How to remove duplicates from a matrix without using unique?

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

[Jos (10584)]

some columns may have more unique elements than other columns. If you delete elements from columns than how will you store the results. In a cell array, perhaps?

[Adam]

What kind of output are you expecting? Obviously if you remove an arbitrary number of elements from each column you will no longer have a 2d numeric matrix because you can't have empty values in a matrix.

Also, I wouldn't recommend using length on a 2d matrix. If you want number of columns explicitly then use

size( A, 2 );

or

size( A, 1 );

to give you the number of rows.

[jeff417]

Adam, the entire goal of this project I'm working on is removing duplicates from each column and rewriting them to an excel spreadsheet. So I guess the output I'm expecting is each column as a separate array with separate variables with all duplicates removed.

[Jos (10584)]

and why can't you use unique ...

[Jan]

http://www.mathworks.com/matlabcentral/answers/38787-what-can-be-programmed-without-any-built-in-functions

Answer 1

[Kuifeng]

% use the function ismember for each column, and it would help

 doc ismember

[jeff417]

Sorry I forgot to mention I can't use built in functions to help me identify unique variables, including unique and ismember.

[Kuifeng]

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]

Kuifeng's suggestion contains the builtin functions: size, subsref, subsasgn, zeros, for, colon, eq, end, nan.

Answer 2

[Andrei Bobrov]

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))));

Answer 3

[Jos (10584)]

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 ...

[Guillaume]

Most likely this is an assignment, and the student is not allowed to use 'built_in functions'.

But then the whole purpose of the assignment would to see whether the student can develop his own unique algorithm, not to see if they can ask questions on matlab answer.

[Jos (10584)]

I think such assignments are made by lazy lecturers. As a carpenter, you need to learn how to use a hammer, not how to build one. As a hammer-builder you have to learn how to build a hammer from wood and metal, not how to chop wood in a forest. etc.

[Adam]

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]

When I use the function 'unique', it changes the order to smallest to largest, which messes up the matrix fpr me.