MATLAB Answers

FInd vector in matrix

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Kris zenitis
Kris zenitis on 14 May 2011
Commented: Esubalew on 25 Jul 2017
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

Azzi Abdelmalek
Azzi Abdelmalek on 19 Feb 2014
[Kris zenitis commented]
Yeah thank you man!!

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Accepted Answer

Jan
Jan on 14 May 2011
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

Andrei Bobrov
Andrei Bobrov on 14 May 2011
Hi Jan! may be it?
I = sum((A(:, 1) == x).* (A(:, 2) ==y));
Jan
Jan on 14 May 2011
@Andrei: You are right. But the multiplication is slower than the AND operator. Therefore I've removed this method.
Esubalew
Esubalew on 25 Jul 2017
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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More Answers (1)

Matt Fig
Matt Fig 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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