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Indexing matrices with matrices in MATLAB

I am trying to pull values ​​from a matrix that serves mainly as a lookup table. For this I have a row coordinate matrix and a column coordinate matrix. Is there a way that I can output the values ​​like this:

A(R, C)

or

A(R(:), C(:))

For example:

A = [ 1, 2, 3, 4; 
      5, 6, 7, 8; 
      9, 0, 1, 2; 
      3, 4, 5, 6 ];

R = [ 1, 2; 
      1 2];
C = [ 1, 1; 
      1 1];

I would like to find some command to

A(R, C) =

1, 5
1, 5

or

A(R(:), C(:)) =

1
1
5
5

What actually happens when you run these commands, you get every permutation of the matrix. So, in my example, you end up with a 4x4 result matrix consisting of

A(1,1)  A(1,1)  A(1,1)  A(1,1)
A(1,1)  A(1,1)  A(1,1)  A(1,1)
A(2,1)  A(2,1)  A(2,1)  A(2,1)
A(2,1)  A(2,1)  A(2,1)  A(2,1)

Is there a way to just do the indexing on each index matrix / vector so that the result shows

A(1,1)  A(2,1)
A(1,1)  A(2,1)

Thank!

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2 answers


Indeed, you are pointing out a subtle issue with Matlab indexing.
To solve this problem, you need to convert rows columns indexes R

, C

in the linear indices . One easy way to do this is sub2ind

:

 A( sub2ind( size(A), R, C ) );

Here's how it works

>> A(sub2ind(size(A),R,C))
ans =
 1     5
 1     5

>> A(sub2ind(size(A),R(:),C(:)))
ans =
 1
 1
 5
 5


If you are working hard at runtime and want to avoid overhead sub2ind

, you can convert the indices directly:

 A( size(A,1)*(C-1)+R )
+4


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you can use arrayfun 

>> arrayfun(@(x,y) A(x,y), R, C)
ans =
     1     5
     1     5


You get the result the same size as R

and C

not as a vector.

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