2 次元ポアソン方程式の 5 点差分近似による連立一次方程式の係数行列の疎行列パターンをプロットする.

---

Sub GenMat_5DF(M As Long, N As Long, A() As Double)
    Dim NN As Long, II As Long, I As Long, J As Long
    NN = M * N
    For II = 0 To NN - 1
        I = Int(II / M)
        J = II - I * M
        If I > 0 Then A(II, II - M) = -1
        If I < N - 1 Then A(II, II + M) = -1
        If J > 0 Then A(II, II - 1) = -1
        If J < M - 1 Then A(II, II + 1) = -1
        A(II, II) = 4
    Next
End Sub

Sub Test_Spy()
    Const M = 10, N = 10, NN = M * N
    Dim A(NN, NN) As Double
    Dim Fig As Figure, Ax As Axs
    Call GenMat_5DF(M, N, A())
    Set Fig = Plt.Figure()
    Set Ax = Fig.Gca()
    Call Ax.Spy(NN, NN, A(), "markersize=5")
    Call Plt.Show
End Sub