MATLAB: how to pass in the diagonal of a matrix as an argument in another matrix?

Question

Let c be 2D array, and x and y be 1D arrays of the same length (for instance, let\'s have x=1:7 and y=3:9).<

Answer 1

You are looking for sub2ind function

res = c( sub2ind(size(c), x, y ) )

Answer 2

There's an easier way. If you're looking for a diagonal, use diag. If you have a matrix c:

c =

    5    8    4    2    9    1    6    1    1
    9    8    7    5    9    3    2    7    5
    2    3    9   10    2    1    4    2    2
    3    2    9    2    4    4    7    2    4
    3    9   10    8    7    5    2    1    8
    5    6    3    7    6    1   10    5    2
    6    1    7    3   10    8    2    4    2

you can find the main diagonal by using diag with no extra arguments:

>> diag(c)
ans =

   5
   8
   9
   2
   7
   1
   2

The second argument, though, indicates which diagonal you want as an offset from the main diagonal. So the default diagonal is equal to 0. If you want the diagonal starting at c(1,3), that's 2 above the main diagonal, so

>> diag(c,2)
ans =

   4
   5
   2
   4
   2
   5
   2

Similarly, if you want the diagonal starting at c(4,1), the offset is -3:

>> diag(c,-3)
ans =

   3
   9
   3
   3