data test;
input x @@;
cards;
. . 1 2 . 3 . . 5
;run;
/* step 1: add a sequence number ID and change the missing value to zero */
data test_1;
    id = _n_;
     set test;
         if x = . then y = 0;
            else y=x;
run;

/* Step 2: reverse the order of the data by ID */
proc sort data=test_1 out=test_revers;
    by descending id;
run;

/* Step 3: create a lag variable */
data test_2;
     set test_revers;
        y_lag = lag(y);
run;

/* step 4: fill the missing value */
data test_3;
    set test_2;
        retain z;
    if _n_ = 1 and y~=. then  z = y;
        else if  y = 0 and y - y_lag < 0 then z = y_lag;
           else if y = 0 and y_lag = 0 then z = z;
        else z = y;
        drop y y_lag;
run;

/* change the order back */
proc sort data=test_3 out = test_4;
     by ID;
run;

data test_result (rename = (x=z));
    set test_4 (keep=z);
run;

感到那个if _n_=1那里很神奇……之前不知道还可以这样运行的~列的方向上处理总是觉得很别扭T_T

很好的补缺方法