Trying to code backward euler?

I am trying to write a code to compute the backward euler approximation of a differential equation. the ODE is dy/dx = -y(x)

so my code looks like

for k = 1:((T/dt)-1)

xb(k+1) = xb(k)+dt;

yb(k+1) = yb(k) + dt*f(yb(k+1),xb(k+1));

end

My problem is that using my equation i am trying to use yb(k+1) to compute itself, which obviously doesnt work, so i get an error 'index exceeds matrix dimension'. How should i resolve this?