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for loop for gradient nested within objective function of fmincon

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Dear all, I am trying to include for-loop to define gradient nested within objective function of fmincon as below. However, I am getting an error saying the number of elements for the gradient has to be 10. How can properly incorporate for-loop in the gradient nested in the objective function?

clc
clear
%%
n = 10;
Q0 = 0;
x0 = zeros(n, 1) + 25;
p0 = 1;
I = 50;
p1 = 1.25;
A1 = -diag(ones(n, 1));
%%
A2 = zeros(n/2, n);
for i = 1:n/2
    w = A2(i, :);
    j = 2*i;
    w(1:2:j) = 1;
    w(2:2:j) = -1;
    A2(i, :) = w;
end
%%
A3 = zeros(n/2, n);
for i = 1:n/2
    w = A3(i, :);
    j = 2*i;
    w(j) = 1;
    A3(i, :) = w;
end
%%
A12 = vertcat(A1, A2);
A = vertcat(A12, A3);
%%
b = zeros(2*n, 1);
for i = 1:2*n
    if i <= n
        b(i) = 0;
    elseif i > n && i <= n + n/2
        b(i) = Q0;
    elseif i > n + n/2 && i <= n + n/2 + floor((n/2)/2) + 1
        b(i) = I/p0;
    else
        b(i) = I/p1;
    end
end
Aeq = [];
beq = [];
lb = [];
ub = [];
nonlcon=[];
options = optimoptions('fmincon','SpecifyObjectiveGradient',true);
sol = fmincon(@myobj,x0,A,b, Aeq, beq, lb, ub, [], options);
%---------------------------------------------------------
function [f, g] = myobj(x)
global I
global p0
global p1
global n
q0 = x(2:2:n/2+1);
a0 = x(1:2:n/2+1);
c0 = ones(length(q0), 1).*I - q0.*p0;
u0= a0.^(1/2).*c0.^(1/2);
q1 = x(n/2+3:2:n);
a1 = x(n/2+2:2:n);
c1 = ones(length(q1), 1).*I - q1.*p1;
u1= a1.^(1/2).*c1.^(1/2);
f = -1*(sum(u0 + u1));
% if nargout > 1
%     g = -x;
% end
if nargout > 1
    g = [];
    for i = 1:n
        if mod(i,2)==0
            dx = -(1/2 * 1/sqrt(x(i)));
        elseif (mod(i,2) ~= 0) && (i <= n/2 + 1)
            dx = -(1/2 * (-p0)/sqrt((I  - p0*x(i))));
        elseif (mod(i,2) ~= 0) && (i > n/2 + 1)
            dx = -(1/2 * (-p1)/sqrt((I - p1*x(i))));
        end
        g = [g;dx];
    end
end    
end
%---------------------------------------------------------    

Accepted Answer

Alan Weiss
Alan Weiss on 20 Apr 2018

It looks to me as if your gradient should have 10 elements.

Did you declare all those variables global in the workspace? Maybe n isn't being passed. For more reliable ways to include extra parameters and avoid global variables, see Passing Extra Parameters.

Alan Weiss

MATLAB mathematical toolbox documentation

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