# Reducing repeated elements in an array by a factor

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Thomas Gvero on 12 Feb 2020
Answered: Matt J on 12 Feb 2020
Is there a way to reduce the number of repeated elements in an array by a factor? For example, say the factor I had was 3, i would want to reduce
[1 2 2 2 2 2 2 5 6]
to
[1 2 2 5 6]
i.e. only leave one third of the repeated elements.
Not sure if this is possible but worth a shot, thanks! :)

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Thomas Gvero on 12 Feb 2020
That would be 2 independent groups.
Matt J on 12 Feb 2020
Do the repeated elements always occur in fixed, known multiples? How would, a reduction by 3 handle this,
[1 1 1 5 2 2 2 2 6]
Thomas Gvero on 12 Feb 2020
As far as I'm aware, in the context i am using it, yes.

Stephen Cobeldick on 12 Feb 2020
Edited: Stephen Cobeldick on 12 Feb 2020
>> V = [1,2,2,2,2,2,2,5,6];
>> N = 3;
>> X = cumsum([1,diff(V)~=0]);
>> F = @(v) {v(1:ceil(numel(v)/N))};
>> cell2mat(accumarray(X(:),V(:),[],F))
ans =
1
2
2
5
6

#### 1 Comment

Thomas Gvero on 12 Feb 2020
thank you so much!

Matt J on 12 Feb 2020
Another method, one which avoids for-loops inherent in cell2mat,
V=[1,2,2,2,0,2,2,2,5,5,5,5,5,5,6,6,6];
N=3;
X = cumsum([1,diff(V)~=0]);
[~,idx]=unique([X,inf],'stable');
result=V( repelem(idx(1:end-1), ceil(diff(idx)/N) ) )
result =
1 2 0 2 5 5 6