Multiple initial points in fmincon optimization

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Hi
Thank you for reading this question!
I am studying "Fmincon" in Matlab. Based on "Fmincon" document, I wrote a code, as shown below.
close all;
clear;
clc;
fun = @(x)100*(x(2)-x(1)^2)^2 + (1-x(1))^2; % objective function
x0 = [-1, 2; -1, 2; -1, 2]'; % initial points
Aeq = [0.5, 0.1, 0, 0, 0, 0; 0, 0, 2, 1, 0, 0; 0, 0, 0, 0, 0.2, 2]; % constraints
beq = [1; 1; 1]; % constraints
[x,fval] = fmincon(fun,x0,[],[],Aeq,beq)
Local minimum found that satisfies the constraints. Optimization completed because 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 = 2×3
1.5310 -0.6000 -1.1386 2.3448 2.2000 0.6139
fval = 0.2821
In this code, x0 contains six elements. Then, Aeq needs six columns of elements. Based on "Fmincon" document, Equality constraint equation can be written as below:
0.5*x(1)+0.1*x(2)+x(3)+x(4)+x(5)+x(6)=1 (1)
x(1)+x(2)+2*x(3)+x(4)+x(5)+x(6)=1 (2)
x(1)+x(2)+x(3)+x(4)+0.2*x(5)+2*x(6)=1 (3)
In fact, objective function only includes two variables that are x(1) and x(2). Therefore, how does this code ("Fmincon") work? I have not found the similar question in the community. Could anyone explain it or share the relevant link?
Many thanks in advance!

采纳的回答

Alan Weiss
Alan Weiss 2023-4-10
fmincon takes the size of the x0 input as determining the number of input variables. Your x0 has 6 elements, so fmincon thinks that you have a 6-variable problem.
You are attempting to vectorize fmincon. However, fmincon cannot be vectorized, meaning it does not accept multiple input points.
Alan Weiss
MATLAB mathematical toolbox documentation
  1 个评论
xin lin
xin lin 2023-4-10
Hi Alan,
Thank you for your reply. This explanation is very helpful to me.
Best regards,
Xin

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更多回答(1 个)

Jon
Jon 2023-4-10
As @Alan Weiss points out it seems that you are misunderstanding how MATLAB interprets the 2 by 3 array you are inputting as x0. You are thinking that you are optimizing to find the optimum value of a two element vector, with elements x(1) and x(2), and you think that each column of x0 is a different initial guess.
Infact MATLAB only allows one initial guess. So it interprets your 6 element array of x0's as telling it that you are optimizing to find 6 values (that happen to be in a 2 by 3 array). It then uses linear indexing (numbering elements columwise) to find the initial values of x, so x(1) = x0(1,1), x(2) = x0(2,1), x(3) = x0(1,2), x(4) = x(2,2) etc. It doesn't mind that your objective function only uses the first two of these six elements, but uses all 6 when evaluating the constraints.
  1 个评论
xin lin
xin lin 2023-4-10
Hi Jon,
Thanks for the detailed explanation. I previously misunderstood the definition of X0. Now, it should be fine.
Best regards,
Xin

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