I can not create a 3D object with the following code

%Generate a volume-swept 3D object
N = 40; %number of increments
z = linspace (-5,5,N);
radius = sqrt(1 +z.^2);
theta = 2*pi*linspace(0,1,N);
X = radius.*cos(theta);
Y = radius.*sin(theta);
Z = z(:,ones(1,N));% <-- Do not understand the code? Create a matrix Z?
surf(X,Y,Z)

Error using surf (line 71)
Z must be a matrix, not a scalar or vector.

axis equal

Error: Z is not a matrix

Thanks in advance!

Answer 1

[Cris LaPierre]

To create a surface, your Z input must contain a value for every X-Y combination organized in a grid (matrix). Columns correspond to X, rows correspond to y.

• Here, z is a row vector
• Z = z(:,ones(1,N)) is the same as z(:,[1 1 1 ... 1]) or repmat(z(:,1),1,N)

Use MATLAB to see what that code is doing:

N=4;
z = linspace (-5,5,N)

z = 1×4

-5.0000 -1.6667 1.6667 5.0000

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z(:,ones(1,N))

ans = 1×4

-5 -5 -5 -5

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repmat(z(:,1),1,N)

ans = 1×4

-5 -5 -5 -5

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It just repeats the first value of your vector N times.

I think you are not getting the results you expect because z is a row vector, not a column vector. Perhaps this is what you intended?

N = 40; %number of increments
z = linspace (-5,5,N)'; % use ' to turn z into a column vector
radius = sqrt(1 +z.^2);
theta = 2*pi*linspace(0,1,N);
X = radius.*cos(theta);
Y = radius.*sin(theta);
Z = z(:,ones(1,N));% <-- Do not understand the code? Create a matrix Z?
surf(X,Y,Z)
axis equal

If you did intend for z to be a vector, you can use any 3D plotting functino to view it. Here is a result using scatter3.

%Generate a volume-swept 3D object
N = 40; %number of increments
z = linspace (-5,5,N);
radius = sqrt(1 +z.^2);
theta = 2*pi*linspace(0,1,N);
X = radius.*cos(theta);
Y = radius.*sin(theta);
Z = z(:,ones(1,N));% <-- Do not understand the code? Create a matrix Z?
scatter3(X,Y,Z)
axis equal

[Long]

I got it. Thanks a lot.