Objective Function must return a scalar value.

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Richard 2020-3-7

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https://ww2.mathworks.cn/matlabcentral/answers/509537-objective-function-must-return-a-scalar-value

评论： Walter Roberson 2020-3-29

采纳的回答： Walter Roberson

Topo_3D.m

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Dear friends,

I am trying to do an optimization for a function using a "fmincon" function. I have got an error and couldn't find the problem.

The following is the error obtained :

Error using fmincon (line 609)
Supplied objective function must return a scalar value.
Error in Topo_3D (line 31)
[x,fval,ef,output,lambda,hessian] = fmincon(objective,x0,A,b,Aeq,beq,lb,ub,nonlincon,options);

A copy of my fmincon function is attached.

I appreciate if somebody help with this issue and how can be solved.

Thank you very much.

RTFQ

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Answer 1

Walter Roberson 2020-3-7

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/509537-objective-function-must-return-a-scalar-value#answer_418979

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f = nf*'u';

That multiplies the column vector nf by the literal character lower-case U. The 'u' will be converted to the character position that is used to encode 'u' which happens to be 117. So the vector nf is multiplied by the scalar 117 which yields a vector. However the first output of the objective function must be a scalar.

Your code happens to ignore the content of the variable you assigned, namely u

Perhaps you want nf*u' if u is a column vector the same length as nf

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Richard 2020-3-29

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Thank you Mr Walter, based on your advises and help my problem is solved.

However, my graph now is showing a 2D truss. Supposedly it should be a 3D space truss.

Below is my function to plot for the graphs.

Hope that Mr Walter can give me some advice on enhancing it to a 3D graph.

% switch, case execute one of several groups of statements
    switch state 
         case 'init'
            hold on
         case 'iter'
            % Link current point and objective function
            % value with history. x must be a row vector.
            history.fval = [history.fval; optimValues.fval];
            history.x = [history.x; x];
            figure(1);
            plot(optimValues.iteration,optimValues.fval,'o');
            xlabel('Iterations');
            ylabel('Objective function');
            title('Sequence of Points Computed by fmincon');
            truss(x);
            % Label points with iteration number and add title.
            % Add .15 to x(1) to separate label from plotted 'o'
            %text(x(1)+.15,x(2),num2str(optimValues.iteration));
         case 'done'
            hold off
         otherwise
     end
 end
 function plottruss = truss(x)
    [node, elem, L] = nodelem(); % Get node and element list
    % multiplication factor for area (to be adjusted)
    mult = 10^4;
    % plot the truss
    Nel = size(elem,1);
    figure(2);
    plot(node(:,1), node(:,2), 'k.')
    hold on; axis equal;
    for iel = 1:Nel
        elnodes = elem(iel, 1:2);
        nodexy = node(elnodes, :);
        plot(nodexy(:,1), nodexy(:,2), 'LineWidth', (x(iel)*mult))
    end
 end
end

Thank you very much!

Richard 2020-3-29

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Mr Walter,

May I know how do I transform this into a 3D plot for:

I have x,y,z for my nodes. It was imported from Opensees.exe.

function plottruss = truss(x)
    [node, elem, L] = nodelem(); % Get node and element list
    % multiplication factor for area (to be adjusted)
    mult = 10^3;
    % plot the truss
    Nel = size(elem,1);
    figure(2);
    plot(node(:,1),node(:,2),'k.')
    hold on; axis equal;
    for iel = 1:Nel
        elnodes = elem(iel, 1:2);
        nodexy = node(elnodes, :);
        plot3(nodexy(:,1), nodexy(:,2), repmat(x(iel), size(nodexy,1), 1))
    end
 end
 end

Thanks.

Walter Roberson 2020-3-29

As far as I can tell from the code that you posted earlier, there is no z coordinate in the truss file. It is, though, possible that the data exists in the file somewhere but is not read in. I do not have documentation on the file format.

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Objective Function must return a scalar value.

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