Extract matrix elements using a vector of column indices per row

Question

I have an MxN matrix and I want a column vector v, using the vector s that tells me for each row in the matrix what column I will take.

Here's an example:

Matrix =
[  4  13  93  20  42;
  31  18  94  64  02;
   7  44  24  91  15;
  11  20  43  38  31;
  21  42  72  60  99;
  13  81  31  87  50;
  32  22  83  24  04]    

s = [4 4 5 4 4 4 3].'

And the desired output is:

v = [20 64 15 38 60 87 83].'

I thought using the expression

Matrix(:,s)

would've work but it doesn't. Is there a solution without using for loops to access the rows separately?

Answer 1

It's not pretty, and there might be better solutions, but you can use the function sub2ind like this:

 M(sub2ind(size(M),1:numel(s),s'))

Answer 2

You can also do it with linear indexing, here is an example:

M=M'; s=s';
M([0:size(M,1):numel(M)-1]+s)