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From: Sumeet on 22 Mar 2010 09:52
"Marvin Bugeja" <mervremovethis(a)ieee.org> wrote in message <grpqbl$r9u$1(a)fred.mathworks.com>...
> Hi,
> I came across this very simply task explained below:
>
> Say a 3D matrix G is defined as follows:
> G(:,:,1) = [1,2;3,4];
> G(:,:,2) = [5,6;7,8];
> How can one achieve a 2D matrix that looks like:
> G2D = [1,2;3,4;5,6;7,8];
>
> In other words, I would like to get the 2x2 blocks from the 3D matrix and concatenate them vertically under each other.
> (N.B: this operation would later be used in a block multiplication operation, but this is the main problem). I solved it by converting the matrix to a cell and then use vertcat(), but is there any other way! (maybe faster!). No for loops please :-)
> - Thanks

Does somebody have a general method that works for any a x b x c (3 dimensional) matrix? Implementing the above kind of methods would require for loops.
From: Sumeet on 22 Mar 2010 10:07
"Sumeet " <sumeetsk(a)gmail.com> wrote in message <ho7sm5$pn9$1(a)fred.mathworks.com>...
> "Marvin Bugeja" <mervremovethis(a)ieee.org> wrote in message <grpqbl$r9u$1(a)fred.mathworks.com>...
> > Hi,
> > I came across this very simply task explained below:
> >
> > Say a 3D matrix G is defined as follows:
> > G(:,:,1) = [1,2;3,4];
> > G(:,:,2) = [5,6;7,8];
> > How can one achieve a 2D matrix that looks like:
> > G2D = [1,2;3,4;5,6;7,8];
> >
> > In other words, I would like to get the 2x2 blocks from the 3D matrix and concatenate them vertically under each other.
> > (N.B: this operation would later be used in a block multiplication operation, but this is the main problem). I solved it by converting the matrix to a cell and then use vertcat(), but is there any other way! (maybe faster!). No for loops please :-)
> > - Thanks
>
> Does somebody have a general method that works for any a x b x c (3 dimensional) matrix? Implementing the above kind of methods would require for loops.

Sorry for answering my own question, but I figured it out. Posting the general method in case anybody else needs it:

function [twodmatrix] = planes2matrix (threedmatrix)
n_columns = size(threedmatrix,2);
n_rows = size(threedmatrix,1) * size(threedmatrix,3);
twodmatrix_1 = permute (threedmatrix,[2,1,3]);
twodmatrix_2 = reshape (twodmatrix_1,n_columns,n_rows);
twodmatrix = twodmatrix_2';
endfunction
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