Here is the MATLAB/FreeMat code I got to solve an ODE numerically using the backward Euler method. However, the results are inconsistent with my textbook results, and sometimes even ridiculously inconsistent. What is wrong with the code?
function [x,y] = backEuler(f,xinit,yinit,xfinal,h)
%f - this is your y prime
%xinit - initial X
%yinit - initial Y
%xfinal - final X
%h - step size
n = (xfinal-xinit)/h; %Calculate steps
%Inititialize arrays...
%The first elements take xinit and yinit corespondigly, the rest fill with 0s.
x = [xinit zeros(1,n)];
y = [yinit zeros(1,n)];
%Numeric routine
for i = 1:n
x(i+1) = x(i)+h;
ynew = y(i)+h*(f(x(i),y(i)));
y(i+1) = y(i)+h*f(x(i+1),ynew);
end
end