how to select an element from each row in a matrix given a vector of column indeces per row (no for loops)

Hello, I saw posts to similar questions, but I still couldn't figure out how to do what I want to do. I have a matrix and a vector of values. This vector contains the column indeces to the element I want to extract from each row. for ex:

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

I tried x=A(:,b), but this returns

x=A(:,b)

x =

     2     1     1
     5     3     3
     8     7     7

But what I was hoping to get was

x= [2;3;7].

I could do it with a for loop, but is there a different/direct way of doing this?

Thank you, Jorge

Answer 1

[jonas]

If you want one value from each row, then you could use sub2ind, like this:

A(sub2ind(size(A),1:size(A,1),b'))

This is useful if you have vectors of subscripts (i.e. rows and columns) and you want to extract the corresponding values from a matrix. In this case you want one value per row, so the 2nd argument is just [1 2 3].

[Jorge Rivé]

perfect...thank you.

[Andy]

what if there are multiple columns to extract ... A(sub2ind(size(A),1:size(A,1),b')) becomes a complete mess:

A(sub2ind(size(A),repmat(1:size(A,1),1,nrColumns),b'(:)))

Having a matrix MxN, where we need to extract 5 elements from each line, on random column IDs ... isn't there a more elegant than a FOR solution?