Doug's MATLAB Video Tutorials

January 10th, 2012

How to do a matrix reshape by ‘blocks’

This week’s video covers a non-standard way of reshaping a matrix. Instead of using the reshape function, this kind of matrix manipulation must be done with for loops. This matrix reshape operation treats submatrices within the original matrix as a unit. I suspect there are many other ways of doing this, and would love to hear about them in the comments.

By Doug Hull

13:11 UTC | Posted in Format: Video, Level: Basic | Permalink | 2 Comments »

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2 Responses to “How to do a matrix reshape by ‘blocks’”

Matt Fig replied on January 12th, 2012 at 21:00 UTC :

Here is one version of the obligatory one-liner.

newMat=cell2mat(reshape(mat2cell(longMat,rSize*ones(1,n),cSize),5,4).');

However, this is only significantly faster for large systems.

I have written optimized, hard-to-read MATLAB code in the past, and in order to help those who might be reading it in the future I often put the straightforward FOR loop(s) version in the comments. This serves several purposes. Among them are that it lets any future user see more easily what is going on and quickly get to maintenance concerns.

Thanks for another great video, Doug!
Sean de Wolski replied on January 17th, 2012 at 11:31 UTC :

And the obligatory one-liner without cells:

%Some parameters
blksz = 3; %block size in row/col
nrep = 20; %block repetitions
nr = 4; %number of rows of blocks
nc = 5; %number of columns of blocks

%Determinant sample data
A = reshape(1:(blksz*blksz*nrep),blksz,[])’;

%Engine one line version:
NewA = reshape(permute(reshape(permute(reshape(A.’,blksz,blksz,[]),[2 1 3]),blksz,blksz*nc,[]),[2 1 3]),blksz*nc,blksz*nr)’;

%Engine in pieces:
B = reshape(A.’,blksz,blksz,[]); %reshape the transpose to 3d with each block a 3×3
C = permute(B,[2 1 3]); %transpose each block
D = reshape(C,blksz,blksz*nc,[]); %reshape to nr planes that need to be stacked vertically
E = permute(D,[2 1 3]); %transpose each plane
F = reshape(E,blksz*nc,blksz*nr)’; %reshape into a matrix and transpose