Fitttype inside app designer

I am having trouble using fittype inside app designer.

ft = fittype("HC(x,Radius,Amp,Background,Pd)");

returns Expression HC(x,Radius,Amp,Background,Pd) is not a valid MATLAB expression, has non-scalar coefficients, or cannot be evaluated:

Error in fittype expression ==> HC(x,Radius,Amp,Background,Pd)

??? Undefined function 'HC' for input arguments of type 'double'.

but I have defined the function HC.

I even tried ft = fittype("HC(x,Radius,Amp,Background,Pd)");

but it doesn't help.

function HC_Fitting(app)
        %28th November 2019 - Shankar Dutt [e-mail]
        %It initiates the fitting procedure
         if exist('Graph')
            delete(app.Graph)
         end
        xData = app.q;
        yData= app.r;
        ft = fittype("HC(x,Radius,Amp,Background,Pd)");
        opts = fitoptions( 'Method', 'NonlinearLeastSquares' );
        opts.Display = 'final';

        opts.MaxFunEvals = 60000;
        opts.MaxIter = 40000;
        opts.TolFun=1e-6;
        opts.TolX=1e-6;
        
        Radius=app.CoreRadiusEditField.Value;
        Amp=app.AmplitudeEditField.Value;
        Background=app.BackgroundEditField.Value;
        Pd=app.PolydispersityEditField.Value;
       
        opts.StartPoint = [Amp Background Pd Radius];
     
        
        opts.Lower = [0  0 0 0];
        opts.Upper = [Inf 50 0.5 Inf];
        % Fit model to data.
        [fitresult, ~] = fit( xData, yData, ft, opts );


        app.CoreRadiusEditField.Value=fitresult.Radius;
        app.AmplitudeEditField.Value=fitresult.Amp;
        app.BackgroundEditField.Value=fitresult.Background;
        app.PolydispersityEditField.Value=fitresult.Pd;
        HC_plot(app);
        end
     
        function y=HC(app,q,Radius,Amp,Background,Pd)

        %28th November 2019 - Shankar Dutt [e-mail]
        %This function incorporates the Hard Cylinder FormFactor and Gaussian
        %Distribution

        syms z;

        %Amp = 0.01;Background=0.001;Radius=400;Pd=20;
        y1 = @(z) HC_formfactor(app,q,z); % This loads the Form_Factor of Hard Cylinder Model


        %In the following we define the distribution and its integration.
        %Limits are technically 0 to infinity but I have chosen them in a way that
        %it gives results without much computational energy.


        va = Radius - app.NumptsEditField.Value*Pd; %This is the lower limit of the integration
        vb = Radius + app.NumptsEditField.Value*Pd;%This is the upper limit of the integration
        if(va<0)
            va=0;
        end
        D =@(z) LN_Distribution(app,z,Radius,Pd);
        fun = @(z) y1(z).*D(z);
        y1=integral(D,va,vb,'ArrayValued',true);
        y=integral(fun,va,vb,'ArrayValued',true)/y1;
        y=abs(y.*Amp-Background);
end

Answer 1

[Abhishek Kumar]

As I understand your are trying to use fittype function, as you have already defined the function HC, please use the following syntax to resolve the error.

ft = fittype(HC(x,Radius,Amp,Background,Pd));

You can refer to the following documentation of fittype for further reference:

https://www.mathworks.com/help/curvefit/fittype.html