"Vector must be strictly increasing or strictly decreasing with no repeated values" error

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Michael Elman 2020-8-31

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/586868-vector-must-be-strictly-increasing-or-strictly-decreasing-with-no-repeated-values-error

编辑： Michael Elman 2020-9-1

采纳的回答： Matt J

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Hello,

I'm trying to use contour to represent some data, but I get this error.

Thank you in advance.

R_a=3;
N=100;
phi=linspace(0,2*pi,N);
rho= linspace(0,R_a,N);
r = randi([50 70],1,length(rho));
A=ones(length(rho)).*r;
[x_c,y_c] = pol2cart(phi,rho);
[Ro_tag, Phi_tag]=meshgrid(rho,phi);
contour(x_c,y_c,A);

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Adam Danz 2020-8-31

编辑：Adam Danz 2020-8-31

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Shot in the dark,

R_a=3;
N=100;
phi=linspace(0,2*pi,N);
rho= linspace(0,R_a,N);
r = randi([50 70],1,length(rho));
A=ones(length(rho)).*r;
[Ro_tag, Phi_tag]=meshgrid(rho,phi);
[x_c,y_c] = pol2cart(Ro_tag,Phi_tag);
contour(x_c,y_c,A);
axis equal

Michael Elman 2020-8-31

Looks a little bit weird to me, I will try the link you posted before.

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采纳的回答

Matt J 2020-8-31

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/586868-vector-must-be-strictly-increasing-or-strictly-decreasing-with-no-repeated-values-error#answer_487511

编辑：Matt J 2020-8-31

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Your meshgrid must be Cartesian:

R_a=3;
N=100;
[X,Y]=meshgrid( linspace(-R_a,+R_a, N) );
[Phi,Rho]=cart2pol(X,Y);             
             
A= ... computaton based on Phi and Rho ...
contour(X,Y,A);

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Matt J 2020-8-31

编辑：Matt J 2020-8-31

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Very well, then I modify my answer to,

R_a=3;
N=100;
[X,Y]=meshgrid( linspace(-R_a,+R_a, N) );
A= ... computaton based on X and Y ...
contour(X,Y,A);

The point is, A needs to be sampled on a normal Cartesian meshgrid for the purposes of doing the contour plot. You cannot, as a starting point, set up your sample locations in polar coordinates because they will not remain monotonic when transformed by pol2cart().

Michael Elman 2020-9-1