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Knut
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Deleting an irregular "row"/"column" from 2-d array

Asked by Knut
on 5 Dec 2017
Latest activity Edited by Matt J
on 5 Dec 2017
I have a 2d array, and I want to eliminate the same number of elements in each row (or column) (e.g. 1 element), but the index of that element is itself av vector. This operation would be relevant for doing seam-carving (content-aware image resize).
Is there any neat way of doing this without my array collapsing into a 1d vector?
A = (1:3)'*(1:3);
A =
1 2 3
2 4 6
3 6 9
b = [1; 2; 2];
for r=(1:3)
tmp = A(r,:);
tmp(b(r)) = [];
Y(r,:) = tmp;
end
Y =
2 3
2 6
3 9
I seem to have a working method, but it is kind of cumbersome as I have to (?) make sure that the linear indexing into Y is strictly monotonic:
Y = A';
Y(sub2ind(size(A'), b, (1:3)')) = [];
Y = reshape(Y, size(A,2)-1, size(A,1))'
Anything neater?

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1 Answer

Answer by Matt J
on 5 Dec 2017
Edited by Matt J
on 5 Dec 2017

I think your solution is probably the best, except that I would avoid the extra calls to size() and ctranspose(). The latter are particularly expensive.
Y = A.';
[m,n]=size(Y);
Y( sub2ind([m,n], b, (1:3)) )=[];
Y = reshape(Y, m-1, n).'

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