How to multiply different elements of a vector(with no value stored) inside a for loop to generate an equation.

I want to repeatedly add one form of the equation i times, in this I want constants as f(1),f(2)... and variables x1, x2...y1,y2...z1,z2...So, the required outcome for i=1 should be g=f(1).*x1.^2+f(2).*y1+f(3).*z1 as seen by program below(not working). It is necessary for its use with nlinfit.

I am trying to implement it using for loop with null vector(supposedly wrong approach). I have also tried declaring it as f(1)=[]... it also gives an error "Deletion requires an existing variable". It works if I use f as a symbol like x,y or z but that will be useless for me.

Please give me a solution to the problem addressed not the problems in my solution. Thanks :)

x=sym('x',[1 3]);
y=sym('y',[1 3]);
z=sym('z',[1 3]);
f=[];                       %first way to declare null vector
%f=int16.empty(3,0);          %second way to declare null vector
g=0;
a=1;
for i=1:1:3
    g=g+f(i).*x(i).^2+f(i+1).*y(i)+f(i+2).*z(i);
    a=a+3;
end 

[darova]

If the vector is null how can you use its elements? null - empty (no elements)

[Vijay Shekhawat]

That is one approach I could think of, if there is any other feasible solution to this problem please let me know.

[KALYAN ACHARJYA]

"different elements of a null vector"

No, please note what @darova said, there are no elements in the null vector.

[KALYAN ACHARJYA]

f=[];

If you multiply it with any,still it reflects a null vector. In this case there is no meaning of f(i) within loop iteration, which is f(1), f(2)...so on.

[Vijay Shekhawat]

I clearly get it f=[] will not work, so please stop pointing at it again and again. Try to move ahead, I just want to add an equation of this form n times.

x=sym('x',[1 3]);
%How to declare f?? It should be a vector f(1), f(2) ...
n=3
g=0
for i=1:n
g=g+f(i).*x(i);
end
g

The above should return f(1).*x1+f(2).*x2+f(3).*x3.