IIR filter as amoving averege

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

Answer 1

[Andrei Bobrov]

EDIT

out = filter([1 1 1]/3, 1, x(:));
out(1:2) = [x(1);sum(x(1:2))/2];

[Andrei Bobrov]

EDIT

Answer 2

[Image Analyst]

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