Flagging rows based on dates across two variables that are on different rows

[joshCRF]

So as the image below shows, this is currently data that is within one data-step.

I'm currently trying to figure out a way in which rows can be flagged which contain the same date or same date + 1; when the end date matches the start date on the following row or if the end date is within a day of the start date on the following row.

Essentially we are checking for when the row has 'chgdt_10' populated and then if the end date matches the start date of the following row or within +1 day(s).

Using the lag function I am able to (testflg) the consecutive records but unable to flag the initial record where the match occurs, so where I've linked the rows with the blue line I would like the very first row to be populated for 'testflg' too.

Thanks in advance for any responses.

[sbxkoenk]

data have;
start='25sep2021'd; end='28apr2022'd; output;
start='28apr2022'd; end='15sep2022'd; output;
start='15sep2022'd; end='03oct2022'd; output;
format start end date9.;
run;
data have; set have; rowid=_N_; run;

/* you probably have this now : */
/*
data want(drop=a);
 set have;
 a=lag(end);
 if start=a then testflg=1;
run;
*/

/* you need this : */
/* just an example -- other solutions are possible !! */
proc timedata data=have out=wanthmmm outarray=work.testflg_array print=(arrays);
   id rowid interval=day;
   vars start end;
   outarrays testflg;
      do t= 1 to dim(end);
         if end[t]   = start[t+1] then do; testflg[t]=1; end;
		 else do; testflg[t]=0; end;
      end;
run;

data work.testflg_array;
 set work.testflg_array;
 keep  start end testflg;
 format start end date9.;
run;
/* end of program */

PROC TIMEDATA is part of SAS/ETS software.

Koen

[data_null__ · Accepted solution]

I think you want to look ahead.  You should be able to find lots of discussion here in the community.

Look-Ahead and Look-Back - SAS Support Communities

Is a good start the part from @mkeintz is really good. 

If you post some useable data in text format not a picture you will get more helpful solutions.

[mkeintz]

Why did you flag the last line (START=05MAR2024), but not flag its predecessor (END=04MAR2024)?

In the absence of usable sample data in the form of a working DATA step, here is my untested guess as to what you want.  

data want (drop=nxt_:);
  set have ;
  by chgdt_10 notsorted;
  merge have
        have (firstobs=2 keep=chgdt_10 start rename=(chgdt_10=nxt_chgdt start=nxt_start)) ;

  retain _beg_series  _end_series 0;
  _beg_series = (first.chgdt_10=1 and chgdt_10^=.);
  _end_series = (last.chgdt_10=1  and nxt_chgdt^=.);

  flag=  (start-1 <= lag(end)  and _beg_series=0)
         or
         (end+1   >= nxt_start and _end_series=0);
run;
  

This assume that each series

1. is sorted by START and
2. starts with a non-missing value for CHGDT_10.