How can we run for loop in order to get several fitting parameters?

Hello, I am the fitting experimental data with custom equation and I want to do it for several numbers to check uncertainty. The code is fitting value for one number but not in the loop. Can you help me regarding this? when I introduce (i) with ft in the code shown below, the following error occurs: double objects can't be assigned to using subscripts

 if
function [fitresult, gof] = createFit(temp, yieldstress)
temp=[1 2 3 4 5];
yieldstress=[10 20 30 40 50];
nsample=100;
for i=1:nsample
    DD=1e12+2e12*rand(1);
    self_coff=0.08+0.04*rand(1);
    line_energy=0.5+0.5*rand(1);
    C_F=0.32+0.06*rand(1);
    threshold_stress=(25.9e9*2.86e-10.*sqrt(DD).*((0.5.*line_energy)+sqrt(self_coff)))./(1e6);
    ave_strainrate=2.86e-10*1e12.*sqrt(DD);
[xData, yData] = prepareCurveData( temp, yieldstress );
% Set up fittype and options.
ft(i) = fittype( strcat('(',num2str(threshold_stress),'+((0.89.*s).*((1-(((-8.617e-5.*x)./F).*',num2str(log(6.667e-4./ave_strainrate)),').^(1/q))^(1/p))))./',num2str(C_F)), 'independent', 'x', 'dependent', 'y');
opts = fitoptions( 'Method', 'NonlinearLeastSquares' );
opts.Display = 'Off';
opts.Lower = [1.1 0.667 1.5 3];
opts.StartPoint = [1.8 0.667 1.5 4];
opts.TolX = 1;
opts.Upper = [3 1 1.5 20];
end 
% Fit model to data.
[fitresult, gof] = fit( xData, yData, ft, opts );
end

Answer 1

[Ayush Aniket]

The error occurs because you are trying to store fittype objects in an array using subscripts. In MATLAB, arrays of objects like fittype cannot be indexed in the same way as numeric arrays. Instead, you should use a cell array to store these objects as show below:

function [fitresult, gof] = createFit(temp, yieldstress)
    temp = [1 2 3 4 5];
    yieldstress = [10 20 30 40 50];
    nsample = 100;

    % Initialize cell array to store fittype objects
    ft = cell(nsample, 1);

    for i = 1:nsample
        DD = 1e12 + 2e12 * rand(1);
        self_coff = 0.08 + 0.04 * rand(1);
        line_energy = 0.5 + 0.5 * rand(1);
        C_F = 0.32 + 0.06 * rand(1);
        threshold_stress = (25.9e9 * 2.86e-10 * sqrt(DD) * ((0.5 * line_energy) + sqrt(self_coff))) / (1e6);
        ave_strainrate = 2.86e-10 * 1e12 * sqrt(DD);

        [xData, yData] = prepareCurveData(temp, yieldstress);

        % Create fittype object and store it in the cell array
        ft{i} = fittype(strcat('(', num2str(threshold_stress), '+((0.89.*s).*((1-(((-8.617e-5.*x)./F).*', ...
            num2str(log(6.667e-4 / ave_strainrate)), ').^(1/q))^(1/p))))./', num2str(C_F)), ...
            'independent', 'x', 'dependent', 'y');
        opts = fitoptions('Method', 'NonlinearLeastSquares');
        opts.Display = 'Off';
        opts.Lower = [1.1 0.667 1.5 3];
        opts.StartPoint = [1.8 0.667 1.5 4];
        opts.TolX = 1;
        opts.Upper = [3 1 1.5 20];

        [fitresult{i}, gof{i}] = fit(xData, yData, ft{i}, opts);
    end
end

This will allow you to perform multiple fits with different parameters in a loop.