how to find common elements of two matrices considering also the repeated values?

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Gessica Cos 2018-6-13

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评论： Gessica Cos 2018-6-14

Hello everybody, i would like to know how to find the position of elements of a matrix who are in another matrix. For example, letting i have two matrix A and B:

I would like the output to provide the position of the elements of the B matrix that are present in the A matrix considering also repeated values.

For example "it finds in which position the element B (1,1) is present in the matrix A (:, 1), in which position the element B (2,1) is present in the matrix A (:, 1), in which position the element B (3,1) is present in the matrix A (:, 1) and so on ". So similarly for the other columns "find in which position the element B (1,2) is present in the matrix A (:, 2) etc"

Regarding the repetition I would like to make sure that if, for example, there are 3 repetitions in a column, each repetition will have an increasing order index, ie:

A= [ 2
4
4
4]
B=[4
2
4
4]
output=[2
1
3
4 ]

I tried to use the MATLAB function "intersect", but it give as outputs only the indexes of the columns whose values are not repeated.

for n=1:size(A,2)
      [C, idx1, idx2] = intersect( B(:,n), A(:,n), 'stable');
end

I do not know if I've been clear. Sorry.

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Answer 1

Stephen23 2018-6-13

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编辑：Stephen23 2018-6-13

在 MATLAB Online 中打开

This is easy with sort, assuming that both A and B contain exactly the same elements:

>> A = [2;4;4;4];
>> B = [4;2;4;4];
>> [~,ida] = sort(A);
>> [~,idb] = sort(B);
>> [~,ida] = sort(ida);
>> [~,idb] = sort(idb);
>> ida(idb)
ans =
     2
     1
     3
     4

EDIT: use a simple loop to do matrices of any size. This is simpler because C is preallocated so we can index into it using idb:

A = [2,1,2;3,2,5;4,7,5;4,8,10];
B = [4,7,5;2,8,10;4,1,2;3,2,5];
C = nan(size(A));
for k = 1:size(A,2)
    [~,ida] = sort(A(:,k));
    [~,idb] = sort(B(:,k));
    C(idb,k) = ida;
end

And checking the output:

>> C % my code
C =
     3     3     2
     1     4     4
     4     1     1
     2     2     3
>> [3 3 2; 1 4 4; 4 1 1; 2 2 3] % requested output
ans =
     3     3     2
     1     4     4
     4     1     1
     2     2     3

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Stephen23 2018-6-13

编辑：Stephen23 2018-6-13

@Gessica Cos: see my edited answer. Neither ismember nor interesect will do what you want.

Gessica Cos 2018-6-14

Thanks so much. You've been really kind. You have been very helpful to me.

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Answer 2

Image Analyst 2018-6-13

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Try ismember().

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Gessica Cos 2018-6-13

I tried to use "ismember" but I have not succeeded.Could you give me an idea of the code I should write?

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Answer 3

Image Analyst 2018-6-13

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在 MATLAB Online 中打开

Try this:

A= [2 1 2
  3 2 5
  4 7 5
  4 8 10]
B= [4 7 5
  2 8 10
  4 1 2
  3 2 5]
% Find unique numbers in each of the two arrays.
ua = unique(A)
ub = unique(B)
% Check each number in B to see where it lives in A.
for k = 1 : length(ub)
  % Get this number from B
  thisB = ub(k);
  % Find out what rows and columns it shows up at in A
  [rows, columns] = find(A == thisB);
  % Save the number we were looking for into column 1 of the cell array.
  locations{k, 1} = thisB;
  % Save the locations that number was found into column 2 of the cell array.
  locations{k, 2} = [rows, columns];
end
celldisp(locations)