Lax wendroff Two-step method

hi i am trying to program a generalised lax wendroff two step method to solve the general equation

du/dt + df(u)/dx = 0

the code i have come up with is as shown below (sorry im not sure how to make it look more tidy)

i belive im doing something wrong in the first step with no F function included, however when i place this in or try to i get a graph that can't possibly be correct

thanks for any help kyle

function compare close all;clc;clear all

%intial values

ntime = 1000000; dt=0.00040; nx=100; time = 0; a=1;output=0.4;

%step size calculation

dx= (1/nx);

%create size of u_int vector

u_int = zeros(nx,2);

%create little u_int vector for the initial values of height begining and end depending which value of A is used

u_int(nx,2) = 1;

u_int(1,1) = 1;

%loop for the two directions needed

for vec = 1:2

% %lax_wendroff

[U] = lax_wendroff_2(nx,a,output, dt, ntime, time,u_int, vec,dx);

figure(vec)

plot(U(:,vec),'r*');grid on;legend('centered');

hold on

end

end

function [U] = lax_wendroff_2(nx,a,output, dt, ntime, time,u_int, vec,dx);

%compute original size matracies

U=zeros(nx,1);

update = U ;

F = U;

if vec == 1;

U(1,vec)=u_int(1,vec);

A=a;

else

U(nx,vec)=u_int(nx,vec);

A=-a;

end

for p = 1:ntime;

if(time + dt>output); dt=output-time;

end

coeff=dt/dx;

%calculate interim steps

for i = 1:(nx-1)

F(i+1,vec)= ((U(i,vec)+U(i+1,vec))/2) - ((A*coeff)*((U(i+1,vec) - U(i,vec))));

end

for i = 2:(nx-1)

update(i,vec) = U(i,vec) - coeff*A*(F(i+1,vec) - F(i,vec));

end

for i = 2:(nx-1)

U(i,vec) = update(i,vec);

end

time = time + dt;

if time==output

break

end

end end