# How to exclude the center/current number when performing movmax?

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liu James on 2 Dec 2016
Answered: Chris Turnes on 5 Dec 2016
Im trying to create the Donchian Channel which uses the preceding 20day high. So I figure to use movmax but it includes the current/center number.
A = [4 8 6 -1 -2 -3 -1 3 4 5];
M = movmax(A,[2 0])
M =
4 8 8 8 6 -1 -1 3 4 5
Is there a way to not include the center value?

#### 1 Comment

Not that I can see using movmean. You can program your own version easily enough though with just a moving window and your own defined function

Chris Turnes on 5 Dec 2016
There's no direct way to exclude the current point in a given window with the moving statistics functions. However, since you want a trailing maximum in this particular answer, it is fairly easy to get around. As Jan pointed out, a 20-point trailing maximum that excludes the current point can be computed as a 19-point trailing maximum with a 1-point shift. So, you should be able to just do:
movmax([-Inf x(1:(end-1))], [18 0]);
I believe this should be equivalent to what you want (although you'll have to decide what should happen for the first point, where you'd have a window only containing the current point).

Andrei Bobrov on 2 Dec 2016
Edited: Andrei Bobrov on 2 Dec 2016
One way
m = 20;
M = conv2(x(:),[zeros(m+1,1); ones(m,1)]/m,'same');
M = M(m+1:end);
Other way
m = 20;
n = numel(x);
M = mean(x(hankel(1:n-m+1,n-m+1:n)),2);

Isn't that a mean?
Andrei Bobrov on 2 Dec 2016