Find index of a matrix of values into another matrix
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Hello,
I have a matrix A 151x1 double and a matrix B 151x1000 double. Each value of A corresponds to a certain value in the same row of B. For example if A=22;14;53;2, then B will have the value 22 in one of its columns in the first row, the value 14 in one of its columns in the second row etc. Is there a way to have the matrix of the index of all the columns in B that contain the values of A (always in their coresponding row?).
3 Comments
Accepted Answer
madhan ravi
on 15 Apr 2019
Indices = cell(numel(A),1);
for k = 1:numel(A)
Indices{k} = find(B(k,:)==A(k));
end
%celldisp(Indices)
% Indices{1} represents first element of A match in first row of B likewise others
9 Comments
Adam Danz
on 15 Apr 2019
Now test it with these inputs and you'll see why the simpler suggestion doesn't work.
A = [1;5;15;22]
B = [1 23 10 1 9; 1 31 51 5 11; 15 22 33 44 55; 20 5 22 22 22]
Jon
on 15 Apr 2019
Yes I agree your approach will cover the case where there is more than one matching entry in a row. I did condition my original response with limitation that for the simple approach to work there had to be exactly one match in each row. In the end I'm not sure whether the OP has multiple matching entries in a row. Thanks for the clarification and discussion.
More Answers (2)
Adam Danz
on 15 Apr 2019
If I understood you correctly, you have a matrix "B" and a column vector "A" and you want the identify the columns of B that are identical to A (if any). Here's a demo that achieves that:
% Create data
A = randi(100, 151,1);
B = randi(100, 151, 1000);
B(:,55) = A; %column #55 of B equals A
B(:,66) = A; %column #66 of B also equals A
% Identify columns of B that match A
% matchIdx is a logical row vectors containing 'true' for columns of B that match A
matchIdx = ismember(B.', A.', 'rows').';
% Find the column numbers
colNums = find(matchIdx);
colNums =
55 66
Jon
on 15 Apr 2019
This should work assuming you have exactly one match in each row. If some rows have no matches or more than one match you will have to do something more elaborate
% find the row and column indices of all the places in B that match A
[row,col] = find(A==B)
% sort the matching column locations by row to get them in the desired (row) order
idx = col(row)
3 Comments
Jon
on 15 Apr 2019
Edited: Jon
on 15 Apr 2019
Actually, it is ok the == operation apparently operates column wise in this situation and then find with two left hand arguments works perfectly for this situation. Please try it to see.
For more info on A==B with A a column with the same number of rows as B please see https://www.mathworks.com/help/matlab/matlab_prog/compatible-array-sizes-for-basic-operations.html
Adam Danz
on 15 Apr 2019
The problem is not that A and B are not the same size. The problem is that this does not address the question. The OP wants to list column numbers where row 'n' of B matches the value of row 'n' of A.
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