Using vector as index columns in matrix rows, in Octave

Question

Using vector as index columns in matrix rows, in Octave

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 it with a for loop, but with large matrices it will take a while. I would also like to use this to make a boolean matrix like this:

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 can give me.

+3

vector matrix indexing octave

tarikki Aug 18 14 at 19:07

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1 answer

carandraug · Accepted Answer · 2014-08-18T21:32:52+0000

For this, you need to create linear indices. 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

Using vector as index columns in matrix rows, in Octave

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