Runge kutta matlab code (method)

Hi guys,

I am having trouble implementing Runge Kutta to solve the following

Lotka-Volterra equations.

dx/dt=x(1-2y) t0=0, x(t0)=1

dy/dt=-y(1-2x) t0=0, x(t0)=2

Below I given what code I have done so far but it only produces an axis with a dot.

My first file has the code below:

function z_out=lotka_volterra_rhs(z)

%create a vector for storing the results;

x=zeros(1,2);

%z(1) is supposed to store the values of x

%z(2) is supposed to store the values of y

x(1)=z(1)*(1-2*z(2));

x(2)=-z(2)*(1-2*z(1));

z_out=x;

my second file has the code below:

t0=0;

t1=40;

Delta=0.005;

t=t0: Delta:t1;

i=1;

y(i)=2;

x(i)=1;

while (x(i)<x)

z_1=lotka_volterra_rhs(y(i),x(i));

z_1x=lotka_volterra_rhs(y(i),x(i));

z_2x=lotka_volterra_rhs((y(i)+Delta*(z_1/(2))),(x(i)+Delta*(z_1x/(2))));

z_2=lotka_volterra_rhs((y(i)+Delta*(z_1/(2))),(x(i)+Delta*(z_1x/(2))));

z_3x=lotka_volterra_rhs((y(i)+Delta*(z_2/(2))),(x(i)+Delta*(z_2x/(2))));

z_3=lotka_volterra_rhs((y(i)+Delta*(z_2/(2))),(x(i)+Delta*(z_2x/(2))));

z_4x=lotka_volterra_rhs((y(i)+Delta*z_3),(x(i)+Del ta*z_3x));

z_4=lotka_volterra_rhs((y(i)+Delta*z_3),(x(i)+Delt a*z_3x));

y(i+1)=(y(i)+Delta*((z_1+2*z_2+2*z_3+z_4)/(6)));

x(i+1)=(x(i)+Delta*((z_1x+2*z_2x+2*z_3x+z_4x)/(6)));

i=i+1;

end;

plot(x,y)

Thank you guys... really appreciate it