data a;
input x1 $ x2;
cards;
a 1
a 1
a 1
a -1
a -1
b 1
b 1
b -1
b -1
b -1
c 1
c 1
c -1
c -1
c -1
;
run;
data b;
do until(last.x1);
set a;
by x1 notsorted;
dif_2=dif2(x2);/*这里是2步差分,如果是12步差分,就是dif12(x2)*/
if x1^=lag2(x1) then dif_2=.; /*如果是12步差分,那么就是x1^=lag12(x1)*/
output;
end;
run;