请问id=3时，A=1.3，跟1.3最接近的应该是1.1和1.2，对应的B的值是2和4，C的值应该是3，为什么是2.5?

抱歉啊，我输错了

try this:

data test;
input id a  b;
cards;
1  1.1   2
2  1.2   4
3  1.3   4
4  1.0   1
;

data step1;
set test;
do i=1 to nobs;
set test1(drop=id rename=(a=a1 b=b1)) nobs=nobs point=i;
  val=abs(a-a1);
  if a ne a1 then output;
end;
run;

proc sort data=step1;
by id val;
run;

data step2;
set step1;
by id val;
if first.id then n=0;
n+1;
if n<=2;
run;

proc sql;
create table wanted as
select distinct id, a, b, mean(b1) as c
from step2
group by id;
quit;

data aaa;
        input id a b;
        cards;
        1 1.1 2
        2 1.2 4
        3 1.3 4
        4 1.0 1
        ;
run;
/*数据集拼接*/
proc sql;
        create table aaa_new as
            select* from aaa,aaa(rename=(id = id1 a = a1 b = b1));
quit;
/*删除ID相同的观测，并求差值*/
data aaa_new;
    set aaa_new;
        if id = id1 then delete;
        diff = abs(a-a1);
run;
/*排序，为后续做准备*/
proc sort data = aaa_new;
        by id diff;
run;
/*让每个ID的第一个DIFF为9999，这是为了使第一行变成最小值，方便使用LAG函数*/
data aaa_new1;
    set aaa_new;
        by id;
        if first.id = 1 then diff = 9999;
run;
proc sql;
        select id, a, b, (b1+lag(b1))/2 as c from aaa_new1
                group by id
                having diff = min(diff);
quit;
/*求变量C*/
data aaa_final;
        set aaa_new1;
        c = (b1+lag(b1))/2;
        output;
run;
/*输出结果，在输出窗口*/
proc sql;
        select id, a, b, c from aaa_final
        group by id
        having diff = min(diff);
quit;

已经有同学回答了，本帖的悬赏到此结束，但是有大侠有其他思路也欢迎讨论，在本楼前回答的同学我会兑付