You can add an additional input argument to your createFit function:
function [fitresult gof] = createFit(z,y,startPoint) % use your existing input and output variable names
% your code
end
You can make it an optional argument by checking whether it was given, and if not, use the default:
function [fitresult gof] = createFit(z,y,startPoint) % use your existing input and output variable names
if nargin < 3
startPoint = [226 26 1E07 0.4]; % whatever default value
end
% your code
end
Then call createFit with 2 or 3 arguments:
[fitresult gof] = createFit(z,y); % no startPoint specified -> createFit will use the default defined in its function definition
[fitresult gof] = createFit(z,y,[226 26 1E07 0.4]); % explicitly using default startPoint -> same as no startPoint specified
[fitresult gof] = createFit(z,y,[0 0 0 0]); % using some other startPoint
