Row-wise individual circshift of a 3D-matrix without using a loop

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Pascal Wielsch
Pascal Wielsch on 16 Jun 2021
Commented: Pascal Wielsch on 17 Jun 2021
Hello dear MATLAB community,
I'm looking for an alternative to perform "circshift" without using a loop to optimize code process time.
Let me first explain you what I want to do. I have several 3D data matrices "A(i,j,k)", that have to be circular shifted row by row for every dimension "k". The individual shift indices "RowsShift" for every single row in each dimension "k" are not equal.
% Input data matrix (3D): every single row should be shifted individually
% with its shifting index "RowsShift"; for example:
% - A(2,:,1) should be shifted by 3 (RowsShift(2,:,1))
% - A(4,:,3) should be shifted by -4 (RowsShift(4,:,3))
A(:,:,1) = [0 0 0 1 0 0 0 0;
1 0 0 0 0 0 0 0;
0 0 0 0 0 1 0 0;
0 1 0 0 0 0 0 0;
1 0 0 0 0 0 0 0];
A(:,:,2) = [0 0 0 1 0 0 0 0;
1 0 0 0 0 0 0 0;
0 0 0 0 0 1 0 0;
0 0 0 0 0 0 0 1;
0 0 0 0 0 0 1 0];
A(:,:,3) = [1 0 0 0 0 0 0 0;
1 0 0 0 0 0 0 0;
0 0 0 0 0 0 1 0;
0 0 0 0 1 0 0 0;
0 0 0 0 0 1 0 0];
% Matrix with circular shift indices
RowsShift(:,:,1) = [0;3;-2;2;3];
RowsShift(:,:,2) = [0;3;-2;-4;-3];
RowsShift(:,:,3) = [0;0;-6;-4;-5];
I can perform the matrix reordering using a loop and the "circshift" function and get the right result "B(i,j,k)":
% Old version with for looI canp and circular shift
for j = 1:size(A,3)
for i = 1:size(A,1)
B(i,:,j) = circshift(A(i,:,j),RowsShift(i,:,j));
end
end
B(:,:,1) =
0 0 0 1 0 0 0 0
0 0 0 1 0 0 0 0
0 0 0 1 0 0 0 0
0 0 0 1 0 0 0 0
0 0 0 1 0 0 0 0
B(:,:,2) =
0 0 0 1 0 0 0 0
0 0 0 1 0 0 0 0
0 0 0 1 0 0 0 0
0 0 0 1 0 0 0 0
0 0 0 1 0 0 0 0
B(:,:,3) =
1 0 0 0 0 0 0 0
1 0 0 0 0 0 0 0
1 0 0 0 0 0 0 0
1 0 0 0 0 0 0 0
1 0 0 0 0 0 0 0
My problem is that I have to circshift more than one 3D matrix with these dimension ("i" & "j" remains the same size) and the dimension "k" can be up to millions. My current version (loop & circshift) takes way to much time, so I'm looking for an alternative way.
Thanks in advance and best regards,
Pascal
  4 Comments
Pascal Wielsch
Pascal Wielsch on 17 Jun 2021
That's right. "RowsShift" is the same for all different 3D arrays. I already integrated your code snippet and calculate "idx" only once and reuse it for all 3D input matrices.

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Accepted Answer

Matt J
Matt J on 16 Jun 2021
Edited: Matt J on 16 Jun 2021
%%% Precomputations - do this block only once
[m,n,p]=size(A);
[I,J,K]=ndgrid(1:m,1:n,1:p);
%%%
Jshift=mod(J-RowsShift-1,n)+1;
idx=sub2ind([m,n,p], I,Jshift,K);
B=A(idx);

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