ReDim是重新定义数组的大小
加上Preserve使数组中已赋值的那些数据不被清空

例如
dim A(3)
A(0)=A(1)=A(2)=A(3)=1
Redim Preserve A(4)   ‘数组变大了，之前赋值的数据不变
A(4)=2                ‘赋值给A(4)
for i = 0 to 4
   print A(i)
next

ReDim Preserve 的用途