Dear Shirin,
in your code, the variable u is not a vector. Try this:
x=8;
b=9;
for i=1:2
w(i)=x+b
u(i)=x+w(i)
end
However, notice that in this way your vector will be increasing; this is a bad practice from the point of view of computational efficiency.
Most importantly, there is strictly no reason for you to obtain the result you posted beause w(i) = x+b = 17 for any index i, hence u(i) = x+w(i) = 2x+b = 25 is constant as well (no matter if u is a vector or a scalar). Are you sure of your algorithm?
