Efficiently converting a 3d matrix to a 2d matrix
569 views (last 30 days)
Show older comments
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.
0 Comments
Accepted Answer
the cyclist
on 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 Comments
More Answers (2)
Andrew Newell
on 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
0 Comments
See Also
Categories
Find more on Matrices and Arrays in Help Center and File Exchange
Products
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!