data a;
input x1 @@;
dif=dif(x1);
obs1=_n_;
if dif=1 then obs2=obs1-1;
cards;
1 2 4 5 7
;

proc sql noprint;
select obs1 into: obs1 separated by ','
from a
where dif=1;
select obs2 into: obs2 separated by ','
from a
where dif=1;
quit;

data wanted;
set a;
if _n_ in (&obs1,&obs2);
keep x1;
run;

data c1;
        input x1 ;
        dif=dif(x1);
        obs+1;
        datalines;
        1
        2
        4
        5
        7
        ;
run;
proc sort data=c1 ;
        by descending obs;
run;
data c1;
        set c1;
        dif1=lag(dif);
run;
proc sort data=c1;
        by obs;
run;
data c1;
        set c1;
        where dif=1 or dif1=1;
        drop obs dif1;
run;

3q a lot .非常有帮助。

学习了，真是感谢，有人支持就有学习的动力~~~