spline.txt

VB常用数值算法集 内含有解线性代数方程组

Sub SPLINE(X(), Y(), N, YP1, YPN, Y2())
    Dim U(100)
    If YP1 > 9.9E+29 Then
        Y2(1) = 0
        U(1) = 0
    Else
        Y2(1) = -0.5
        AAA = (Y(2) - Y(1)) / (X(2) - X(1))
        U(1) = (3# / (X(2) - X(1))) * (AAA - YP1)
    End If
    For I = 2 To N - 1
        SIG = (X(I) - X(I - 1)) / (X(I + 1) - X(I - 1))
        P = SIG * Y2(I - 1) + 2#
        Y2(I) = (SIG - 1#) / P
        AAA = (Y(I + 1) - Y(I)) / (X(I + 1) - X(I))
        BBB = (Y(I) - Y(I - 1)) / (X(I) - X(I - 1))
        CCC = X(I + 1) - X(I - 1)
        U(I) = (6# * (AAA - BBB) / CCC - SIG * U(I - 1)) / P
    Next I
    If YPN > 9.9E+29 Then
        QN = 0#
        UN = 0#
    Else
        QN = 0.5
        AAA = YPN - (Y(N) - Y(N - 1)) / (X(N) - X(N - 1))
        UN = (3# / (X(N) - X(N - 1))) * AAA
    End If
    Y2(N) = (UN - QN * U(N - 1)) / (QN * Y2(N - 1) + 1#)
    For K = N - 1 To 1 Step -1
        Y2(K) = Y2(K) * Y2(K + 1) + U(K)
    Next K
End Sub