data aaa; aa = 1; b = 3; output; run; data ccc; aa = 2; output; aa = 3; output; aa = 3; output; aa = 3; output; aa = 4; output; aa = 5; output; run; data aab; put _all_; set aaa ccc; /* by aa;*/ if aa = 3 then do; b = 1; b1 = 2; end; put _all_; run;   As above, when I comment the by statement, b is retained as 1  when aa = 4, 5. However when I uncomment the by statement, the value of b becomes missing. I wonder what happended when by xxx is used with set statement?   By the way, if the set statement is replaced by merge statement, no matter whether commenting the by statement or not, the value of b never become 1 when aa = 4,5 ... View more

  I am learning SAS data setp flow recently, and I found a interesting question. Please see the following programs. The only difference between data bb and data cc is that " put _all_" was a comment statement in data bb, which  was not in data cc.   However, it made a big difference to the result. I could underatand the number of dataset bb is zero, but I wonder what happened to dataset cc.  Can someone explain it?  Thanks!   data aa; do A = 1 to 20; output; B = A; end; run;   data bb; do until (i >= 5); set aa; /* put _all_*/ i = _N_; i + 1; end; run;   data cc; do until (i >= 5); set aa; put _all_ i = _N_; i + 1; end; run; ... View more