Is there faster ranking for matrices than double sort()?
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I have a uint16 matrix, where I want to determine the rank of each element with respect to its column in sorted form. The sort function doesn't quite do this, but using the second output twice does it. I need to repeat this routine many times in my code, and profiling reveals that it's by far the slowest part. So, I was wondering if there is a faster way to accomplish this (double sort() seems a bit of an overkill for this perhaps).
Here is a MWE:
M = 50;
m = 50;
n = 2000;
A = randi(M,m,n,'uint16'); % generate an example matrix
% Ranking via double sort():
tic;
[~,I] = sort(A,1);
[~,R] = sort(I,1);
toc;
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Accepted Answer
Bruno Luong
on 16 Sep 2022
Edited: Bruno Luong
on 16 Sep 2022
Not sure it is faster, but on the theorical complexity it is better (single sort).
A = randi(10,5,3)
[~,I] = sort(A,1);
[m,n] = size(A);
R = zeros(m,n);
R(I+(0:n-1)*m) = repmat((1:m)',1,n)
9 Comments
Bruno Luong
on 16 Sep 2022
Are you sure? it looks fine to me for toy example
A = randi(10,5,3);
[~,I] = sort(A,1);
[m,n] = size(A);
% double sort
[~,R] = sort(I,1)
% accumarray
R=accumarray(reshape(I+(0:n-1)*m,[],1),repmat((1:m)',n,1),[m*n,1]); % EDIT BUG FIX
R=reshape(R,m,n)
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