flag participants with multiple observations

[JLang055]

Hello. I have a patient dataset with observations by participant. I want to flag participants with multiple observations in the dataset. See below for an example.

FID	Flag (want)
1	1
1	0
2	0
3	0
4	1
4	0
4	0
5	0

[Kurt_Bremser]

data want;
set have;
by fid;
flag = (first.fid and not last.fid);
run;

[JLang055]

Thanks @Kurt_Bremser. It seems that your code flagged the first observation for every participant. Is there a way to change the code so that it only flags those with multiple observations. If a participant has only one observation I dont want them flagged. 

[Kurt_Bremser]

Simply cannot be with the data you posted and my code:

data have;
input fid;
datalines;
1
1
2
3
4
4
4
5
;

data want;
set have;
by fid;
flag = (first.fid and not last.fid);
run;

proc print data=want noobs;
run;

Result:

id	flag
1	1
1	0
2	0
3	0
4	1
4	0
4	0
5	0

Matches exactly with your expected result from your initial post:

FID	Flag (want)
1	1
1	0
2	0
3	0
4	1
4	0
4	0
5	0