Error using fmincon function

% Define constants for Antoine's equation
A = 8.07131;
B = 1730.63;
C = 233.426;

% Define the operating temperature at the outlet of the evaporator (in Celsius)
T = 70; % You can adjust this value based on the actual temperature

% Define the partial pressure of water at the outlet of the evaporator (kPa)
Pi = 30;

% Define the mass flow rate of seawater entering the evaporator (in kg/s)
Fseawater = 0.020; % kg/s

% Define the objective function to maximize water recovery
objective = @(x_vap) -calculate_water_recovery(x_vap, A, B, C, T, Pi, Fseawater);

% Define lower and upper bounds for the decision variable (mole fraction of water in vapor phase)
lb = 0; % Lower bound
ub = 1; % Upper bound

constraint = @(x) 20 - calculate_water_recovery(x, A, B, C, T, Pi, Fseawater);

% Perform optimization
options = optimoptions('fmincon', 'Display', 'iter');
[x_vap_opt, ~, exitflag] = fmincon(objective, 0.5, [], [], [], [], lb, ub, [], [], options);

Error using solution>@(x_vap)-calculate_water_recovery(x_vap,A,B,C,T,Pi,Fseawater)
Too many input arguments.

Error in objfunEvaluator (line 5)
fval = feval(Objfun, x, self.FunArgs.AdditionalParameters{:});

Error in OptimFunctions/objective (line 271)
[fval_, fgrad_, hess_] = self.ObjectiveFunAndGrad(self,self.FunFcn{3},...

Error in OptimFunctions/objective_first_eval (line 614)
[fval,self] = self.objective(X0);

Error in fmincon (line 500)
[initVals.f,initVals.g,HESSIAN,funObj] = funObj.objective_first_eval(X);

Caused by:
Failure in initial objective function evaluation. FMINCON cannot continue.

if exitflag > 0
    % Calculate water recovery and display results
    [Lwater, Vwater] = calculate_water_recovery(x_vap_opt, A, B, C, T, Pi, Fseawater);
    disp(['Optimal mole fraction of water in vapor phase: ', num2str(x_vap_opt)]);
    disp(['Optimal mass flow rate of freshwater product: ', num2str(Vwater), ' kg/s']);
    disp(['Optimal percent water recovered: ', num2str(Lwater), '%']);
else
    disp('Optimization failed to converge.');
end

function [Lwater, Vwater] = calculate_water_recovery(xi_vap, A, B, C, T, Pi, Fseawater)
% Calculate the saturation vapor pressure using Antoine's equation
Pisat = exp(A - B / (C + T));

% Calculate the mole fraction of water in the liquid phase using Raoult's law
xi_liq = Pi / Pisat;

% Calculate the mole fraction of water in the vapor phase
xi_vap = 1 - xi_liq;

% Calculate the mass flow rate of freshwater product
Vwater = xi_vap * (0.0180153); % kg/s

% Calculate the percent water recovered
Lwater = (1 - (Fseawater - Vwater) / Fseawater) * 100;
end

Answer 1

[Lokesh]

Hi Nazia,

It appears you are encountering the "Too many input arguments" error while using the 'fmincon' function.

The correct syntax for the 'fmincon' function is as follows: "x = fmincon(fun, x0, A, b, Aeq, beq, lb, ub, nonlcon, options)." Based on your code, it seems that you are passing an extra argument in your function call. To resolve this issue, you should modify your function call from:

%Existing function call
[x_vap_opt, ~, exitflag] = fmincon(objective, 0.5, [], [], [], [], lb, ub, [], [], options);

%Modified function call 
[x_vap_opt, ~, exitflag] = fmincon(objective, 0.5, [], [], [], [], lb, ub, [], options);

This adjustment removes the extra argument, aligning the call with the expected syntax and should solve the issue.

Refer to the following MATLAB documentation for more information on 'fmincon':

https://www.mathworks.com/help/optim/ug/fmincon.html

[Nazia]

Thank you. I also want to include the constraint in my optimization problem, in the modified function call, but that does not work. Do you know how to go about this?

[Steven Lord]

Then you should include constraint instead of the last [] in the call.

[x_vap_opt, ~, exitflag] = fmincon(objective, 0.5, ... % objective function and x0
    [], [], [], [], ... % linear inequality and equality constraints
    lb, ub, ... % lower and upper bounds
    constraint, ... % nonlinear constraints
    options); % options

Answer 2

[Malay Agarwal]

Hi @Nazia,

I understand that you are trying to use the “fmincon” function and are receiving an error.

The error is “Too many input arguments”, which is raised when a function is given more input arguments than it expects.

According to the documentation of “fmincon”, the function accepts at most 10 arguments: https://www.mathworks.com/help/optim/ug/fmincon.html#d126e98025.

In your code, you are passing 11 arguments to the function, which results in the “Too many input arguments” error. Please replace the call to “fmincon” with the following call:

[x_vap_opt, ~, exitflag] = fmincon(objective, 0.5, [], [], [], [], lb, ub, [], options);

This does not raise the error since there are exactly 10 arguments and the code has the following output:

Hope this helps!