[求助]如何在圆形区域中画三维曲线（曲面）

[slgu]

clear
[x,y]=meshgrid(-3:.05:3);
r=4;n=size(x)
for i=1:n
for j=1:n
if x(i,j)^2+y(i,j)^2>r^2
x(i,j)=nan;y(i,j)=nan;
end
end
end
z=(r-y).*(x.^2-(r-y).^2)./((x.^2+(r-y).^2).^2)+(r+y).*(x.^2-(r+y).^2)./((x.^2+(r+y).^2).^2)
figure(1)
surf(x,y,z)
shading interp