MATLAB: How to extract values from a matrix using a row – column matrix

indexmatrix manipulation

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

Best Answer

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
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