You initialize a(1) to 1 and a(2) to 1, but then on line 7 you run
a = 1:z;
This changes a(1) to 1 and a(2) to 2. You can remove the line 7 (which makes a grow on each loop iteration), or you can put a=1:z before line 5. Even better, initialize it to a = zeros(1, z), again before line 5.
