Removing zeros that may be in columns or rows, in efficient fast way

Question

Mohamed Abd El Raheem 2021-12-10

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1608460-removing-zeros-that-may-be-in-columns-or-rows-in-efficient-fast-way

编辑： Mohamed Abd El Raheem 2021-12-11

Greetings:

I have some matrices in a big computaion task. Some of these matrices will have zero rows or zero columns as part of them. I can't decide in priori.

We want an efficient way to remove any whole zero row or whole zero column from any matrix that may appear during the computation.

The methods I found, to my knowledge, are all:

1- require a priori knowledge of : Are there whole zero rows or whole zero columns?

2- require knowledge of the required dimensions of the output matrix after zeros removal.

These two pieces of information are not available.

Example:

If computations yield X = [ 0 0 0 ; 1 1 1; 2 2 2 ] , make it X = [1 1 1; 2 2 2]

If they yield Y = [0 1 2; 0 1 2 ; 0 1 2] make it [1 2; 1 2 ; 1 2] and so on..

We don't know if the zeros appear on rows or columns. We don't know the number of zero rows or columns.

Any ideas?

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Answer 1

Voss 2021-12-10

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https://ww2.mathworks.cn/matlabcentral/answers/1608460-removing-zeros-that-may-be-in-columns-or-rows-in-efficient-fast-way#answer_852595

在 MATLAB Online 中打开

X(:,~any(X,1)) = []; % remove columns which are all 0
X(~any(X,2),:) = []; % remove rows which are all 0

or an equivalent but maybe more clear way:

X(:,all(X == 0,1)) = []; % remove columns which are all 0
X(all(X == 0,2),:) = []; % remove rows which are all 0

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Mohamed Abd El Raheem 2021-12-10

Thank you.

Your answer worked very well

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Answer 2

Image Analyst 2021-12-10

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1608460-removing-zeros-that-may-be-in-columns-or-rows-in-efficient-fast-way#answer_852590

在 MATLAB Online 中打开

Try this (untested)

X = [ 0 0 0  ; 1 1 1; 2 2 2 ] 
X = 3×3
     0     0     0
     1     1     1
     2     2     2
rowsToRemove = all(X == 0, 2);
X(rowsToRemove, :) = [];
columnsToRemove = all(X == 0, 1);
X(:, columnsToRemove) = []
X = 2×3
     1     1     1
     2     2     2
Y = [0 1 2; 0 1 2 ; 0 1 2]
Y = 3×3
     0     1     2
     0     1     2
     0     1     2
rowsToRemove = all(Y == 0, 2);
Y(rowsToRemove, :) = [];
columnsToRemove = all(Y == 0, 1);
Y(:, columnsToRemove) = []
Y = 3×2
     1     2
     1     2
     1     2

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Image Analyst 2021-12-11

Yes, he posted his answer (essentially the same answer) just two minutes after mine. We were probably composing our answers at the same time. His second code sample is probably the more efficient one. Personally I think mine is less cryptic because I used well-named intermediate variables which will make it more maintainable should you pass the code on to others. Though he did use comments which will help make it less cryptic. Maybe you can at least "Vote" for mine.

I'm not sure what you're saying in your second paragraph. It takes however long as it takes, but it should be pretty fast, at least for matrices that are a few thousand rows or columns. How big are yours? Do they have millions of rows and columns?

Mohamed Abd El Raheem 2021-12-11

编辑：Mohamed Abd El Raheem 2021-12-11

No, my matrices are not that big. But the process of zeros deletion may repeat in nested loops, over matrices that may be thousands of rows or columns. That's why I care about time.

I accepted Benjamin answer, and tried to accept yours also. I thought it was possible to accept the two answers. Now I noticed the vote button and pressed it. Your answer is certainly appreciated.

Thank you!

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