Using a vector as column-indices into rows of a matrix, in Octave

Question

Let's say I have a matrix A and a vector B. Is it possible to use the values in vector B as indices to select one value from each row in matrix A? Example:

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

B = [1;3;1]

C = A(B) or C = A(:,B) 

giving: 

C = [1; 6; 7]

Of course I could do this with a for loop but with bigger matrices it will take a while. I would also like to use this to make a logical matrix in the following fashion:

A = zeros(3,3)

B = [1;3;1]

A(B) = 1

A = [1, 0, 0;
     0, 0, 1;
     1, 0, 0]

Thanks for any advice you are able to give me.

score 9 · Accepted Answer · answered Aug 18 '14 at 21:32

9

You need to create linear indices for that. Following your example:

octave-3.8.2> a = [1  2  3
                   4  5  6
                   7  8  9];
octave-3.8.2> b = [1 3 1];
octave-3.8.2> ind = sub2ind (size (a), 1:rows (a), b);
octave-3.8.2> c = a(ind)
c =

   1   6   7

answered Aug 18 '14 at 21:32

carandraug

12,938
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1

Thanks man. I was looking at sub2ind but couldn't figure it out on my own. Now I can rewrite my code snippet and ditch the loop. :) – tarikki Aug 19 '14 at 11:58

score 3 · Answer 2 · edited Mar 05 '19 at 19:11

3

As per my understanding, the way to go about creating a logical matrix is below:

>A = eye(3,3)

>B = [1;3;1]

>A(B,:) =
>
>[ 1   0   0;
>  0   0   1;  
>  1   0   0; ]

edited Mar 05 '19 at 19:11

Alex Myers

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answered Mar 05 '19 at 17:18

Shugeeth

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Using a vector as column-indices into rows of a matrix, in Octave

2 Answers2