Euler's method to solve the heat equation

hello,

I want to plot the exact and proximative curves for the solution of the heat equation

but my code has a problem:

x1=0;
a = 1; 
x2=a;

T = 1; % length of time for solution

n = 599;   
N = 150;

dx = a/(n+1.); 

dt = T/N; 

t=0.; % initial time = 0

beta = 1.e-5; 

s = beta*dt/(dx^2);  

if s > 0.5
   fprintf('gain parameter > 0.5');
end

Adiag = (1-2*s)*ones(n,1);

Asubs = s*ones(n,1);

Asuper = s*ones(n,1);

A = spdiags([Asubs,Adiag,Asuper],[-1 0 1],n,n);

A(n,n) = 1-s;

b= zeros(n,1);

b(1) = 0;
b(n) =  0;

x = linspace(0,a, n+1)';

f=inline('sin(pi*x)');
U_tk = f(x);

a=1 ;
v = inline('exp(-a^2*pi*t)*sin(pi*x)','x','t','a'); % exact solution 

for n=1:N
    plot(x,U_tk,'b',x,v(x,t,a),'r')
    axis([0,1.5,-0.5, 1.5])
    title(['lambda=',num2str(s)])
    pause,
end;

also, I have to use Euler's explicit method here: