SAS do循环里的set语句用法请教-经管之家官网！

data data1;
input a b c;
cards;
1 0 12
1 1 13
2 0 10
2 1 8
3 0 0
3 1 7
;
run;
data data2;
input a c;
datalines;
1 25
2 18
3 7
;
run;
data data3;
merge data1(rename=c=Nil2) data2(rename=c=total);
by a;
Nil1=total-Nil2;
if Nil1=0 or b=1 then
output;
/* else if b=1 then*/
/* output;*/
/* drop b;*/
run;
data data4;
do i=1 to _n_;
set data3 ;
c=a+b;
/*output;*/
end;
run;
data data5;
do _n_=1 to _n_;
set data3;
sum0=sum(sum0,a);
put sum0;
output;
end;
run;
data data6;
do _n_=1 by 1 until (last);
set data3 end=last;
sum0=sum(sum0,a);
put sum0;
output;
end;
run;
其中，data5和data6中结果不一样，并且去掉output语句也不一样，哪位高手指教一下！谢谢哈！