Error when using fminsearch to find least squares fit of data using a given equation

I have a dataset and a nonlinear equation that should fit it. The goal is to minimize the least squares error using fminsearch. I've gotten this code to work with a simple equation (just finding one variable) but can't make it work when the output is a vector. The data 'lag' and 'vario' are both 20-element vectors. The code I'm working with is below. When I run it, I get an error saying "unable to perform assignment because the size of the left side is 1-by-1 and the size of the right side is 1-by-20." I've been trying to figure this out for ages -- any thoughts? I'm a super beginner at this, so if the answer is obvious I apologize in advance!

ypred = zeros(1,length(lag));

for k = 1:length(lag)

if lag(k)<m(1)

ypred(k) = m(2)*(1.5*(lag(k)/m(1))-0.5*(lag(k)/m(1)).^3);

else

ypred(k) = m(2);

end

end

yerr = (vario-ypred);

yerr = sum(yerr.^2);

end

m = fminsearch(@(m) varerr(m,lag,vario),[2 2]);

plot(lag, varerr(m, lag, vario)); shg

Answer 1

[Alan Weiss]

I suggest that you use the debugger.

Alan Weiss

MATLAB mathematical toolbox documentation

[Michael Hasson]

So when I do that, it takes me to line 200 of the actual fminsearch.m file, at which point it's using a variable "fv" that's defined within the fminsearch function. So I guess I'm not sure how to think about debugging when the issue is within the function I'm calling, if that makes sense.

Answer 2

[Star Strider]

I can’t figure out from your code what your independent and dependent variables are.

As a general rule, the argument to fminsearch for data-fitting problems is:

objfcn = @(b,x) ...;                                    % Model
fitfcn = @(b) norm(y - objfcn(b,x));                    % Returns Norm Of Residuals
[B,resnrm] = fminsearch(fitfcn, [2 2])                  % Estimate Parameters

ypred = objfcn(B,x);                                    % Calculate Fit

figure
plot(x, y, 'p')
hold on
plot(x, ypred, '-r')
hold off
grid

Here, ‘x’ are the independent variables, ‘y’ are the dependent variables, and ‘b’ is the parameter vector.