6
6
6
6
6
6

Want this:

ID	Start
1	1/1/2022
1	1/1/2022
1	1/1/2022
1	1/1/2022
4	3/5/2022
4	3/5/2022
4	3/5/2022
6	2/8/2022
6	2/8/2022
6	2/8/2022
6	2/8/2022
6	2/8/2022
6	2/8/2022
6	2/8/2022

Thanks!

[Kurt_Bremser · Accepted solution]

Do this:

data TOSStart (drop=_:);
set toscount (rename=(start=_start));
by id;
retain start;
format start mmddyy10.;
if first.id then start = _start;
run;

[EpiNovice]

Worked like a charm, thanks!!