Burgers equation using finite differeance and Runge-Kutta 4

I'm solving for a the pde u_t=alpha*u_xx+beta*u*u_x

in the file called evers_code.m I solved for the pde using ode45,however it wouldn't calculate for negative alpha and beta. I'm not to sure why ode45 doesn't allow me to do this.

I have now tried to merge my function called odes(t,y) from file "evers_code.m" with a running RK4 in file "rk4.m".

I keep getting a errors that say

""Index exceeds matrix dimensions.

Error in Burgers_Draft>odes (line 44) yp(1)=u(2);

Error in Burgers_Draft (line 16) f=fxt(T,xrk);""

I need help debugging the code to get it running

[Jim Riggs]

It appears to me that "f" is being used as a function inside the "for" loop, but it is not defined as a function.