fitting to a curve defined by an integral

Is it possible to use 'fit' to fit data to a function that involves an integral? Here's the code I'm using.

 g = fittype('a*integral(@(tau) exp((-x-tau).*(a/b)),0,x)','coeff',{'a','b'});
 fit(X,Y,g)

but this gives me the following error:

 Error using fittype (line 356)
 Expression a*integral(@(tau) (exp((-x-tau).*(a/b)),0,x) is not a valid MATLAB expression, has non-scalar
 coefficients, or cannot be evaluated:
 Error in fittype expression ==> a.*integral(@(tau) exp((-x-tau).*(a./b)),0,x)
 ??? A and B must be floating point scalars.

Does anybody know how to do this? Thanks in advance

[Jin Ow]

Have you solved this problem?

I'm now fitting a curve defined by an integral as well, but error keeps coming out. T_T

Answer 1

[Michael Soskind]

Hi James and Jin,

Would you both be willing to take a bit of a different approach? The reason the fit is not working for you is that you are trying to input an array into the integral function. Instead of an array, I recommend making a function which can take an array in, and then using this to output an array of the integrated values. This is what I do in the code below:

% Defining coefficients for a sample plot
a = 10;
b = 12;
tau = 0.5;

% Creating the data to fit to
x = -1:0.1:1;
y = func(a,b,x,tau);
plot(x,y, 'x'); hold on;        % plotting the data as points

% Implementing lsqcurvefit, which I prefer to fittype
coeff = lsqcurvefit(@(c, xdata) func(c(1), c(2), xdata, tau), [5,7], x, y);
plot(x,func(coeff(1), coeff(2), x, tau), '--')      % plotting the fit as a dashed line

% defining the function that returns an array of the integrated values with an arry of input x values
function [y] = func(a, b, x, tau) 
    y = zeros(1, numel(x));
    for i = 1:numel(x)
        y(i) = a.*integral(@(x) exp((-x-tau).*(a./b)),0,x(i));
    end
end

Hopefully the above code can get you both on the right path towards successfully fitting your function.

Best,

Michael

[Jin Ow]

thx. Actually I've solved this problem a week ago, and the method is similar as yours.

But still, thanks for providing a detailed example to this fitting problem. ;)