如何对缺失值取上一条观测的值

shiyiming

id     value
01     0.2
01     .

怎么让第二条观测的值取上一条观测的值0.2 阿？

shiyiming

[code:3nb04tjh]data a;
input id value;
cards;
01 0.2
01 .
02 0.1
02 0.3
02 .
;
run;

data b;
set a;
by id;
retain _value;
if first.id then _value=.;

if value^=. then _value=value;
else value=_value;
drop _value;
run;

proc print;run; [/code:3nb04tjh]

shiyiming

thank U very much!