How to plot 3D suraface and contour lines?

Question

ANCY S GEORGE 2022-6-2

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https://ww2.mathworks.cn/matlabcentral/answers/1732220-how-to-plot-3d-suraface-and-contour-lines

回答： Bjorn Gustavsson 2022-6-2

I want to plot the function

X0=[0;0];
lb=[-10;10];
ub=[-10;10];
f=@(x)(2*x(1)+20/x(1)+2*x(2)+20/x(2)+120);
x=fmincon(f,X0,[],[],[],[],lb,ub)
Initial point is a local minimum that satisfies the constraints.

Optimization completed because at the initial point, the objective function is non-decreasing 
in feasible directions to within the value of the optimality tolerance, and 
constraints are satisfied to within the value of the constraint tolerance.
x = 2×1
   -10
    10

How to plot the 3D surface and also the contour lines?

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Answer 1

Bjorn Gustavsson 2022-6-2

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https://ww2.mathworks.cn/matlabcentral/answers/1732220-how-to-plot-3d-suraface-and-contour-lines#answer_976545

在 MATLAB Online 中打开

To effectively use matlab sometimes one has to make functions vectorized - such that you can feed the function arrays and get all results at once. This is such a case. Your f works well for the optimization. But if you define it this way:

f=@(x1,x2)(2*x1+20./x1+2*x2+20./x2+120);

You can still run it through fmincon:

X0=[1;1]; % Note that your function is undefined at [0, 0] due to the 2 1/x-terms
lb=[-10;-10]; % Note that I've changed the bounds. Yours constrained the solution to be
ub=[10;10];   % X1 = -10 and X2 = 10
x=fmincon(@(X) f(X(1),X(2)),X0,[],[],[],[],lb,ub);

and then effectively plot the function with surf and contour:

[x1,x2] = meshgrid(linspace(lb(1),ub(1),100),linspace(lb(2),ub(2),100));
surf(x1,x2,f(x1,x2)),shading flat
hold on
contour(x1,x2,f(x1,x2),-200:30:400)

HTH