How to use an index of a vector as a value in another matrix?

Question

Syed Fahad Hassan 2017-8-3

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关闭： MATLAB Answer Bot 2021-8-20

Hi, I have a vector x=[1 1 2 2 3] and a matrix [1 3 ; 1 4 ; 2 3 ; 2 4]

The logic that I want to create says that the index of the latter of the similar values is extracted. Then, wherever the matrix has that value, that row becomes zero. For example, the indices of first two similar values in vector x are 1 and 2. I want those rows in the matrix that contains the value 2, should become zero. Same is the case for 3 and 4.

In the end, the matrix should only contain a single row i.e. [1 3]

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Matthew Eicholtz 2017-8-3

I think when you say "becomes zero", you mean "is removed", correct?

Syed Fahad Hassan 2017-8-4

Yes Matthew, I want the row to be removed

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Answer 1

dpb 2017-8-3

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>> x=[1 1 2 2 3] ;
>> m= [1 3 ; 1 4 ; 2 3 ; 2 4];
>> m(any(ismember(m,find(diff(x))),2),:)=[]
m =
   1     3
>>

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Stephen23 2017-8-4

+1 nice use of indexing.

Answer 2

Alex Kerzner 2017-8-3

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I'm sure someone can come up with something more clever and efficient, but here's something hack-y that might give you some ideas for improvements. Assuming the matrix is called M :

    repeats = [];
        while ~isempty(x)
            if numel(find(x == x(1))) >= 2 %If we have multiple of them
                repeats(end+1) = max(find(x==x(1))); %add the largest index to the list
                x(find(x==x(n))) = []; %remove all the duplicates
            else
                x(1) = []; %If no duplicates, just get rid of it
            end
        end
    for r = repeats
        [i,~] = find(M == r);
        M(i,:) = [];
    end

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Syed Fahad Hassan 2017-8-4

what is 'n' in your logic?

Answer 3

Matthew Eicholtz 2017-8-3

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How about this?

Assume you have the inputs:

x = [1,1,2,2,3];
y = [1,3; 1,4; 2,3; 2,4];

Then, you can find indices to remove by:

[~,ind,~] = unique(fliplr(x));
ind = length(x)-ind+1;

Then, remove the rows containing any of those indices:

y(any(ismember(y,ind),2),:) = [];

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Matthew Eicholtz 2017-8-3

Note, my approach will remove indices of values that exist only once in x (e.g., the 3). It is unclear from the question whether this should be allowed or not.