Efficiently converting a 3d matrix to a 2d matrix

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Omar Mian
Omar Mian le 21 Mar 2011
Commenté : Sem Diaz le 4 Déc 2019
Hello,
I would like to convert a 3d matrix into a 2d matrix. I want the 3rd dimension to be concatenated along dimension 1 in the 2d matrix. In the code below, the variable 'desired' illustrates what I want to achieve, but I want to do it more efficiently than via a for a loop. I tried doing the same with reshape (see variable B) but can't get it to produce the output I desire.
A(:,:,1) = ones(2,2);
A(:,:,2) = 2*ones(2,2);
A(:,:,3) = 3*ones(2,2);
desired = [];
for n = 1:size(A,3)
desired = cat(1,desired,A(:,:,n));
end
B = reshape(A,[],size(A,2),1);
Any advice would be much appreciated. Thanks for reading.

Réponse acceptée

the cyclist
the cyclist le 21 Mar 2011
You can permute the 2nd and 3rd dimension before doing the reshape:
>> C = permute(A,[1 3 2]);
>> C = reshape(C,[],size(A,2),1)
  9 commentaires
Yue Zhang
Yue Zhang le 23 Fév 2019
Thank you.
Sem Diaz
Sem Diaz le 4 Déc 2019
How can you convert it back?

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Plus de réponses (2)

Andrew Newell
Andrew Newell le 21 Mar 2011
You can do it with cell arrays:
Acell = num2cell(A,[1 2]); % put each page of A into a cell
Acell = reshape(Acell,size(A,3),1); %make the cell array a vector
desired = cell2mat(Acell);
EDIT: the solution by @the cyclist is nicer for this particular problem, but be careful about generalizing it. For example, suppose you want to pack your matrices into a 2x2 array:
A(:,:,1) = ones(2,2);
A(:,:,2) = 2*ones(2,2);
A(:,:,3) = 3*ones(2,2);
A(:,:,4) = 4*ones(2,2);
Acell = num2cell(A,[1 2]);
Acell = reshape(Acell,2,2);
desired = cell2mat(Acell)
desired =
1 1 3 3
1 1 3 3
2 2 4 4
2 2 4 4
C = permute(A,[1 3 2]);
C = reshape(C,[],4,1)
C =
1 3 1 3
1 3 1 3
2 4 2 4
2 4 2 4

Omar Mian
Omar Mian le 21 Mar 2011
Thank you Andrew & cyclist,
Both solutions work. Cyclist's permute solution is more efficient, taking approx approx 17% of the time used by the cell array solution.

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