Different type loops evaluate loop index at different stage. Take a look examples below.

data _null_;
do n=1 to 5;
        put n=;
end;
put n=;
run;

data _null_;
n=0;
do until (n=5);
    n+1;
        put n=;
end;
put n=;
run;

data _null_;
n=0;
do while (n<=5);
    n+1;
        put n=;
end;
put n=;
run;

还是再个解释一下吧，SAS beginner.谢谢。