How can I write the Matlab “filter”-function myself?

Question

I would like to use a Butterworth filter on a 1D-Signal. In Matlab the script would look like this:

 f=100;
 f_cutoff = 20; 
 fnorm =f_cutoff/(f/2);
 [b,a] = but         


        

Answer 1

BlackEagle's solution does not reproduce the same results as MATLAB with other arrays. For example:

b = [0.1 0.1]
a = [2 -2.5 1]
u = [1, zeros(1, 30)];
z = filter(b, a, u)

Gives you completely other results. Be careful.