# How to extract values from a matrix using a row - column matrix?

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VIJENDRA on 22 Apr 2015
Commented: VIJENDRA on 23 Apr 2015
I have a matrix like
A =
0.2180 0.5714 0.2476 0.3942 0.9253 0.5082
0.3906 0.5881 0.6529 0.3806 0.3819 0.7947
0.3381 0.0287 0.0652 0.3696 0.3735 0.1351
0.7459 0.0911 0.0163 0.7531 0.3298 0.2050
0.3690 0.5010 0.5201 0.6119 0.9038 0.1902
0.4232 0.9183 0.6523 0.2392 0.4661 0.0019
and a matrix having row and column indices
loc =
1 2
2 5
4 3
6 6
I want the extract the values of locations (1,2), (2,5), (4,3) and (6,6) from matrix A when I do this
val = A(loc(:,1),loc(:,2));
it gives me
val =
0.5714 0.9253 0.2476 0.5082
0.5881 0.3819 0.6529 0.7947
0.0911 0.3298 0.0163 0.2050
0.9183 0.4661 0.6523 0.0019
I can use diag(val); to get the locations values but is there any way so that i could get values directly without getting that square matirx
val =
0.5714
0.3819
0.0163
0.0019

Stephen23 on 22 Apr 2015
Edited: Stephen23 on 22 Apr 2015
Yes! You can use sub2ind to achieve this:
A = [...
0.2180 0.5714 0.2476 0.3942 0.9253 0.5082
0.3906 0.5881 0.6529 0.3806 0.3819 0.7947
0.3381 0.0287 0.0652 0.3696 0.3735 0.1351
0.7459 0.0911 0.0163 0.7531 0.3298 0.2050
0.3690 0.5010 0.5201 0.6119 0.9038 0.1902
0.4232 0.9183 0.6523 0.2392 0.4661 0.0019];
loc = [...
1 2
2 5
4 3
6 6];
X = sub2ind(size(A), loc(:,1), loc(:,2));
and then run this in the command window:
>> A(X)
ans =
0.5714
0.3819
0.0163
0.0019
VIJENDRA on 23 Apr 2015
Thanks Stephen Cobeldick

Punit on 22 Apr 2015
[nRows nCols ] = size(A);
A(nRows*(loc(2)-1)+loc(1))
Guillaume on 22 Apr 2015
This is what sub2ind does and I'd recommend using it as it works regardless of the number of dimensions of A.