Nonlinear parameters search

Question

need to find a set of optimal parameters P of the system y = P(1)*exp(-P(2)*x) - P(3)*x where x and y are experimental values. I defined my function

f = @(P) P(1)*exp(-P(2)*x) - P(3)*x

and

guess = [1, 1, 1]

and tried

P = fminsearch(f,guess)

according to Help. I get an error

Subscripted assignment dimension mismatch.

Error in fminsearch (line 191) fv(:,1) = funfcn(x,varargin{:});

I don't quite understand where my y values would fall in, as well as where the function takes P from. I unfortunately have no access to nlinfit or optimization toolboxes.

Answer 1

You should try the matlab function lsqnonlin(@testfun,[1;1;1]) But first make a function and save in an m-file that includes all the data points, lets say your y is A and x is x like here below:

function F = testfun(P)
A = [1;2;3;7;30;100];
x = [1;2;3;4;5;6];
F = A-P(1)*exp(-P(2)*x) - P(3)*x;

This minimized the 2-norm end gives you the best parameters.