SAS task. Need to pick records conditionally

Question

I have been given a task to solve, I'm quite new to this programming language. When the flag is 0, I have to pick the next immediate record, provided the flag variable has three or more consecutive records with value 1. I have been going at it for hours. Below is the datastep. Please suggest.

data two;
input usubjid visit flag;
cards;
1001 1 1
1001 2 1
1001 3 0
1001 4 1
1001 5 1
1001 6 1
1002 1 1
1002 2 1
1002 3 0
1002 4 1
1002 5 1
1003 1 0
1003 2 1
1003 3 1
1003 4 1
1003 5 1
;
run;

output will be:-

usubjid visit flag
1001     4     1

1003     2     1

This is what I have tried so far.

proc sort data = two ;
        by usubjid ;    
run;
  proc transpose data = two out = tran ;
        by usubjid ;
        id visit ;var flag ;
run;
data b ;
    set tran ;
    if ( ( _1 = _2 ) and ( _1 = _3 ) ) or ( ( _2 = _3 ) and ( _2 = _4 ) ) or ( ( _3 = _4 ) and (     _3 = _5 ) ) or
        ( ( _4 = _5 ) and ( _4 = _6 ) ) ;
run;
proc sort data = b ;
    by usubjid ;
run;
data c ;
    merge a ( in = a ) b ( in = b ) ;
    by usubjid ;
    if a ;                                                                
run;                                                                                   

Answer 1

Ok. Deleted my old answer as I didn't fully understand what you were looking for.

data two;
set two;
n = _n_;
run;

proc sort data=two;
by descending n ;
run;

data two;
set two;
retain count  0;
if flag then
    count = count + 1;
else 
    count = 0;
run;

proc sort data=two;
by n;
run;

data two (drop=count n);
set two;

if lag(flag) = 0 and count >=3;
run;

First, I add a row number variable.

Then sort by that value in reverse order.

Add a variable that counts the number of consecutive records where flag=1.

Resort back to the original order.

Finally, subset down to what you are looking for.

Answer 2

This could be the fastest and simplest way to this

1. Count the continuous "1"s after each "0", if you found 3 of them then proceed to step2.
2. Look back/up two records and extract it.

3. Do explicit Output.

   data two(keep=usubjid visit flag);
   set two;
   retain temp 0;
   
   if flag ne 0 then temp=temp+flag; else temp=0; 
   
   lag2_usubjid= lag2(usubjid);
   lag2_visit=lag2(visit);
   lag2_flag=lag2(flag);
   
   if temp=3 then do;
        usubjid=lag2_usubjid;
        visit=lag2_visit;
        flag=lag2_flag;
        output;
   end;        
   

   run;