# Least square fitting of experimental data

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Zine on 12 Dec 2013
Commented: Zine on 14 Dec 2013
I have experimantal data xdata,ydata; and I want to fit a model to these data, the model shape is like Y(xdata)=[A*exp(C*D/xdata)]/[E+F*exp(C*D/xdata)+...]; where Y should be very near to ydata (curve fitting), my problem is that the parameters A B C D... are function of xdata too with sub-parameters to be determined, I proceeded to write a huge function of sub-parameters but the process takes time of writing code and checking and if there is error it is difficult to locate it, so my question is : can I declare these parameters as parametric variables and how to do it if possible? and; is there other shortcut to bypass the long functions which increase the possibility of typing errors and facilitate the replacement of the variable function like I input A=exp(a-c); and if does not give good results I change easily to A=(1/d+e)?... thanks in advance

Alan Weiss on 12 Dec 2013
Edited: Alan Weiss on 12 Dec 2013
If you have Symbolic Math Toolbox, you can use matlabFunction to generate your function handles or program files. For examples related to optimization (not exactly curve fitting, but close, you would use lsqnonlin or lsqcurvefit instead of fmincon), see this example or this example. There is also an example on the file exchange that uses data in a symbolic test, and might be somewhat relevant.
Alan Weiss
MATLAB mathematical toolbox documentation
Zine on 14 Dec 2013
thanks very much, solved

### More Answers (1)

John D'Errico on 12 Dec 2013
Edited: John D'Errico on 12 Dec 2013
This just seems a problem of modular programming. It means you need to learn a good programming style, so that you CAN modify component parts of your function. But there is very little we can do for you in that respect.
• Break the problem into pieces.
• Learn to use nested functions.
• Test every piece separately. If they all work separately, then they will work as a whole.
• Document your code, so it will be easy to debug. If the code is easy to read, it will be easy to repair too.
Beyond that, there is no magic. There are no shortcuts to writing good code.

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