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FInd vector in matrix

Asked by Kris zenitis on 14 May 2011
Latest activity Commented on by Esubalew on 25 Jul 2017
Accepted Answer by Jan
Lets assume that I have a matrix A 300x2 with x y coordinates. I want to find how many times a specific [x y] exists in this matrix A. I ve tried this but find can search only single values.
i = find(A == [x y]);
Any idea?????

  1 Comment

[Kris zenitis commented]
Yeah thank you man!!

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2 Answers

Answer by Jan
on 14 May 2011
 Accepted Answer

Another method:
I = sum(A(:, 1) == x & A(:, 2) == y);
EDITED: Buggy multiplication method removed. Thanks Andrei!
This method is about 50 times faster than ISMEMBER(A, b, 'rows').

  3 Comments

Hi Jan! may be it?
I = sum((A(:, 1) == x).* (A(:, 2) ==y));
Jan
on 14 May 2011
@Andrei: You are right. But the multiplication is slower than the AND operator. Therefore I've removed this method.
Hi Jan,
The method you suggested above is a good one, but incase if A is 1000 by 100 matrix or a bigger matrix, it there a better way to cind the row of A, A_i such that A_i ==v for some vector v of size =size(A,2)?
Thank you!

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Answer by Loginatorist on 14 May 2011

For example:
A = reshape(1:12,6,2) % A sample matrix for demonstration...
I = ismember(A,[4 10],'rows');
If you want to find how many there are, use:
S = sum(I)

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