example:
data a;
x=1;output;
x=2;output;
x=3;output;
x=4;output;
run;

data b (drop=i j);
do i=1 to  nobs;
  set a end=last nobs=nobs;
  output;
end;
  if last and (i-1)<6 then do;
    do j=1 to (6-i+1);
     x=. ;output;
        end;
        end;
run;

如果变量不止一个呢？

Moving the  call missing(of _all_) into the loop makes it logically better and save (19) times unwanted execution of  "call missing(of _all_)".

data test(drop=i);
    set sashelp.class end=eof ;
    output;
   
    if eof then do;
      do i=1 to 6;
          call missing(of _all_);
      output;
    end;
    run;

    proc print noobs;run;

Everyone knows this logic. Did you test your code? Did it really works except missing an end? hehe~