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.
