HOW TO SLOVE ODE WITH AN CONTROL INPUT?

Hello I'm trying this Question :

dx/dt = 0.5*x + 10*u + 30*sin(t)

error signal e(t) = 3

if x>=e(t), u = -0.45*x

if x<e(t), 'u' retains its previous value

I've tried this code:

function dx=sys(t,x)
dx=0.5*x + 10*u + 30*sin(t)
end

Please save the above function in an MATLAB file and name is as sys.m

Then, in a separate MATLAB script file or in the command window, you can simulate the system by executing the following

>> u(0) = 0;
>> tspan = [0 10];
>> iniCon = [0]
>> [t, x] = ode45(@sys, tspan, iniCon);
>> for k=1:10
>> if x(k)>=et
>>     u(k)=-0.45*x(k);
>> else
>>     u(k)=u(k-1);
>> end
>> end
>> figure(1)
>> plot(t,x)

but I,m getting errors like :

"Subscript indices must either be real positive integers or logicals.

Error in firstplot (line 1) u(0) = 0;"

please help me out for solving this problem