# How to write my own filter function with initial condition similar to Filter.m?

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Hossein on 5 Feb 2013
I have read the documents on how MATLAB implements Filter(b,a,y,z0). To fully understand the concept and learn more about Signal Processing, I tried to implement it by myself, but the answer is different from MATLAB filter.m
Could anyone let me know what I am missingg?! :-(
function y =myfilter(num, den, x, zi)
N = length(x);
% assuming order of Num and Den are the same.
ord = (length(num)-1)/2;
y = zeros(N,1);
for i=1:N
c = 0;
for j=1:2*ord
if i-j < 1
c = c+1;
xo(j) = zi(c);
yo(j) = 0;
else
xo(j) = x(i-j);
yo(j) = y(i-j);
end
end
y(i) = num(1)*x(i) + sum(num(2:end).*xo') - sum(den(2:end).*yo');
end

Hossein on 6 Feb 2013
Ok, I searched online and found out that I was wrong on the concept :) Here' the correct formulation of 1D filtering with delay initial values: SciPY Filtering Documentation, and it outputs the same result as MATLAB filter() does.
Mariasole Laureti on 7 Oct 2019
Hi, I have the same problem, can I ask you if you can show me your code? I would be very grateful. Thank you very much

ssp on 12 Feb 2017
Hi! Greetings!
I was also trying to write my filter function. However y=filter(b,a,x) seems easy to implement, but I am stuck at implementing y=filter(b,a,x,zi). What would be the new sequence of the following sequence after applying initial condition zi? y(n) = b(1)*x(n) + b(2)*x(n-1) + ... + b(nb+1)*x(n-nb) - a(2)*y(n-1) - ... - a(na+1)*y(n-na)
Thanks! SSP
Jan on 12 Feb 2017
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