function [n, v1] = FDM(a, h)%h為步長
px = 2 * a / h + 1; %長邊上的點數
py = a / h + 1; %短邊上的點數
v1 = ones(py, px); %設置一10*20的矩陣
v1(1, :) = ones(1, px) * 10; %初始化上邊界
v1(py, :) = zeros(1, px); %初始化下邊界
for i = 1 : py %初始化左右邊界
    v1(i, 1) = 0;
    v1(i, px) = 0;
end
v2 = v1;
maxt = 1;
t = 0;
n = 0;
while(maxt > 1e-6)
    maxt = 0;
    n = n + 1;
    for i = 2 : py - 1
        for j = 2 : px - 1
            v2(i, j) = 0.25 * (v1(i, j + 1) + v1(i + 1, j) + v1(i - 1, j) + v1(i, j - 1)); %計算邊界內的值
            t = abs(v2(i, j) - v1(i, j));
            if(t > maxt) %誤差判斷
                maxt = t;
            end
        end
    end
    v1 = v2;
end
axis([0, 20, 0, 10, 0, 10]);
contour(v1, 30);
end