Please advise - Error 'constraints' Too many output arguments

I have two .m files

function [c,ceq] = constraint(x)

c = [x(1)+x(2)-1;
     x(2)-x(1);
     x(2)-(1/2);
    -x(1);
    -x(2)];
ceq = [];
end

and

function f = model_stictionptt(x)

global y uv OP_N PV_N  models

%-------load data test--------%
load normalize_data.mat
OP_N=OPN;
PV_N=PVN;

%----Time-----%
TT=0:1:length(PV_N)-1;
Time=TT';

%--estimate MV by He's madel 3 parameter--%
%----parameter-----%
K=1.99;
n=length(OPN);
uv(1)=0;
for i=2:n
u(i)=OP_N(i);
e(i)=u(i)-uv(i-1);
T(i)=Time(i);
if(abs(e(i))> x(1))
   uv(i)=uv(i-1)+K*(e(i)-sign(e(i))*x(2));
   TT1(i)=T(i);
else
    uv(i)=uv(i-1);
    TT1(i)=T(i);
end
    
end

%------ process model by using uv --------%
 z = iddata(PV_N,uv',1,'Tstart',0);
%-------ARX model-------------------------%
   na=1;
   nb=3;
   nc=1;
 models = arx(z,[na nb nc]);
 yy=compare(z,models);
 y=yy.OutputData;

%-----cost function-------%
nn=length(PV_N);
   f= (1/nn)*sum((y-PV_N).^2);
end

Using fmincon

options = optimoptions('fmincon','Algorithm','sqp');
[x,fval]=fmincon(@model_stictionptt,x0,[],[],[],[],[],[],@constraint,options)

gives the error

Error using constraint
Too many output arguments.

Error in fmincon (line 623)
        [ctmp,ceqtmp] = feval(confcn{3},X,varargin{:});

Error in run_optimizeptt (line 21)
[x,fval]=fmincon(@model_stictionptt,x0,[],[],[],[],[],[],@constraint,options)

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

Answer 1

[Alan Weiss]

Do not give bounds and linear inequality constraints as a nonlinear constraint function. Your first two nonlinear constraints should be represented in A and b arguments as

A = [1 1;-1 1];
b = [1;0];

(I assume that your variable x is 2-D.) The other constraints should be represented by bounds:

lb = [0 0];
ub = [Inf 1/2];

So you should not use that constraint function at all.

As to why you are getting your current error, I don't know. I suggest that you evaluate the following and see if it leads to an error:

[ctest,ceqtest] = constraint(x0)

where x0 is the same value you use in your fmincon call.

Alan Weiss

MATLAB mathematical toolbox documentation