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How to remove duplicates from a matrix without using unique?

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jeff417 on 14 Apr 2016
Commented: Jan on 14 Apr 2016
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));
AA = A(Ab == 0);

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Answers (3)

Kuifeng on 14 Apr 2016
% use the function ismember for each column, and it would help
doc ismember
Jan on 14 Apr 2016
Kuifeng's suggestion contains the builtin functions: size, subsref, subsasgn, zeros, for, colon, eq, end, nan.

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Andrei Bobrov
Andrei Bobrov on 14 Apr 2016
Edited: Andrei Bobrov on 14 Apr 2016
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) on 14 Apr 2016
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 ...
Adam on 14 Apr 2016
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!

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