Autocorrelation function in matlab without using built-in function

Question

I am trying to write a code for the colculate autocorrelation function. Could you suggest what I'm doing wrong. Thanks in advance

 i = 1:199;
    U =sin(0.3*i);
    sum_1 = 0;
    for tau = 1:length(U)
       for   t = tau:length(U)-1
           
             sum_1 = sum_1+sum(U(t).*U(t+1-tau));
             r(tau)=sum_1;
        end
    end

Answer 1

You can do something like this. This is an implementation of the original algorithm with only a small optimization; calculating half the sequence and copying the remaining half.

n = length(x);
m = 2*n - 1;
rxx = zeros(1,m);
for i = 1:length(x)
    rxx(i) = dot(x(n-i+1:n), x(1:i).');
    rxx(m-i+1) = rxx(i)';
end

Example:

x = [2, 3, -1];

rxx =
    -2     3    14     3    -2

% MATLAB built-in
xcorr(x,x)
ans =
    -2     3    14     3    -2