FMINCON - Failure in initial objective function evaluation. FMINCON cannot continue

I am trying to solve a bar optimization problem, passing the initial conditions (the area of each bar) and all variables as a struct but I am facing the following problem:

 "Failure in initial objective function evaluation. FMINCON cannot continue."

fem = struct(...
    'x',x0,...                   % Initial set of points
    'NODE',NODE,...              % [NNode x 2] array of nodes
    'ELEM',ELEM,...              % [NElem x 2] array of elements
    'v',{v},...                  % Cell defining the variables
    'NNode',size(NODE,1),...     % Number of nodes
    'NElem',size(ELEM,1),...     % Number of elements
    'NVar',size(v,1)...          % Number of variables
     );

    Plot10bar(fem);
    mass = GetMass10bar(fem);
    fprintf('Initial Mass = %f\n',mass);

    [x,fval,exitflag,output,lambda,grad,hessian] = ...
        fmincon(@GetMass10bar,x0,[],[],[],[],lb,ub,@GetConstraints10bar,options);   
    mass = GetMass10bar(fem);
    fprintf('Final Mass = %f\n',mass);
    Plot10bar(fem);

Thank you,

André Leitão

[Matt J]

If you evaluate the objective function at the initial point from the command line

 >> GetMass10bar(x0)

you should see it fail.

[AXL]

GetMass10bar evaluates the mass of 10-bar-truss! If I want to evaluate the initial mass, before the optimization process happens, I have to use, as argument of this function, the struct fem, which contains cross-section areas and nodal coordinates (used in to optimized the truss).

Answer 1

[AXL]

I solved it using this:

    [x,fval,exitflag,output,lambda,grad,hessian] = ...
    fmincon(@GetMass18bar,x0,[],[],[],[],lb,ub,...
    @GetConstraints18bar,options,fem);

Answer 2

[Walter Roberson]

You assign a struct to fem and then you call

      mass = GetMass10bar(fem);

this suggests that the function GetMass10bar normally expects to be passed a single parameter that is a struct.

You then have

   [x,fval,exitflag,output,lambda,grad,hessian] = ...
        fmincon(@GetMass10bar,x0,[],[],[],[],lb,ub,@GetConstraints10bar,options);

fmincon passes a numeric vector with the same number of elements as x0; it does not pass a struct.

If your code for GetMass10bar cannot handle either a struct or a numeric vector, then the fmincon call to GetMass10bar is going to fail.

GetMass10bar appears to be your own code; I do not find any reference to it anywhere on the Internet.

[AXL]

Yeah, GetMass10bar is my code! I could solve it (after a couple of hours trying to figure out if fmincon returns more than one output) by adding the struct as an input in the fmincon, like that:

[x,fval,exitflag,output,lambda,grad,hessian] = ...
    fmincon(@GetMass18bar,x0,[],[],[],[],lb,ub,...
    @GetConstraints18bar,options,fem);

But, thanks anyway

[Walter Roberson]

Please instead refer to http://www.mathworks.com/help/matlab/math/parameterizing-functions.html

[AXL]

I will take a look at this carefully.