Removing duplicate rows (not "unique")
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Michael Siebold
on 4 May 2016
Commented: saad sulaiman
on 5 Nov 2022
I have a matrix with many (1e5+) rows and I want to remove both copies of all duplicate rows. Is there a fast way to do this? (This function needs to be run many times.)
4 Comments
jgg
on 4 May 2016
You can use the other calling methods to get replicate counts.
a = [1 2; 1 2; 2 3; 2 4; 2 5; 4 2; 4 2; 1 3; 1 3; 4 5];
[C,ia,ic] = unique(a,'rows');
[count key] = hist(ic,unique(ic));
Then you can just select the keys with non-unit counts and drop them.
Accepted Answer
Roger Stafford
on 5 May 2016
Edited: Roger Stafford
on 5 May 2016
Let A be your matrix.
[B,ix] = sortrows(A);
f = find(diff([false;all(diff(B,1,1)==0,2);false])~=0);
s = ones(length(f)/2,1);
f1 = f(1:2:end-1); f2 = f(2:2:end);
t = cumsum(accumarray([f1;f2+1],[s;-s],[size(B,1)+1,1]));
A(ix(t(1:end-1)>0),:) = []; % <-- Corrected
6 Comments
saad sulaiman
on 5 Nov 2022
greetings.
how could we apply this code to a mesh where we have coordinate points for each triangle, such that we remove the internal edges, or edges shared by two triangles?
thanks in advance.
More Answers (2)
Azzi Abdelmalek
on 4 May 2016
Edited: Azzi Abdelmalek
on 4 May 2016
A=randi(5,10^5,3);
tic
A=unique(A,'rows');
toc
The result
Elapsed time is 0.171778 seconds.
3 Comments
Azzi Abdelmalek
on 4 May 2016
Edited: Azzi Abdelmalek
on 4 May 2016
You said that unique function will leave a copy of duplicate rows. With this example, I show you that there is no duplicates rows stored! And also it doesn't take much time
Mitsu
on 3 Aug 2021
I reckon your answer does not address OP's question because running the following:
A=[1 1 1;1 1 1;1 1 0];
tic
A=unique(A,'rows');
toc
Will yield:
A = 1 1 0
1 1 1
Therefore, A still contains one instance of each row that was duplicate. I believe Michael wanted all instances of each row that appears multiple times be removed.
GeeTwo
on 16 Aug 2022
%Here's a much cleaner way to do it with 2019a or later!
[B,BG]=groupcounts(A);
A_reduced=BG(B==1); % or just A if you want the results in the same variable.
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