Error in lsqcurvefit with 3 unknown parameters involved

Hi. Would like to ask for any enlightenment here. I am using lsqcurvefit as a curvefitting method.... However, I could't decipher how the error keeps on appearing on the parameters even if I changed the values. As you can see below, the resulting equation is:

but it should be . Hence the error regarding array.

Hoping for some help here. Thank you.

syms i_x
syms parameter [1 2 3]

YYx=exp((-parameter(1).*valx./parameter(2))+parameter(3).*i_x)

YYHCl = 

fun4=matlabFunction(YYx,'vars',{parameter,i_x});
obj=@(P4,I_x) arrayfun(@(i_x) fun4(P4,i_x), I_x);
P40=0.003;
P4=lsqcurvefit(obj,P40,I_x(:),Y_x(:));

Index exceeds the number of array elements. Index must not exceed 1.

Error in symengine>@(in1,i_HCl)exp(i_HCl.*in1(3)-in1(1)./in1(2))

Error in solution (line 14)
obj=@(P4,I_HCl) arrayfun(@(i_HCl) fun4(P4,i_HCl), I_HCl);

Error in solution (line 14)
obj=@(P4,I_HCl) arrayfun(@(i_HCl) fun4(P4,i_HCl), I_HCl);

Error in lsqcurvefit (line 225)
initVals.F = feval(funfcn_x_xdata{3},xCurrent,XDATA,varargin{:});

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

Y_HCl_calcc=obj(P4,I_HCl(:));

plot(I_HCl, Y_HCl, 'k*', I_HCl, Y_HCl_calcc, 'g-o')
legend({'Y_HCl exp)', 'Y_HCl calc)'}, 'location', 'best')

parameter1=P4(1)
parameter2=P4(2)
parameter3=P4(3)

[Star Strider]

There are a number of posts on fitting ODEs to data. See for example Coefficient estimation for a system of coupled ODEs - MATLAB Answers - MATLAB Central

.

[lvenG]

Thank you so much

Answer 1

[Matt J]

If your model has 3 unknown parameters, then your initial guess P40 must be a vector of length 3.

[lvenG]

Thank you so much that finally made sense, didn't realize it at first.