Supplied objective function must return a scalar value in fminunc

I write the following code for AR(2) process maximum likelihood estimation

global y t;

y = [1 2 4 6 4 12 11 13 11 14 16 17]';
T = length(y);
t = [1:T]';

x0 = [1, 1, 0.1, 0.1];
[x, loglval] = fminunc(@(x) logl(x(1), x(2), x(3), x(4)), x0);
disp(x)

and the log likelihood function

function val = logl(a, b, p, s)
    global y t T
    f = y(2:T) - a*ones(T-1,1) - b*t(2:T) - p*y(1:T-1) - p*a*ones(T-1,1) - p*b*t(1:T-1);
    val = -log(2*pi)*T/2 - T*log(s)/2 + log((1-p^2)^(1/2)) - (1/(2*s))*((1-p^2)*(y(1)-a-b*t(1))^2 + sum(f.*f));
end

But I get Supplied objective function must return a scalar value error. ı do not understand the error. I run the code with isscalar(val) and it returns 1. Could you help me?

Answer 1

[Walter Roberson]

%driving script
global y t;

%your function
global y t T

Since you do not initialize T within your function or within the context of a global, the value of it will be empty by default, giving you an empty result.

We recommend firmly against using global, as these kinds of mistakes are too easy to make with global.

http://www.mathworks.com/help/matlab/math/parameterizing-functions.html