HELP needed! (Error using surf, Z must be a matrix, not a scalar or vector)

Hi guys,

I'm quite new to matlab and I'm required to plot this paraboloid graph. I have spend hours trying to solve but no result. Don't think it's a matter of cap's as well.

A= 1./1500;

X_r2 = 1;

Z_r2 = 0;

x = -10:.1:10;

y = -10:.1:10;

[X,Y] = meshgrid(x,y);

syms A X Y Z X_r2 Z_r2

Temp_Z = solve(sqrt((X-X_r2).^2 + Y.^2 + (Z-Z_r2).^2) - sqrt(X.^2 + Y.^2 + Z.^2) - A == 0 , Z );

Z = Temp_Z;

surf(X,Y,Z)

Please help!

[Wang]

yes,the size of Z is just same as X and Y;you can try this:remove the operator '.' in the equation

[Tonny]

Hi Wang. I've just tried but it's still the same error.

[Thomas]

You are overwriting your meshgrid matrices with symbolic vars

syms A X Y Z X_r2 Z_r2

...?

[Tonny]

Hi Thomas and all,

I think my whole method of trying to plot a surface graph of a symbolic eqn is flawed.

Does anyone know how I can plot this the eqn above? It's suppose to be a paraboloid. I've tried ezsurf as well but having other errors there.

Thanks in advance. Tonny

Answer 1

[cr]

Whoa, I think thats not quite what you want to do. What is the eqn you want to plot? If i'm plotting the eqn z = sin(sqrt(x^2+y^2))/sqrt(x^2+y^2), this is what I do:

a = -10:.1:10;
b=a;
[x,y] = meshgrid(a,b);
z =  sin(sqrt(x.^2+y.^2))./sqrt(x.^2+y.^2);
surf(x,y,z)

[Tonny]

Hi. What I needed to do is to solve the eqn above for z, then plot the graph of z.

What I was thinking of doing is to use the solve(eqn) function of Matlab to solve the eqn for z for me. Then use it to get a surface plot.

Apparently, using the solve(eqn) function will require me to use symbolic variables. So when I tried to use surf(x,y,z), an error "z must be a matrix, not a scalar or vector".

Answer 2

[cr]

ok, that clarifies that you need to use solve. if solve function is giving you a closed form solution for your eqn, use subs to substitute for various values of x & y inside two loops. Or better, manipulate the result you get to vectorize it so that you can use meshgrid matrices. Then comes surf.

Answer 3

[Walter Roberson]

After you solve(), use matlabFunction() to convert the symbolic form to an anonymous function. You can then apply that to the meshgrid matrices.

There are going to be at least two solutions for each point. You need to select one of them or separate out the two parts before doing the matlabFunction()