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removing zero element rows in a m×n matrix?

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i want to remove the rows which contain all zeros. so finally i expect a matrix containing with the remaining rows. I use a 'for loop' to represent each row, but then i get a error "Index of element to remove exceeds matrix dimensions". help me to get rid of these zero elemental rows. thank you. example: a =[0 0 0 0
1 2 3 4
2 0 1 1
0 0 0 0]
new_a =a;
for i=1:4
if a(i,:)==0
new_a(i,:)=[];
end
end
new_a

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Accepted Answer

Walter Roberson
Walter Roberson on 11 Nov 2011
That is a common error. After you assign [] to remove a row, the matrix now has fewer rows, but you proceed as if it still has as many rows as it used to have. Eventually you will index a row that no longer exists.
Easiest fix: loop backwards, so that the rows you have not examined yet will not have been moved.
Better fix: use logical indexing and do everything at the same time:
new_a = a(any(a,2),:);
or
new_a = a;
new_a(~any(a,2),:) = [];

  2 Comments

Rakesh Praveen
Rakesh Praveen on 11 Nov 2011
Thanks now its working. when u say any(a,2) what does the 2 mean ?
Walter Roberson
Walter Roberson on 11 Nov 2011
The 2 means to work across the second dimension, which is across rows in MATLAB. The first dimension is columns and is the default direction for the majority of MATLAB operations.

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