Mapping a shifted and sequence to a given output axis

Hi,

I have a sequence x1, which is the same lenght of an output vector (nx). However, the output vector goes from -12890:13975. I need to define the output (y[n]) of the system along the axis (nx). I used circshift( ) and made conditions for replacing the back/front fill of the shifted array with zeros, according to the direction of the shift. I think the output might be correct, but I doubt the output is defined along the output axis properly. is the system. Input is x1. Output vector is nx. I've arbitrarily chosen shifts of -30:30 to test plot. Thank you

x1 = x1';
y = zeros(1,length(nx));
k_vals = -30:30;
for k_idx = 1:length(k_vals)
    k=k_vals(k_idx);
    x = circshift(x1,Nb*k);
    if k<0
        x(end + k: end) = 0;
    elseif k==0;
    elseif k > 0;
        x(1:k)=0;
    end
    y = y + (Ab^k).*x;
end