I'm attempting to solve a difference equation y(k+1) = -0.5*y(k) + x(k) where y(0) = 0 and x(k) is in my case a unit step function ie = 1 well, i'm trying to solve this using a for loop, but am having some trouble. The code i've generated gets an error, "Index into matrix is negative or zero." Any ideas? My code is below. k = 0 ; %counter variable y = 0; %y(k) x = 1; %x(k), unit step function for k = 0 : 5 y(k+1) = -.5*y(k) + x; y(k) = y(k+1) end Thanks for any help.