How do I extract the odd and even rows of my matrix into two separate matrices and reconstruct it back using vectorized code?

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MathWorks Support Team 2013-8-17

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https://ww2.mathworks.cn/matlabcentral/answers/102465-how-do-i-extract-the-odd-and-even-rows-of-my-matrix-into-two-separate-matrices-and-reconstruct-it-ba

I would like to reconstruct my original matrix from the odd and even matrices using vectorized code.

I have a matrix M:

n = 7;
M = repmat((1:n)',1,5)

I split matrix M into two matrices. One contains the odd rows and another the even ones. Is it possible to reconstruct the initial matrix without using a FOR loop?

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Answer 1

MathWorks Support Team 2013-8-17

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https://ww2.mathworks.cn/matlabcentral/answers/102465-how-do-i-extract-the-odd-and-even-rows-of-my-matrix-into-two-separate-matrices-and-reconstruct-it-ba#answer_111811

Here is an example on how to extract the odd and even rows of a matrix into two separate matrices:

n = 7;   % number of rows
M = repmat((1:n)',1,5) 
A = M(1:2:end,:)  % odd matrix
B = M(2:2:end,:)  % even matrix

To reconstruct the original matrix you can preallocate the result matrix and then use indexing similar to what was used above for splitting:

C = ones(size([A;B]))
C(1:2:end,:) = A
C(2:2:end,:) = B

As an alternative, you can use the SORT function to create an index for rearranging rows:

% concatenate odd and even matrices
C = [A;B] ;
% get odd row indices
oddI = 1:2:size(C,1) ;
% get even row indices
evenI = 2:2:size(C,1) ;
% concatenate odd and even row indices, then use SORT to find appropriate reordering
[~,reorderI] = sort([oddI,evenI])
% reorder matrix to reconstruct original matrix
M0 = C(reorderI,:)