mmid.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 MMID(Y(), DYDX(), NVAR, XS, HTOT, NSTEP, YOUT())
    Dim YM(10), YN(10)
    H = HTOT / NSTEP
    For I = 1 To NVAR
        YM(I) = Y(I)
        YN(I) = Y(I) + H * DYDX(I)
    Next I
    X = XS + H
    Call DERIVS(X, YN(), YOUT())
    H2 = 2# * H
    For N = 2 To NSTEP
        For I = 1 To NVAR
            SWAP = YM(I) + H2 * YOUT(I)
            YM(I) = YN(I)
            YN(I) = SWAP
        Next I
        X = X + H
        Call DERIVS(X, YN(), YOUT())
    Next N
    For I = 1 To NVAR
        YOUT(I) = 0.5 * (YM(I) + YN(I) + H * YOUT(I))
    Next I
End Sub