efficient variable circshift on 3D matrix

Question

Jona Gladines 2022-3-2

2
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1661690-efficient-variable-circshift-on-3d-matrix

回答： Jona Gladines 2022-3-2

Hello,

I have a working method of circularly shifting every 60 element vector in a 3D matrix A (300x300x60) over its corresponding value in 2D shift matrix B (300x300) which is relatively slow. I hope there is a faster method than the methods I currently have.

The shifting works as follows: If B(1,1) for example is 10, I want to shift A(1, 1, :) over 10 samples. Every value in B can be different.

My first approach was the following:

for i=1:size(B, 1)
    for j=1:size(B, 2)
        A(i, j, :) = circshift(A(i, j, :), B(i, j));
    end
end

which works, but is relatively slow (0.2s). A second approach was to first reshape matrices A and B to 2D and 1D respectively and get rid of the nested for loop.

a = reshape(A, size(A, 1)*size(A, 2), size(A, 3))';
b = reshape(B, size(B, 1)*size(B, 2), 1);
for i = 1:length(b)
    a(:, i) = circshift(a(:, i), b(i));
end
A = reshape(a', size(fm2, 1), size(fm2, 2), size(fm2, 3));

Which also works and is already little bit faster (0.1s).

Is there any other method to do this that would be much faster?

Thanks.

5 个评论
显示 3更早的评论隐藏 3更早的评论

Jan 2022-3-2

编辑：Jan 2022-3-2

在 MATLAB Online 中打开

Providing inputs would be very useful. It matters e.g. if the values of B are unique or if there are typically many same values. Optimizing code can exploit such patterns of the input.

For the test data DGM hast provided, this is twice as fast:

s = size(A);
a = reshape(A, [], s(3))';
b = reshape(B, [], 1);
ub = unique(b);
for i = 1:numel(ub)
   m       = (b == ub(i));
   a(:, m) = circshift(a(:, m), ub(i));
end
A = reshape(a', s);

Jona Gladines 2022-3-2

Ishift.mat

The shift data is part of a 3D recovery method, of which I cannot disclose more information at this point in time. however I've attached a shift matrix from one of the measurements. However since every item that is measured is different, these values wil also be different for every measurement. Sometimes there will be lots of the same values, other times values might differ more.

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Jan 2022-3-2

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1661690-efficient-variable-circshift-on-3d-matrix#answer_908125

在 MATLAB Online 中打开

In this code:

s = size(A);
a = reshape(A, [], s(3))';
b = reshape(B, [], 1);
ub = unique(b);
for i = 1:numel(ub)
   m       = (b == ub(i));
   a(:, m) = circshift(a(:, m), ub(i));
end
A = reshape(a', s);

40% of the computing time is spent for transposing. So if you store the data directly in a way, which let the operations work on the first dimension, the computing time is reduced also.