CODEWITHSUNDEEP
arraysmatlabmatrix
Christina
Christina

Reputation: 935

Automatic test for the equality of columns between two matrices

I have two matrices:

X =

     1     2     3
     4     5     6
     7     8     9

`Y` =

     1    10    11
     4    12    13
     7    14    15

I know that if I want to find the index of a specific element in X or Y, I can use the function find. For example: index_3 = find(X==3)

What I want is to find or search in a very automatic way if a column in X is also present in Y. In other terms, I want a function which can tell me if a column in X is equal to a column in Y. In fact to to try this, one can use the function ismember which indeed has an optional flag to compare rows:

rowsX = ismember(X, Y, 'rows');

So a simple way to get columns is just by taking the transpose of both matrices:

rowsX = ismember(X.', Y.', 'rows')
rowsX =

   1
   0
   0

But how can I do that in other manner?

Any help will be very appreciated!

Upvotes: 2

Views: 69

Answers (1)

Luis Mendo
Luis Mendo

Reputation: 112769

You can do that with bsxfun and permute:

rowsX = any(all(bsxfun(@eq, X, permute(Y, [1 3 2])), 1), 3);

With

X = [ 1     2     3
      4     5     6
      7     8     9 ];
Y = [ 1    10    11
      4    12    13
      7    14    15 ];

this gives

rowsX =
     1     0     0

How it works

permute "turns Y 90 degrees" along a vertical axis, so columns of Y are kept aligned with columns of X, but rows of Y are moved to the third dimension. Testing for equality with bsxfun and applying all(...,1) gives a matrix that tells which columns of X equal which columns of Y. Then any(...,3) produces the desired result: true if a column of X equals any column of Y.

Upvotes: 1

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