How to use surf to plot sphere function in matlab

Question

I'm trying to plot sphere function below, But I'm getting wrong result

Here is the code I'm using

x1 = [-10:1:10];
x2 = [-10:1:10];
y = zeros(1,21);

for i = 1:21
    y(i) = sphere([x1(i) x2(i)]);
end
Y = meshgrid(y);
surf(x1,x2,Y);
colormap hsv;

sphere.m

function [y] = sphere(x)
d = length(x);
sum = 0;
for i = 1:d
    sum = sum + x(i)^2;
end
y = sum;
end

Answer 1

sphere(10)

It is a MatLab built in function.

Please enjoy responsibly.

If you need to see the source code use: edit sphere or help sphere when your sphere function is not on the path.

Answer 2

For the sake of completness your code is not working because you are only evaluating your function on the pairs (x,x) for some x \\in [-10,10] so you don't cover the whole domain. It would work with this:

x1 = [-10:1:10];
x2 = [-10:1:10];
y = zeros(1,21);

for i = 1:21
    for j=1:21
        Y(i,j) = sphere([x1(i) x2(j)]);
    end
end
surf(x1,x2,Y);
colormap hsv;

or way faster (because you should always avoid unnecessary loops for computation time reasons):

x1 = meshgrid([-10:1:10]);
x2 = x1';

Y = x1.^2+x2.^2;

surf(x1,x2,Y)