Mesh plot of the function sqrt(y-x^2)

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Dimitrios Anagnostou
评论： Star Strider 2021-4-6
I want to plot the function The function is defined for . With
f = @(x,y)sqrt(y-x.^2);
fmesh(f,[-3 3 0 9])
I get a mesh plot of the function in the rectangle [-3,3]X[0,9]. I want to plot the function using meshgrid. The following code results to complex values error message.
x = -3:1:3
y = 0:1:9
[X, Y] = meshgrid(x,y)
Z = sqrt(Y-X.^2)
mesh(X,Y,Z)
The problem is that Z is evaluated for not suitable pairs of (X,Y). What am I doing wrong?

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采纳的回答

Star Strider
Star Strider 2021-4-6
编辑：Star Strider 2021-4-6
Try this:
x = -3:1:3;
y = 0:1:9;
[X, Y] = meshgrid(x,y);
Y = Y.*(Y>=X.^2); % Select Y ≥ X^2
Z = sqrt(Y-X.^2);
Zre = Z.*(imag(Z)==0); % Select Only Real Values Of ‘Z’
figure
mesh(X,Y,Zre)
EDIT — (5 Apr 2021 at 15:56)
Added:
Y = Y.*(Y>=X.^2); % Select Y ≥ X^2
to accord with the edited Question.
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Dimitrios Anagnostou
Dimitrios Anagnostou 2021-4-6
So, I guess, the easiest solution is to use fmesh here:-)! Thank your very much for your feedback. I was puzzled today during a lesson how to have mesh return a similar graph with fmesh.
Star Strider
Star Strider 2021-4-6
As always, my pleasure!
The easiest solution is to use fmesh. It is possible to examine the fmesh code using edit or type (however that is not very revealing, since it calls fsurf, that it is posible to examine in some detail) to see exactly how it works. The fsurf function then calls a number of other funcitons that I have not taken the time to examine closely. My information on the ‘f’ plotting functions derives from other discussions in MATLAB Central that I have read over the years.

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更多回答（1 个）

Bruno Luong
Bruno Luong 2021-4-6
Here is the surface in meshgrid form
s = -1:0.01:1;
y = 0:0.1:9;
[S, Y] = meshgrid(s,y);
X = S.*sqrt(Y);
Z = sqrt(max(Y-X.^2,0));
figure
mesh(X,Y,Z)

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