data fishdata4;
    set chapt6.fishlength(keep=location date _name_ measurement);
    by location date;
        array tr[1:4] length1-length4;
    retain length1-length4;
    if first.date then do i=1 to 4;
        tr(i)=0;
    end;
    select(_name_);
      when ('length1') length1=measurement;
      when ('length2') length2=measurement;
      when ('length3') length3=measurement;
      when ('length4') length4=measurement;
      otherwise;
    end;
        if last.date;
        keep location date length1-length4;
run;
原fishlength中，对应的每个length，measurement都是有相应的具体数据，为什么最后输出的表格中都变成了0，这个程序是照搬书本上，是哪里跑错了？？