You defined v=1, as scalar, but you called the function as vector v(i)
vh=v(i)-dt*k*z(1,i)/ (2*m);
%....^.........
When i=1, then v(1)=??
When i=2, then v(2)=?? in iterations.
Also the while loops runs multiple times
As t < tend-2*dt, where t=0 and tend=1000, dt=0.1, On such case the size of z also not sufficient
vh=v(i)-dt*k*z(1,i)/ (2*m);
%...............^.........
Because z(:,1)=[0,1] allows only two iterations.
