spring mass using ode45

I wrote this program for solving a spring mass equation using ode45 but it shows an error at the ode45

M = 1.0;
K = 10;
C = 1;
l = 0.5;
g = 9.8;

x0 = [0;
    0];

tspan = [0, 10];

[t, x] = ode45(@func, tspan, x0);

subplot(2,1,1)
plot(t, x(:, 1));
grid('on');
%axis([0 10 -1 1]);
xlabel('time [s]');
ylabel('x [m]');

subplot(2,1,2)
plot(t, x(:, 2));
grid('on');
%axis([0 10 -1 1]);
xlabel('time [s]');
ylabel('$\dot{x}$ [m/s]');

% 導関数の定義
function xdot = func(t, x)
M = 1.0;
K = 10;
C = 1;
l = 0.5;
g = 9.8;
xdot = [x(2);
    -K*(x-l)*x(1)/M-C*x(2)/M+g]*dt;
end

Answer 1

[Alan Stevens]

Like this

x0 = [0;
      0];
tspan = [0, 10];
[t, x] = ode45(@func, tspan, x0);
subplot(2,1,1)
plot(t, x(:, 1));
grid('on');
%axis([0 10 -1 1]);
xlabel('time [s]');
ylabel('x [m]');
subplot(2,1,2)
plot(t, x(:, 2));
grid;
%axis([0 10 -1 1]);
xlabel('time [s]');
ylabel('dx/dt [m/s]');

function xdot = func(~, x)
  M = 1.0;
  K = 10;
  C = 1;
  l = 0.5;
  g = 9.8;
  xdot = [x(2);
         -K*(x(1)-l)*x(1)/M-C*x(2)/M+g];  % x(1) not just x.  No need for dt
end