How to use user input for fittype function

I want to take user inputs for a function and in turn use it in the fittype function. I realize that the fittype uses only string values so I converted my input to a string. However it is still not working and it keeps giving me this:

Error using fittype Expected a string for the parameter name, instead the input type was 'cell'.

Here is my code:

 g=input('input your function','s')
f = fittype('g',{g});
nargs = numargs(f)
args = argnames(f)
args=sym(args)

and this is my input that I'm using for g:

 a*x^2+b*exp(n*x)

Thanks

Answer 1

[Javier Bastante]

Try this:

ft = fittype( 'a*x^2+b*exp(n*x)', 'independent', 'x', 'dependent', 'y' )

[Josiah McCarther]

Thanks for the response, but I don't think this is exactly what I'm looking for. I want to be able to read out the variables of a function based on user input instead of typing the equation directly into the code.

[Javier Bastante]

well you are right, but then use this:

g=input('Input your function: ','s');
ft = fittype( g, 'independent', 'x', 'dependent', 'y' )

and then introduce your input function, a*x^2+b*exp(n*x), or whatever.

I use this way, what I saw on your code us that you introduced {g} as input argument, and that's why MATLAB says that you are introducing a cell because {} are used for cell variables. I hope it works this time. Regards