首先这个是我的Code：
data base;
    infile  'C:\Users\mayan\Documents\Statistics 224\Final_Data\*.txt' delimiter='@';
    input stud $ semester $ course $ score  GPA $;
    Abb_Course = substr(course,1,4);
    School_Year= substr(semester,2,2);
    run;
    data base;
    set base;
    if substr(GPA,1,1)='A' then new_GPA=4;
    else if substr(GPA,1,2)='A+' then new_GPA=4.33;
    else if substr(GPA,1,2)='A-' then new_GPA=3.66;
    else if substr(GPA,1,2)='B+' then new_GPA=3.33;
    else if substr(GPA,1,2)='B' then new_GPA=3;
    else if substr(GPA,1,2)='B-' then new_GPA=2.66;
    else if substr(GPA,1,2)='C+' then new_GPA=2.33;
    else if substr(GPA,1,2)='C' then new_GPA=2;
    else if substr(GPA,1,2)='C-' then new_GPA=1.66;
    else if substr(GPA,1,2)='D+' then new_GPA=1.33;
    else if substr(GPA,1,2)='D' then new_GPA=1;
    else if substr(GPA,1,2)='D-' then new_GPA=0.66;
    else if substr(GPA,1,2)='E' then new_GPA=0;
    else new_GPA=.;
    run;
    data base;
    set base;
    if substr(semester,1,1)='1' then Semester_Season='Fall';
    else if substr(semester,1,1)='2' then Semester_Season='Spr';
    else if substr(semester,1,1)='3' then Semester_Season='Sum';
    else if substr(semester,1,1)='5' then Semester_Season='Win';
    else Semester_Season=.;
    run;
    data base;
    set base;
    if substr(semester,1,1)='1' then Semester_Order='1';
    else if substr(semester,1,1)='2' then Semester_Order='3';
    else if substr(semester,1,1)='3' then Semester_Order='4';
    else if substr(semester,1,1)='5' then Semester_Order='2';
    else Semester_Order=.;
    run;
    proc sort data=base;
    by stud School_Year Semester_Order;
    run;
    proc sql;
        create table base1
        as select stud,School_Year,Semester_Season,mean(new_GPA) as mean_GPA,
        sum(score) as sum_score,
        count(*) as num_course from base
        group by stud,semester,School_Year,Semester_Season;
    quit;
    proc print data=base1;
    run;

最后打出来的表如下：

如何才能在每行的最后加上一个Cumulative GPA (学期累计成绩），如图第一个就是那个学期成绩，第二个就是两个学期的平均成绩[(3.47571+3.33167)/2]，第三个就是3个学期的平均成绩。而且一定要按学生（学生号是第一列）区分。

我大概猜到是不是应该用一个loop？但是不知道怎么下手，请高手帮忙 ORZ