Question

A question about matlab and randomisation of a 3d matrix respecting the rows and columns.

I have a n x n x s matrix M and I want to mess it up a bit, but with some control.

I can achieve my wish with a for loop

for j=1:size(M,3)
r=randperm(size(M,1));  
random_M(:,:,j)=M(r,r,j); 
end

Is there a way to perform this without having to loop over j? I need many randomisation iterations and could afford the benefits of indexing.

Cheers!

edit: Some more thoughts following Alexandrew’s comments

I have created a function that randomises a squeezed version of M:

function randomMat=randomiseMat(Mat)
[rows,cols]=size(Mat);
r=randperm(rows);
randomMat=Mat(r,r);

then, using arrayfun I seem to get what I want:

randomM=arrayfun(@(x) randomiseMat(M(:,:,x)),1:size(M,3),'UniformOutput', false)

however, randomM is now a cell array of size (1,size(M,3)) with each cell containing randomised array.

Is there a way to make it in a 3d matrix just like the input M?

Answer 1

You can calculate all the values for r in one go, and then use arrayfun:

[nRows,nCols,nPages] = size(M);
[~,r]=sort(rand(nRows,nPages));

%# you should test on a realistic example whether a for-loop
%# isn't faster here
outCell = arrayfun(@(x) M(r(:,x),r(:,x),x), 1:nPages,'UniformOutput',false);
randomM = cat(3,outCell{:});