how to get values of a matrix in MATLAB where the indices are given in a nx2 array

Question

I have a matrix A of size nRows x nCols.

I have a nx2 matrix B which contains indices of the matrix A. I want to get the values of A at the indices given in B.

lets say,

B = [1, 2;
     2, 3;
     3, 4]

A(1,2) = 1
A(2,3) = 2
A(3,4) = 1

I want to know any Matlab command which gives the following, given A and B (I don't want to use loops):

[1 2 1]

Ok I got it. I need to use sub2ind. A(sub2ind(size(A), B(:,1), B(:,2))) — Swagatika, Oct 27 '12 at 10:00
general solution (for any-dimensional matrices) is [here](http://stackoverflow.com/questions/10146082/indexing-of-unknown-dimensional-matrix) — Gunther Struyf, Oct 27 '12 at 11:56

score 6 · Accepted Answer · answered Oct 27 '12 at 10:26

6

I guess this is what you are looking for:

A(sub2ind(size(A),B(:,1),B(:,2)))

answered Oct 27 '12 at 10:26

mola

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...yes, thanks. I was not able to put an answer so I put it as comment. – Swagatika Oct 28 '12 at 14:13

Ali · Answer 2 · 2012-10-27T10:53:47.207

2

This is what you want:

A = [1,2; 3, 4; 5, 6; 7,8; 9,0]; % this is your N by 2 matrix
B = [1,1; 1,2; 2,1; 3, 1; 4,2]; % these are your indexes
A(sub2ind(size(A), B(:,1), B(:,2)))

A =

 1     2
 3     4
 5     6
 7     8
 9     0



B =

 1     1
 1     2
 2     1
 3     1
 4     2

ans =

 1
 2
 3
 5
 8

edited Oct 27 '12 at 10:53

answered Oct 27 '12 at 09:57

Ali

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what do you mean? I edit the answer to show you how general it is. – Ali Oct 27 '12 at 10:36

how to get values of a matrix in MATLAB where the indices are given in a nx2 array

2 Answers2