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 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').


Andrei Bobrov
Andrei Bobrov on 14 May 2011
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.
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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