How comes first.ParkName = 1 and ParkName. col = 1

[AMIN_ALJALLAD]

proc sort data=pg2.np_acres 
          out=sortedAcres(keep=Region ParkName State GrossAcres);
    by Region ParkName;
run;
	
data multiState singleState;
    set sortedAcres;
    by Region ParkName;
    if First.ParkName=1 and Last.ParkName=1 
        then output singleState;
    else output multiState;
    format GrossAcres comma15.;
run;

[RichardDeVen · Accepted solution]

last.<variable> (and corresponding first.<variable>) are two automatic temporary variables created for each BY variable and indicate if the current row is at a group edge row; either the first or last row of a by-group level.

You can infer information about where the current observation is within the group

FIRST.  LAST.   where
  1             at first in group
  0             not at first in group
          1     at last in group
          0     not at last in group
  1       0     group has >= 2 rows and currently at first in group
  0       0     group has >= 3 rows and currently in the middle part
  0       1     group has >= 2 rows and currently at last in group
  1       1     group has only 1 row

Learning about the underlying premises of the implicit loop fundamental to DATA/SET processing will make you a better SAS coder:

• A good starter is SUGI 30 paper "A Hands-On Introduction to SAS® DATA Step Programming" Debbie Buck, D. B. & P. Associates, Houston, TX". 
• Ian Whitlock's papers are some of the best. Try his SUGI 31 "How to Think Through the SAS® DATA Step "

[AMIN_ALJALLAD]

Thank you for your contribution.

My concern at first is how the both is set to 1 at the same time. However, I discovered that when the value id unique, then first and last variable will be 1