MATLAB: How to quickly obtain the row indices in the original matrix A for a sub-matrix B

row indicex

Dear All,

I have an original matrix A and a sub-matrix which is formed by selected rows from A. Now given A and B, how can I find out the row indices of B in A?

For example,

A = [

0 0 1 0 -1

2 -1 0 0 0

-1 0 -1 0 0

0 0 3 -1 -1

0 0 -1 1 0

0 0 -1 0 1

];

B = [

0 0 1 0 -1

0 0 3 -1 -1

0 0 -1 1 0

0 0 -1 0 1

];

The indices should be ind = [1 4 5 6].

Thanks a lot.

Benson

Best Answer

[~,Y] = ismember(B,A,'rows')
Y = 4×1
     1
     4
     5
     6

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MATLAB: How to delete a row from a matrix

When you use vectorized code then this task is simple, and there is no need for complicated loops:

>>B(any(B,2),:)
ans =
   1     0     0     0     0     0     0
   0     0     0     1     0     0     0
   0     1     0     1     0     0     0
   0     0     0     0     1     1     0
   0     1     0     0     0     0     1
   0     0     0     0     0     0     1
   0     0     1     0     0     0     0

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You can't chain indexing operations, so you can either create a temporary variable

subarr = arr(i:i+n-1, j:j+n-1);
x = somefn(subarr(:), a);

Or, since the colon here just does a reshape, call reshape explicitly:

x = somefn(reshape(arr(i:i+n-1, j:j+n-1), [], 1), a);

Best Answer

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MATLAB: How to delete a row from a matrix

MATLAB: Manipulating a sub-matrix

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