How to Select one element from each row of a matrix A based on a column matrix B.

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vishwanath
vishwanath 2012-1-30
Hello Everyone,
I need to select one element from each row of matrix A of size 6400 X 8 based on another column matrix of size 6400 X 1.
For e.g. lets say A( 6400X8) = [ 1 0 1 0 0 0 1 1; 0 1 0 1 1 1 0 0; 1 0 1 0 0 0 1 0; 0 1 0 0 0 0 0 1; 1 0 0 1 0 0 1 1; 1 0 1 0 0 0 1 0; ...... ..... ];
and let B (6400X1) = [ 6; 7; 1; 4; 5; 8; .; .; .; ]; Now I need a matrix C which has elements from each row of matrix A based on matrix B.
A( 6400X8) = [ 1 0 1 0 0(0) 1 1;
0 1 0 1 1 1 (0)0;
(1) 0 1 0 0 0 1 0;
0 1 0 (0)0 0 0 1;
1 0 0 1 (0)0 1 1;
1 0 1 0 0 0 1 (0);
......;
......;
];
Therefore C = [ 0; 0; 1; 0; 0; 0; .; .; .; .; ];
Please help me. Thanks in advance.

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Bård Skaflestad
Bård Skaflestad 2012-1-30
So, if I understand correctly, your vector B contains column indices (i.e., numbers between 1 and size(A,2) inclusive) such that you want to extract the elements C(i)=A(i,B(i)) for all i=1:size(A,1). Is that correct?
If so, this is a perfect case for linear indexing. Much more about this technique can be found in Steve Eddins' blog at http://blogs.mathworks.com/steve/2008/02/08/linear-indexing/. In this case I'd write something along the lines of
I = (1 : size(A, 1)) .';
J = reshape(B, [], 1);
k = sub2ind(size(A), I, J);
C = A(k);

更多回答（1 个）

Sean de Wolski
Sean de Wolski 2012-1-30
You could use sub2ind():
idx = sub2ind(size(A),(1:numel(B)).',B); %not tested in ML
AB = A(idx);
for more info:
doc sub2ind

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