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

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Benson Gou il 2 Feb 2021
Commentato: Benson Gou il 3 Feb 2021
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
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### Risposta accettata

Stephen23 il 2 Feb 2021
[~,Y] = ismember(B,A,'rows')
Y = 4×1
1 4 5 6
##### 1 CommentoMostra -1 commenti meno recentiNascondi -1 commenti meno recenti
Benson Gou il 3 Feb 2021
Hi, Stephen,
Thanks a lot for your great help. It works very well.
Benson

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