Reshape matrix by taking two consecutive rows every two rows

Question

Pascal Weller 2021-3-23

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https://ww2.mathworks.cn/matlabcentral/answers/781007-reshape-matrix-by-taking-two-consecutive-rows-every-two-rows

评论： Pascal Weller 2021-4-7

Hello everyone,

I have seen quite some questions here on the forum that are about matrix reshaping. However, for my particular problem I did not find an answer. So, here we are:

I have got a matrix that looks like

Is it somehow possible to reshape it to

[1 1 1 1 3 3 3 3;
2 2 2 4 4 4 4;
5 5 5 7 7 7 7;
6 6 6 8 8 8 8]

with the built-in functions like reshape, permutate, flip,...?

There would always be the way with for loops but I thought it might be neater (and possibly faster) to do it with the built-in functions.

Thanks for your help.

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Answer 1

Adam Danz 2021-3-23

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/781007-reshape-matrix-by-taking-two-consecutive-rows-every-two-rows#answer_656118

编辑：Adam Danz 2021-4-7

在 MATLAB Online 中打开

x must be an n*m matrix where n is divisible of 4.

M is the reformatted matrix with size k*j where k=n/2 and j=m*2.

x = [1 1 1 1;
2 2 2;
3 3 3;
4 4 4;
5 5 5;
6 6 6;
7 7 7;
8 8 8];
rowIdx =  [1 3 2 4]' + [0:4:size(x,1)-1];
M = reshape(x(rowIdx(:),:)', size(x,2)*2, [])'
M = 4×8
   1     1     1     3     3     3     3
   2     2     2     4     4     4     4
   5     5     5     7     7     7     7
   6     6     6     8     8     8     8

Another example,

x = [1 1 1 1;
2 2 2;
3 3 3;
4 4 4;
5 5 5;
6 6 6;
7 7 7;
8 8 8
9 9 9
10 10 10
11 11 11 
12 12 12];
rowIdx =  [1 3 2 4]' + [0:4:size(x,1)-1];
M = reshape(x(rowIdx(:),:)', size(x,2)*2, [])'
M = 6×8
   1     1     1     3     3     3     3
   2     2     2     4     4     4     4
   5     5     5     7     7     7     7
   6     6     6     8     8     8     8
   9     9     9    11    11    11    11
  10    10    10    12    12    12    12

To get from M back to x,

k = reshape(M',size(M,2)/2,[]);
rowIdx =  [1 3 2 4]' + (0:4:size(k,2)-1);
xOrig = k(:,rowIdx(:))';

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Adam Danz 2021-4-7

See updated solution.

Pascal Weller 2021-4-7

Amazing! Huge thanks! You make it look so easy...

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Answer 2

Taimoor Hasan Khan 2021-3-23

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/781007-reshape-matrix-by-taking-two-consecutive-rows-every-two-rows#answer_656013

在 MATLAB Online 中打开

A = [1 1 1 1;
2 2 2;
3 3 3;
4 4 4;
5 5 5; 
6 6 6;
7 7 7;
8 8 8];
 
A = cat( 2, [ A(1:2, :); A(5:6, :) ], [ A(3:4, :); A(7:8, :) ] )
            % vertical concat. #1     % vertical concat #2

Not sure how you could generalize the process without loops though, sorry...

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Pascal Weller 2021-3-31

In my case the matrix is some million rows long so I would definitely need to do the indexing with the help of a for loop. However, for smaller matrices your answer is a simple solution and much appreciated. I am sure it will be of good use for someone out there. So thanks for your contribution!

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