elimination of consecutive regions

Question

Michal 2018-9-27

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https://ww2.mathworks.cn/matlabcentral/answers/421062-elimination-of-consecutive-regions

评论： Michal 2018-9-29

I need to effectively eliminate consecutive regions in vector "a" or better in rows/columns of matrix "A" with length of separate ones regions greater than positive integer N <= length(A):

See following example:

N = 2    % separate consecutive regions with length > 2 are zeroed
a =      [0 1 1 0 0 1 1 1 0 0 1 1 1 1 0 1]
a_elim = [0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 1]

or 2D case:

N = 2
A =    [1 0 1 …
        1 1 0 …
        1 1 0 …
        0 0 1 …
        1 1 1]
% elimination over columns
A_elim= 0 0 1 
        0 1 0
        0 1 0
        0 0 1
        1 1 1
% elimination over rows
A_elim= 1 0 1 
        1 1 0
        1 1 0
        0 0 1
        0 0 0

I am looking for effective vectorized function performing this task for size(A) ~ [100000, 1000] (over columns case).

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Answer 1

Matt J 2018-9-27

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https://ww2.mathworks.cn/matlabcentral/answers/421062-elimination-of-consecutive-regions#answer_338734

编辑：Matt J 2018-9-27

在 MATLAB Online 中打开

    e=ones(N+1,1); 
if mod(N,2) %even mask
      mask=~conv2(conv2(A,e,'valid')>=N+1 ,[zeros(N,1);e])>0;
      mask=mask(N+1:end,:);
else %odd mask
   mask=~(conv2( conv2(A,e,'same')>=N+1, e,'same')>0);
end
A_elim=A.*mask;

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Matt J 2018-9-27

I've modified it to handle odd N.

Michal 2018-9-29

Hi Matt … now your solution works very well. Is faster and significantly less memory consuming than Bruno's code. Moreover, the "mask" is very useful to know.

Thanks!!!

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Answer 2

Bruno Luong 2018-9-27

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/421062-elimination-of-consecutive-regions#answer_338743

编辑：Bruno Luong 2018-9-27

在 MATLAB Online 中打开

You could use Huffman encoding (there might be some on the FEX), but the idea is similar to this direct code:

N = 2
A = [1 0 1;
     1 1 0;
     1 1 0;
     0 0 1;
     1 1 1];
% Engine for working along the column (1st dimension)
[m,n] = size(A);
z = zeros(1,n);
Apad = [z; A; z];
d = diff(Apad,1,1);
[i1,j1] = find(d==1);
[i0,j0] = find(d==-1);
lgt = i0-i1;
keep1 = lgt <= N;
keep0 = keep1 & i0 <= m;
i1 = [i1,j1];
i0 = [i0,j0];
C1 = accumarray(i1(keep1,:),1,[m n]);
C0 = accumarray(i0(keep0,:),-1,[m n]);
Aclean = cumsum(C1+C0,1);
Aclean

If you want to filter along the 2nd dimension, transpose A, apply the above, then transpose the result Aclean.

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Bruno Luong 2018-9-27

编辑：Bruno Luong 2018-9-27

It depends how it's implemented. If it's a C-MEX file might be, pure MATLAB Huffman, no chance.

Otherwise my code is probably quite fast (but it creates few big intermediate arrays, you might add "clear...) while the code is running when an array is finished to be used).

Why not test yourself with different methods the link you have found?

Michal 2018-9-27