Attempted to access a(3); index out of bounds because numel(a)=2.

I try to use fminsearch to solve for coefficients of the equation below: accelerate = A*exp(B*time)*cos(C*time+D), and I get this error:

Attempted to access a(3); index out of bounds because numel(a)=2. Any one know why? help is much appreciated.

data = load('project3_accel.dat')
t = data(1:end,1);
a = data(1:end,2);
model = @(t,A,B,C,D)A.*exp(B*t).*cos(C*t+D)
fun = @(r,ti,ai) sum((ai-model(ti,r(1),r(2),r(3),r(4))).^2);
options = optimset('TolX',1e-8,'TolFun',1e-8);
r = fminsearch(fun,[-50,50],options,t,a);
A = r(1);
B = r(2);
C = r(3);
D = r(4);

St = sum((a-mean(a)).^2);
Sr = sum((a-model(t,A,B,C,D)).^2);
r_squared2 = 1-Sr/St

Answer 1

[Star Strider]

You will have to change parts of your code to accommodate this change, but this should work:

% b(1) = A,  b(2) = B,  b(3) = C,  b(4) = D
model = @(b,t) b(1).*exp(b(2).*t).*cos(b(3).*t+b(4));
fun = @(b) sum((ai-model(b,t)).^2);

and:

b0 = [-50 0.5 0.5 50];                   % Choose Good Starting Values For All 4 Parameters
r = fminsearch(fun, b0, options);

[Jack13]

Thanks, my problem is not providing enough initial guesses.

[Star Strider]

My pleasure.

It was a bit more involved than that. The curve fitting functions have to have a single vector of parameters to estimate. You have to provide as many initial guesses as you have parameters to be estimated. (I consider the start values as initial parameter estimates.)

I didn’t test the code I posted because I didn’t have your data to test it on.