how to create a matrix when using surf function？

I want to solve this problem：

figure(1);

[u,v]=meshgrid(-40:1:40);

D1=sqrt((u-40)^2+v^2);

D2=sqrt((u+40)^2+v^2);

if (D1<=40)&(D2<=40)

F=0;

else

F=1;

end

surf(u,v,F);

shading interp;

however, in this way, matlab tells me ''

错误使用 surf (line 71)

Z 必须为矩阵，不能是标量或向量。''

how can I use surf correctly?

Answer 1

[Voss]

What you are doing there is making F a scalar, but F needs to be a matrix the same size as u and v, so something like this:

F = ones(size(u));
F(D1 <= 40 & D2 <= 40) = 0;

Here logical indexing is used to set the value of F based on conditions involving D1 and D2.

[博 王]

Thank you for you help. "F needs to be a matrix the same size as u and v" is really helpful. However, when I use this code:

F(D1 <= 40 & D2 <= 40) = 0;

I find it does not work.

So I change it this way.

figure(1);

[u,v]=meshgrid(-80:1:80);

F = ones(size(u));

for j=1:160

for i=1:160

D1=sqrt((i-80-40)^2+(j-80)^2);

D2=sqrt((i-80+40)^2+(j-80)^2);

if (D1<=80 & D2<=80)

F(i,j)=0;

else

F(i,j)=1;

end

end

end

surf(u,v,F);

shading interp;

and the result is shown in figure. But I think there is a better way to realize it especially for the part of "i j". Can you give me some advice.