? Subscript indices must either be real positive integers or logicals. 什么错误啊?

? Subscript indices must either be real positive integers or logicals. 什么错误啊?
n=[1:15];x=zeros(1,15);x=rand(1,10);z=zeros(1,15);y=[x,0,0,0,0,0];
forj=3:1:12
z(j)=y(j-2)
end
subplot(2,1,1);stem(n,y);subplot(2,1,2);stem(n,z)