how to find common elements of two matrices considering also the repeated values?
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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:
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]
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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Accepted Answer
Stephen23
on 13 Jun 2018
Edited: Stephen23
on 13 Jun 2018
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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More Answers (2)
Image Analyst
on 13 Jun 2018
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)
Be sure to read the FAQ https://matlab.wikia.com/wiki/FAQ#What_is_a_cell_array.3F so you understand what I did.
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