rzextr.txt

用VB实现解常微分方程组 包括定步长四阶龙格-库塔法、自适应变步长的龙格-库塔法、改进的中点法、外推法等

Sub RZEXTR(IEST, XEST, YEST(), YZ(), DY(), NV, NUSE)
    Dim FX(7)
    Static X(11), D(10, 7)
    X(IEST) = XEST
    If IEST = 1 Then
        For J = 1 To NV
            YZ(J) = YEST(J)
            D(J, 1) = YEST(J)
            DY(J) = YEST(J)
        Next J
    Else
        M1 = IEST
        If NUSE < IEST Then M1 = NUSE
        For K = 1 To M1 - 1
            FX(K + 1) = X(IEST - K) / XEST
        Next K
        For J = 1 To NV
            YY = YEST(J)
            V = D(J, 1)
            C = YY
            D(J, 1) = YY
            For K = 2 To M1
                B1 = FX(K) * V
                B = B1 - C
                If B <> 0# Then
                    B = (C - V) / B
                    DDY = C * B
                    C = B1 * B
                Else
                    DDY = V
                End If
                If K <> M1 Then V = D(J, K)
                D(J, K) = DDY
                YY = YY + DDY
            Next K
            DY(J) = DDY
            YZ(J) = YY
        Next J
    End If
    Erase FX
End Sub