Problem with Autoregressive AR(3) Model. Take a look please

I need to plot an AR(3) process, with the following constraints.

I`m typing this

z=randn(1,1020);
z(1)=0;
phi = [-0.3, 0.9, -0.15];
x(1)=0;
for t=3:1023
 x(t) = phi(1)*x(t-1) + phi(2)*x(t-2) + phi(3)*(t-3)+z(t); 
end
plot(x);  % code

Where is my mistake? Please help

Answer 1

[Mark Whirdy]

x is of size 1 so need to "declare" it first as zeros(1,1023); also it is trying to access z(1023) but z has only 1020 elements

z=randn(1,1021);
x= zeros(1,1023);
z(1)=0;
phi = [-0.3, 0.9, -0.15];
x(1)=0;
for t=3:1023
 x(t) = phi(1)*x(t-1) + phi(2)*x(t-2) + phi(3)*(t-3)+z(t-2); 
end
plot(x);  % code

[MAT-Magic]

Inside the for loop, I think the x is missing, it should be like phi(3)*x(t-3). Isn't it?