how to give a value to a flag variable. like when flag=0 then lag(flag)=yes

[lindamtl]

I have this raw data,  actually , need create a new var, named flag. Only give flag a value when each AVALC=Y and next AVALC=N

data have;
input usubjid $ ADY AVALC $;
datalines;
A 15 N
A 36 N
A 57 Y
A 60 N
A 61 Y
A 62 N
A 63 Y
A 64 Y
A 65 Y
B 15 Y
B 36 Y
B 57 N
B 58 N
B 60 Y
; run;

output like below

usubjid $ ADY  AVALC  FLAG $;

A 15 N       .
A 36 N       .
A 57 Y CHECK
A 60 N    .
A 61 Y CHECK
A 62 N   .
A 63 Y   .
A 64 Y  . 
A 65 Y  .
B 15 Y   .
B 36 Y CHECK
B 57 N  . 
B 58 N  .
B 60 Y  .
; run;

I try this code below, the first code, it marked the N,  with flat=o, then i want to give the flag before it the value'check' but faile. 

DATA FLAG;
SET HAVE;
BY usubjid;
length flag $6.;
retain flag;
if first.usubjid then flag = .;
IF  AVALC = 'N' and lag(AVALC) = 'Y' then FLAG='o';
ELSE FLAG=' ';
RUN;
data flag1;
set flag;
BY usubjid;
if flag='o' then lag(flag)='check';
run;

[LinusH]

Please share the log.
Also, I see you are mixing numerical and character missing calue for your flag variable, stick to char.
Then you shoudl only do lag() when it's not first.usubjid, otherswise it will cross BY groups.

[LinusH]

proc sort data=have;
	by usubjid descending ady;
run;

DATA FLAG;
	SET HAVE;
	BY usubjid;
	length flag $6.;
	if first.usubjid then flag = ' ';
	else IF  AVALC = 'Y' and lag(AVALC) = 'N' then FLAG='check';
RUN;

[yabwon]

data have;
input usubjid $ ADY AVALC $;
datalines;
A 15 N
A 36 N
A 57 Y
A 60 N
A 61 Y
A 62 N
A 63 Y
A 64 Y
A 65 Y
B 15 Y
B 36 Y
B 57 N
B 58 N
B 60 Y
; run;



data _null_;
  call symputX("N",nobs,"G");
  stop;
  set have nobs=nobs;
run;

data want;

array test[&n.] $ 1 _temporary_;
do _N_=1 to &n.;
  set have curobs=curobs;
  test[curobs]=AVALC;
end;

do _N_=1 to &n.;
  set have curobs=curobs;
  by usubjid;

  if not last.usubjid then
    do;
      if AVALC="Y" and test[curobs+1]="N" then flag="CHECK";
      else flag=" ";
    end;
  else flag=" ";
  output;
end;

stop;
run;

[yabwon]

data want;
  merge 
    have 
    have(firstobs=2 
         keep=usubjid AVALC 
         rename=(AVALC=NEXT_AVALC usubjid=NEXT_usubjid)
        );

  if usubjid=NEXT_usubjid 
     and
     AVALC="Y" and NEXT_AVALC="N" then flag="CHECK";
  drop NEXT_:;
run;