function adv_LF(a)

% 1D advection equation u_t + u_x = 0 with Dirichlet boundary conditions
% Leap Frog

N = 2^11;
h = 1/N;
x = (0:h:1)';
u0 = 10 * (1-x) .* exp(-200*(.5-x).^3) .* sin(30.*exp(-100*(x-.2).^2));
figure(1); clf; plot(x,u0);
pause


T = 1;
dt = a/N;       % choose a good time step
r = dt/h;
I = 2:N;
U = u0;

% 1st step: Runge Kutta 2
Up = U;
Utemp = zeros(size(U));
Utemp(I) = U(I) - r/4*(U(I+1)-U(I-1));    % like U at time dt/2
U(I) = U(I) - r/2*(Utemp(I+1)-Utemp(I-1));
V = zeros(size(U));

for n = 1:400

    V(I) = Up(I) - r*(U(I+1) - U(I-1));
    Up = U;
    U = V;
    
    figure(1); clf;
    plot(x,U); axis([0 1 -3 5]);
    
end