surface plot with a matrix

I am trying to graph a surface with a diagonal matrix, the equation I am trying graph is f = x^TDx, x is a 2 by 1 vector and D is a 2 by 2 matrix.

Here is what have so far, but I keep getting error.

x = linspace(-10,10);
y = linspace(-10,10);
[X,Y] = meshgrid(x,y);
D = [2 1; 1 2];
f = @(x,y) [x,y]*D*[x,y].'; % [x,y] is 1 by 2 
contour (X,Y,f(X,Y))

I know that would give me an erros since the dimension of [x,y] and D not macth, I know that if D is a identity matrix, I could use eye(2*numel(x)) to create a 200x200 matrix to match [x,y].

I don't know how to do it with a random and a given diagonal symmetric matrix. Can someone show me how do it? Thanks.

Answer 1

[Azzi Abdelmalek]

http://www.mathworks.com/matlabcentral/answers/123643#answer_131349

[konoha]

Thanks. how about for a given matrix, such as [1,2;2,1]

[Azzi Abdelmalek]

about what?

[konoha]

to apply f = x^T*D*x

[Azzi Abdelmalek]

The sizes must match