Finding the index of dupplicate rows in a matrix

I have matrix M 275935x2 . I want to remove duplicate rows. I tried two methods: Method 1)

     M2=unique(M,'rows') % and it gave M2 179109x2 double
Method 2)

x0=find(hist(M,unique(M))>1); % it gave only 8301 duplicate values.

Which method is correct? I want to find th eindices of duplicate rows and not simply remove them. Any help is appreciated

Answer 1

[Stephen23]

>> A = randi(1e4,275935,2);
>> [B,~,Y] = unique(A,'rows','stable');
>> [C,X] = hist(Y,unique(Y));
>> Z = ismember(Y,X(C>1)); % indices of repeated rows of A

For example this random data set had

 >> nnz(Z)
ans =
   816

row that occur most than once. To get the indices of the duplicate rows, try this:

 [U,W] = unique(A,'rows','stable');
 D = setdiff(1:size(A,1),W); %indices of duplicate rows.

[Danielle Leblance]

I am sure there is something wrong. I am attaching the data.the unique function gives a matrix B which is different than the one that I obtain if I remove the duplicates Z

[Stephen23]

This works for me:

>> load matlab.mat
>> [B,W] = unique(M,'rows','stable');
>> D = setdiff(1:size(M,1),W); % indices of duplicate rows.

And now compare:

>> M(D,:) = [];
>> isequal(M,B)
ans =
     1