追赶法求三次样条插值，其中n为全局变量
Private Function spline(num As Single) As Single
Dim k As Integer
Dim h() As Single, lamda() As Single, miu() As Single, d() As Single
Dim q() As Single, r() As Single, u() As Single, m() As Single

ReDim h(1 To n) As Single, lamda(0 To n - 1) As Single, miu(1 To n) As Single, d(0 To n) As Single '矩阵中的数组
ReDim q(0 To n) As Single, r(0 To n) As Single, u(0 To n) As Single, m(0 To n) As Single   '追赶法求解过程中的数组

If num < x(0) And num > x(n) Then
        MsgBox "错误：所输入数据超出插值区间", vbCritical + vbOKOnly
        Text6.Text = ""
        Text6.SetFocus
End If

For k = 1 To n  '基本参数的确定
    h(k) = x(k) - x(k - 1)
Next

For k = 1 To (n - 1)
    lamda(k) = h(k + 1) / (h(k) + h(k + 1))
    miu(k) = 1 - lamda(k)
    d(k) = 6 * ((y(k + 1) - y(k)) / h(k + 1) - (y(k) - y(k - 1)) / h(k)) / (h(k) + h(k + 1))  '这个循环中也出现了k=3的情况，n是全局变量
Next


If Option1.Value = True Then  '端点条件的判定（两端输入导数值）
      d(0) = 6 / h(1) * ((y(1) - y(0)) / h(1) - Val(Text4.Text))
      d(n) = 6 / h(n) * (Val(Text5.Text) - (y(n) - y(n - 1)) / h(n))
      
      For k = 1 To (n - 1)
          q(0) = 2
          lamda(0) = 1
          miu(n) = 1
          r(0) = lamda(0) / q(0)
          u(0) = d(0) / q(0)
          q(k) = 2 - miu(k) * r(k - 1)           'L(3)=0
          r(k) = lamda(k) / q(k)
          u(k) = (d(k) - miu(k) * u(k - 1)) / q(k)  '最后k应该是2，但是却是3
      Next
      q(n) = -miu(n) * r(n - 1) + 2
      u(n) = (d(n) - miu(n) * u(n - 1)) / q(n)
      m(n) = u(n)
      
      For k = n - 1 To 0  '计算二阶导数数组
          m(k) = -r(k) * m(k + 1) + u(k)
      Next
   
Else: m(0) = Val(Text4.Text)  ’两端输入二阶导数值
      m(n) = Val(Text5.Text)
      
      For k = 1 To n - 1
          r(0) = 0
          u(0) = 0
          q(k) = -miu(k) * r(k - 1) + 2
          r(k) = lamda(k) / q(k)
          u(k) = (d(k) - miu(k) * u(k - 1)) / q(k)
      Next
      
      m(n - 1) = u(n - 1)
      
      For k = n - 2 To 1  '计算M数组（这里的K最后是2而不是1）
         m(k) = -r(k) * m(k + 1) + u(k)
      Next
      
End If

For k = 1 To n  '参数确定后的计算
    If num <= x(k) And num >= x(k - 1) Then
        spline = m(k - 1) * ((x(k) - num) ^ 3) / 6 / h(k) + m(k) * ((num - x(k - 1)) ^ 3) / 6 / h(k) + _
                (y(k - 1) - m(k - 1) * (h(k) ^ 2) / 6) * (x(k) - num) / h(k) + _
                (y(k) - m(k) * (h(k) ^ 2) / 6) * (num - x(k - 1)) / h(k)
        Exit For
    End If
Next
   
End Function

,怎么摸不清你想做什么。。。。。。。。

彻底被搞晕，一堆变量，你到底想用这段代码干嘛？！

是哪个循环越界呀？好好看一下redim语句数组的说明。