# IIR filter as amoving averege

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Rica on 22 Mar 2013
hi! I want to use the filter function as a moving averege filter. the Type of the filter should be IIR Filter. Te window length is 3. for exemple:
%
X=rand(1:300)
B=1/3
A=1-B;
Y=filter(B,A,X)->First order IIR filter
the filter functi should start after reaching the window lenght. for the first 2 value (before reaching the window lenght of 3) , i wan to calculate just the mean value.
%
Y(1)=X(1)
Y(2)=mean(Y(1)+X(2));
Y(3:end)=filter(B,A,X(3:end))
could someone help me ?
thank you

Andrei Bobrov on 22 Mar 2013
Edited: Andrei Bobrov on 22 Mar 2013
EDIT
out = filter([1 1 1]/3, 1, x(:));
out(1:2) = [x(1);sum(x(1:2))/2];
Andrei Bobrov on 22 Mar 2013
EDIT

Image Analyst on 22 Mar 2013
Edited: Image Analyst on 22 Mar 2013
To get the middle chunk, do this:
yMiddle = conv(x, [1 1 1]/3, 'valid');
Then prepend and append the endpoints as you already calculated.
Edit: Haven't heard from you, so here's a full-blown demo for you. It shrinks the window as it gets closer to the edges. Let me know if you need a version robust enough to handle any window size.
x = 1:10
% Get the first and last elements.
yFirst = mean(x(1:2))
yLast = mean(x(end-1:end))
% Now get the middle with a moving average.
yMiddle = conv(x, [1 1 1]/3, 'valid')
yFinal = [yFirst yMiddle, yLast]
In the command window you see:
x =
1 2 3 4 5 6 7 8 9 10
yFirst =
1.5000
yLast =
9.5000
yMiddle =
2.0000 3.0000 4.0000 5.0000 6.0000 7.0000 8.0000 9.0000
yFinal =
1.5000 2.0000 3.0000 4.0000 5.0000 6.0000 7.0000 8.0000 9.0000 9.5000