Add 1 to flag if previous row value doesn't match with current flag following certain condition

[John04]

I have a table that is being created, we will say that column 1 is Id, column2 is YearMonth and column3 is Flag.

data have; input 
Id YearMonth Flag; datalines;
1  200101    1
1  200102    1
1  200103    0
1  200104    1
1  200105    1
1  200106    0
1  200107    1
1  200108    0
2  200101    0
2  200102    0
2  200103    1
2  200104    1
2  200105    0
2  200106    0
2  200107    1
2  200108    0
;

I want to add 1 to flag if current flag doesn't match with previous row value along with two major conditions (explained after output for a better explanation).

The output should be:

Id	YearMonth	Flag	Stage
1	200001	1	1
1	200002	1	1
1	200003	0	2
1	200004	1	3
1	200005	1	3
1	200006	0	4
1	200007	1	3
1	200008	0	4
2	200001	0	2
2	200002	0	2
2	200003	1	3
2	200004	1	3
2	200005	0	4
2	200006	0	4
2	200007	1	3
2	200008	0	4

Condition-1: There are only 4 stages. Hence if a flag is repeated after stage 4, then it should not increment and should give output as either stage=3 if flag=1 or stage=4 if flag=0.

Condition-2: If first entry of an Id is 0 then the stage should start with 2 not 1.

I am trying something like this (helped by Richard user:1249962 (stackoverflow))

data want(drop=maxstage_reached);
set have;
if flag ne lag(flag) then stage+1;                * increment per rules;
if stage = 4 then maxstage_reached=1;             * tracking flag (retained);
if maxstage_reached then stage = 4 - flag;        * special rule;
retain maxstage_reached;
run;

Currently I am struggling with:

1)group by Id; I am trying to use "by" but it doesnt work

2)Condition-2

Thank you!

[hhinohar]

data have;
	input Id YearMonth Flag;
	datalines;
1  200101    1
1  200102    1
1  200103    0
1  200104    1
1  200105    1
1  200106    0
1  200107    1
1  200108    0
2  200101    0
2  200102    0
2  200103    1
2  200104    1
2  200105    0
2  200106    0
2  200107    1
2  200108    0
;
run;

data want;
	do until(last.id);
		lagflag=_N_;
		set have;
		by id;
		_N_=flag;

		if first.id then
			do;
				lagflag=flag;
				cnt=id;
			end;
		else if flag ne lagflag then
			do;
				cnt+1;
			end;

		if cnt>4 and lag(cnt)=4 and flag=1 then
			do;
				cnt=3;
			end;
		else if cnt>4 and lag(cnt)=4 and flag=0 then
			do;
				cnt=4;
			end;
		output;
	end;
run;

[novinosrin · Accepted solution]

Hi @John04  Seems nothing more than a pretty straight forward BY GROUP processing and counter logic i.e. assuming I correctly understand your requirement, which I can believe I have?-

data have;
	input Id YearMonth Flag;
	datalines;
1  200101    1
1  200102    1
1  200103    0
1  200104    1
1  200105    1
1  200106    0
1  200107    1
1  200108    0
2  200101    0
2  200102    0
2  200103    1
2  200104    1
2  200105    0
2  200106    0
2  200107    1
2  200108    0
;
run;

data want;
 _n_=0;
 do until(last.id);
   set have;
   by id flag notsorted;
   if first.id and first.flag and not flag then _n_=2;
   else if first.flag and _n_<4 then _n_=sum(_n_,1);
   if _n_=4 then stage=ifn(flag,3,4);
   else stage=_n_;
   output;
 end;
run;

[John04]

I understand what you have done! I am still trying to understand why my code is not working. Can you suggest? Thanks