MATLAB程序错误?Subscript indices must either be real positive in

MATLAB程序错误?Subscript indices must either be real positive integers or logicals.>> for i=1:1:100x(i)=x(i-1)+vx(i)*1y(i)=y(i-1)+yx(i)*1r(t)=sqrt(x(i)^2+y(i)^2)ax(i)=-G*M*x(i)/r(i)^3ay(i)=-G*M*y(i)/r(i)^3vx(t+1)=vx(t)-ax(i)*1vy(t+1)=vy(t)-ay(i)*1end