Create a flag for all rows with the same ID based on comparing variables

[meriS]

Hello all, 

I have a dataset that looks like this:;

ID	Month	Right	Left
1	1	10	5
1	2	11	6
1	3	8	10
2	1	7	10
2	2	8	10
2	3	8	11
3	1	7	7
3	1	10	7

I'd like to add a flag for each user showing whether right or left is higher. If right and left start as equal, I'd like it to check at the second month and check which is higher there. So, I'd like the result to look like this (note that the flag stays the same even if right/left switch later on, see ID 1, Month 3):

ID	Month	Right	Left	right flag	leftflag
1	1	10	5	1	.
1	2	11	6	1	.
1	3	8	10	1	.
2	1	7	10	.	1
2	2	8	10	.	1
2	3	8	11	.	1
3	1	7	7	1	.
3	1	10	7	1	.

So far, I can get the flags to show up for the first month, looking like this: 

data result;
set result;
if month = 1 and (right > left) then rightflag = 1;
if month = 1 and (left > right) then leftflag = 1;
run;

ID	Month	Right	Left	right flag	leftflag
1	1	10	5	1	.
1	2	11	6	.	.
1	3	8	10	.	.
2	1	7	10	.	1
2	2	8	10	.	.
2	3	8	11	.	.
3	1	7	7	.	.
3	1	10	7	.	.

I'd like to know how I can get that flag to fill in for all rows having the same ID, and how to implement the "move to second month" check if right = left at month 1. Thank you!

[Ksharp]

data have;
infile cards expandtabs truncover;
input ID	Month	Right	Left;
cards;
1	1	10	5
1	2	11	6
1	3	8	10
2	1	7	10
2	2	8	10
2	3	8	11
3	1	7	7
3	1	10	7
;

data temp;
 set have;
if (right > left) then _rightflag = 1;
if (left > right) then _leftflag = 1;
run;

data want;
 do until(last.id);
   set temp;
   by id;
   if not missing(_rightflag) and not found then do;found=1;rightflag=_rightflag;end;
   if not missing(_leftflag) and not found then do;found=1;leftflag=_leftflag;end;
 end;
 do until(last.id);
   set temp;
   by id;
   output;
 end;
drop _rightflag _leftflag found;
run;

[andreas_lds]

Is it correct that both obs with id=3 have month=1?

This seems to work, too:

data want;
   if 0 then set have;
   length r_flag l_flag 8;
   
   do _n_ = 1 by 1 until (last.id);
      set have;
      by id;
   
      if first.id or (sum(l_flag, r_flag) = 0) then do;
         r_flag = Right > Left;
         l_flag = Left > Right;
      end;  
   end;
      
   do _n_ = 1 by 1 until (last.id);
      set have;
      by id;
      output;
   end;
run;