2 views (last 30 days)

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).

Bruno Luong
on 27 Sep 2018

Edited: Bruno Luong
on 27 Sep 2018

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.

Bruno Luong
on 27 Sep 2018

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?

Sign in to comment.

Michal Kvasnicka
on 27 Sep 2018

Edited: Michal Kvasnicka
on 27 Sep 2018

Sign in to comment.

Sign in to answer this question.

Opportunities for recent engineering grads.

Apply Today
## 0 Comments

Sign in to comment.