# Find index of a matrix of values into another matrix

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Georgios Tertikas on 15 Apr 2019
Commented: Jon on 15 Apr 2019
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?).
madhan ravi on 15 Apr 2019

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
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.

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
##### 2 CommentsShowHide 1 older comment
Adam Danz on 15 Apr 2019
madhan ravi's solution should do what you're describing. If not, you can elaborate.

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)
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.