How to remove duplicates from a matrix without using unique?

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jeff417
jeff417 2016-4-14
评论： Eduardo Negredo Martin 2021-10-15
I have to be able to remove all duplicates from each column of a matrix A = [1,2,3;1,3,3;4,2,1], while also not using unique and not changing the order.
I got the code to work for a single column, I'm just not sure how to do it for a matrix.
z = length(A);
Ab = zeros(size(A));
for i = 1:(z-1)
Ab(i) = sum(A(i) == A(i+1:end));
end
AA = A(Ab == 0);
end
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回答（3 个）

Kuifeng
Kuifeng 2016-4-14
% use the function ismember for each column, and it would help
doc ismember
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Kuifeng
Kuifeng 2016-4-14
then the following is ok based on your available column code,
[rows cols] = size(A);
Ab = zeros(rows, cols);
for j = 1: cols
for i = 1:rows-1
Ab(i, j) = sum(A(i,j) == A(i+1:end, j));
end
end
A(Ab == 1) =nan;
Jan
Jan 2016-4-14
Kuifeng's suggestion contains the builtin functions: size, subsref, subsasgn, zeros, for, colon, eq, end, nan.

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Andrei Bobrov
Andrei Bobrov 2016-4-14
编辑：Andrei Bobrov 2016-4-14
A = [1,2,3;1,3,3;4,2,1];
[a,ii] = sort(A);
a([false(1,size(a,2));diff(a)==0]) = nan;
[~,i1] = sort(ii);
out = a(sub2ind(size(A),i1,ones(size(A,1),1)*(1:size(A,2))));
Jos (10584)
Jos (10584) 2016-4-14
I do not see any reason why you can't use unique
A = randi(5,5,10) % some data
C = arrayfun(@(k) unique(A(:,k),'stable'),1:size(A,2),'un',0)
C{k} now holds the unique values of column k of A in preserved order ...
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Adam
Adam 2016-4-14
编辑：Adam 2016-4-14
These questions do always seem odd to me too. Not using builtin functions makes using Matlab itself kind of pointless. If you come to use Matlab in a real work environment your employers would expect you to make use of every helpful function available to you.
It also begs the question of what counts as a builtin function.
length, zero and sum in the author's original answer are all builtin, as are numerous hidden function such as subsref that get called when you do almost anything with arrays!
But I agree with Guillaume that the only purpose (though even then a questionable one) of being asked to not use builtin functions is to work out your own method!
Eduardo Negredo Martin
Eduardo Negredo Martin 2021-10-15
When I use the function 'unique', it changes the order to smallest to largest, which messes up the matrix fpr me.

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