3d plot error

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www 2017-1-20

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/321286-3d-plot-error

评论： Star Strider 2017-1-20

Hi All!

I have 3 functions and I need to plot them individually on a plot window. However, I kept getting the error 'Z must be a matrix, not a scalar or vector.' Even though I have change my function into 2d (dependent on x and y only). It still doesnt work.

f1(x,y,z) = 8*x.^3+36*y.^3+z.^2-36; f2(x,y,z) = 4x.^2-y.^2-21*z; f3(x,y,z) = 4x.^2-2y.^2+10z.^2;

xxx = linspace(-3,3); yyy = linspace (-3,3);

a = sym(f1); b= sym(f2); c= sym(f3); s = subs(a,'z',1); t = subs(b,'z',1); u = subs(c,'z',1);

[x,y] = meshgrid(xxx,yyy); mesh(x,y,s); hold on mesh(x,y,t); hold on mesh(x,y,u);

Thanks in advance!

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John Chilleri 2017-1-20

编辑：John Chilleri 2017-1-20

在 MATLAB Online 中打开

To reproduce the error, run:

f1 = @(x,y,z) 8*x.^3+36*y.^3+z.^2-36; 
f2 = @(x,y,z) 4*x.^2-y.^2-21*z; 
f3 = @(x,y,z) 4*x.^2-2*y.^2+10*z.^2;
xxx = linspace(-3,3); 
yyy = linspace (-3,3);
a = sym(f1); 
b= sym(f2); 
c= sym(f3); 
s = subs(a,'z',1); 
t = subs(b,'z',1); 
u = subs(c,'z',1);
[x,y] = meshgrid(xxx,yyy); 
mesh(x,y,s); 
hold on mesh(x,y,t); 
hold on mesh(x,y,u);

When you give mesh(x,y,s), s is just a symbolic function. I'm guessing you need to say:

[x,y] = meshgrid(xxx,yyy);
s = subs(s,'y',y);
s = subs(s,'x',x); 
mesh(x,y,s)

but it's taking a while to run so I can't confirm if this is correct. It feels like this produces an s that is far too large.

www 2017-1-20

I'm still getting the same error :/

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Answer 1

Star Strider 2017-1-20

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/321286-3d-plot-error#answer_251365

在 MATLAB Online 中打开

You were missing some multiplication operators in your functions. (MATLAB does not recognise implicit multiplication.) I also created your functions as anonymous functions (see Function Basics (link) for details), and then did the plots. The Symbolic Math Toolbox is not always the best option unless you want to specifically do symbolic calculations. Here, it is not necessary.

The Code:

f1 = @(x,y,z) 8*x.^3+36*y.^3+z.^2-36;
f2 = @(x,y,z) 4*x.^2-y.^2-21*z;
f3 = @(x,y,z) 4*x.^2-2*y.^2+10*z.^2;
xxx = linspace(-3,3);
yyy = linspace (-3,3);
[x,y] = meshgrid(xxx,yyy);
z = 1;
figure(1)
mesh(x,y,f1(x,y,z))
hold on
mesh(x,y,f2(x,y,z))
mesh(x,y,f3(x,y,z))
hold off
grid on